fix: harden remaining review findings

Reviewed-on: #211

README.md

@@ -140,6 +140,7 @@ Config negotiation is sender-driven: the client serializes transfer options and
 | `--cert <path>` | TLS certificate file (PEM) |
 | `--key <path>` | TLS private key file (PEM) |
 | `--ca <path>` | TLS CA certificate file for verification (PEM) |
+| `--client-cn <name>` | Required TLS client certificate common name |
 
 ### Server
 
@@ -151,6 +152,9 @@ Config negotiation is sender-driven: the client serializes transfer options and
 | `--cert <path>` | TLS certificate file (PEM) |
 | `--key <path>` | TLS private key file (PEM) |
 | `--ca <path>` | TLS CA certificate file for verification (PEM) |
+| `--destination-root <path>` | Authorized destination root (default: `.`) |
+| `--allow-delete` | Permit manifest deletion |
+| `--allow-unauthenticated` | Permit plaintext TCP clients |
 | `-v, --verbose` | Enable debug logging |
 | `--help` | Show help |
 
@@ -188,7 +192,7 @@ Config negotiation is sender-driven: the client serializes transfer options and
 7. **Metadata restoration** — `chmod()`, `chown()`, `utimensat()` on the receiving side
 8. **`--delete`** — sender tracks all sent paths; receiver walks destination tree and removes unlisted files/directories
 9. **SSH transport** — `socketpair()` + `fork()` + `execvp("ssh", ...)` with `ControlMaster` and port support
-10. **TLS transport** — OpenSSL `SSL_CTX` with TLS 1.2 minimum, optional CA verification, transparent `SSL_read`/`SSL_write` via `io_set_ssl()`
+10. **TLS transport** — OpenSSL `SSL_CTX` with TLS 1.2 minimum, mutual CA verification, transparent `SSL_read`/`SSL_write` via `io_set_ssl()`
 11. **Path traversal protection** — `has_path_traversal()` rejects any file path containing `..` components, preventing directory escape attacks
 12. **Connection limiting** — server tracks active connections and rejects new ones beyond `max_connections` (default 100)
 13. **Keep-alive** — idle connections receive periodic `STATUS_KEEPALIVE` to detect half-open TCP connections
@@ -202,7 +206,7 @@ Config negotiation is sender-driven: the client serializes transfer options and
 All received file paths are validated by `has_path_traversal()` before any disk operation. Any path containing `..` components is rejected with `STATUS_ERROR`, preventing directory escape attacks.
 
 ### TLS Certificate Verification
-When `--ca` is provided, the server performs mutual TLS verification (`SSL_VERIFY_PEER` with depth 4). Without `--ca`, TLS is still encrypted but peer certificates are not verified.
+TLS requires `--ca` and performs mutual TLS verification (`SSL_VERIFY_PEER` with depth 4). Connections without certificate verification are rejected.
 
 ### Connection Limits
 The server enforces a maximum of 100 concurrent connections (configurable via `max_connections` in `Server`). When the limit is reached, new connections are immediately rejected and closed.
@@ -244,12 +248,12 @@ cmake -B build -S . && cmake --build build -j$(nproc)
 
 ### Server (TCP mode)
 ```bash
-./build/server
+./build/server --allow-unauthenticated
 ```
 
 ### Server with TLS
 ```bash
-./build/server --tls --cert server.pem --key server-key.pem
+./build/server --tls --cert server.pem --key server-key.pem --ca ca.pem --client-cn fastsync-client
 ```
 
 ### Server via SSH
@@ -265,6 +269,9 @@ Place the `fastsync-server` binary in the remote `$PATH`. The client runs `ssh u
 ./build/client --source-dir /path/to/send --dest-dir /path/to/receive --save-to-disk
 ```
 
+Plain TCP requires the explicit `--allow-unauthenticated` server option. Use TLS for
+authenticated network connections.
+
 ### Client — TCP with TLS
 ```bash
 ./build/client --tls --cert client.pem --key client-key.pem --ca ca.pem \

src/client/client_cli.c

@@ -400,8 +400,8 @@ static bool validate_config(const Config* config) {
     return false;
   }
   if (config->use_tls) {
-    if (!config->tls_cert || !config->tls_key) {
-      fprintf(stderr, "Error: --tls requires --cert and --key\n");
+    if (!config->tls_cert || !config->tls_key || !config->tls_ca) {
+      fprintf(stderr, "Error: --tls requires --cert, --key, and --ca\n");
       return false;
     }
   }

src/server/server.c

@@ -19,10 +19,28 @@
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <openssl/x509.h>
 
 static char* authorized_root;
 static int authorized_root_fd = -1;
 static bool allow_delete;
+static bool allow_unauthenticated;
+static const char* required_client_cn;
+
+static bool tls_client_identity_allowed(SSL* ssl) {
+  if (!ssl || !required_client_cn)
+    return false;
+  X509* certificate = SSL_get1_peer_certificate(ssl);
+  if (!certificate)
+    return false;
+  char common_name[256];
+  int length = X509_NAME_get_text_by_NID(X509_get_subject_name(certificate), NID_commonName,
+                                         common_name, sizeof(common_name));
+  bool allowed = length >= 0 && (size_t)length < sizeof(common_name) &&
+                 strcmp(common_name, required_client_cn) == 0;
+  X509_free(certificate);
+  return allowed;
+}
 
 static bool path_is_within(const char* root, const char* path) {
   size_t n = strlen(root);
@@ -48,7 +66,7 @@ static bool __attribute__((unused)) configure_authorization(const char* root) {
     return false;
   }
   file_set_authorized_root(authorized_root_fd, authorized_root);
-  utils_set_authorized_root_fd(authorized_root_fd);
+  utils_set_authorized_root(authorized_root_fd, authorized_root);
   return true;
 }
 
@@ -118,9 +136,9 @@ int receive_files(Config* config, int fd) {
           send_status(fd, STATUS_ERROR);
           return -1;
         }
-        char* full_path = path_cat(config->receive_root_directory, check_path);
         struct stat st;
-        bool has_old = full_path && lstat(full_path, &st) == 0;
+        char* full_path = path_cat(config->receive_root_directory, check_path);
+        bool has_old = full_path && file_stat_secure(full_path, &st);
         bool match = has_old && (unsigned long long)st.st_size == check_size &&
                      (long long)st.st_mtime == check_mtime;
         bool sent = send_status(fd, match ? STATUS_OK : STATUS_NEXT);
@@ -179,6 +197,18 @@ void handler(int file_descriptor) {
     close(file_descriptor);
     return;
   }
+  if (!allow_unauthenticated && ssl == NULL) {
+    log_message(LOG_LEVEL_ERROR, "Rejected unauthenticated plaintext connection");
+    config_delete(config);
+    close(file_descriptor);
+    return;
+  }
+  if (ssl && required_client_cn && !tls_client_identity_allowed(ssl)) {
+    log_message(LOG_LEVEL_ERROR, "Rejected TLS client with unauthorized identity");
+    config_delete(config);
+    close(file_descriptor);
+    return;
+  }
   char resolved_destination[PATH_MAX];
   char* canonical_destination = realpath(config->receive_root_directory, NULL);
   const char* destination =
@@ -281,8 +311,10 @@ static void print_server_usage(void) {
   printf("  --cert <path>       TLS certificate file (PEM)\n");
   printf("  --key <path>        TLS private key file (PEM)\n");
   printf("  --ca <path>         TLS CA certificate file (PEM)\n");
+  printf("  --client-cn <name>  Required TLS client certificate CN\n");
   printf("  --destination-root <path>  Authorized destination root (default: .)\n");
   printf("  --allow-delete      Permit manifest deletion\n");
+  printf("  --allow-unauthenticated  Allow plaintext/anonymous network clients\n");
   printf("  -v, --verbose       Enable debug logging\n");
   printf("  --help              Show this help\n");
 }
@@ -313,10 +345,14 @@ int main(int argc, char* argv[]) {
       tls_key = argv[++i];
     } else if (strcmp(argv[i], "--ca") == 0 && i + 1 < argc) {
       tls_ca = argv[++i];
+    } else if (strcmp(argv[i], "--client-cn") == 0 && i + 1 < argc) {
+      required_client_cn = argv[++i];
     } else if (strcmp(argv[i], "--destination-root") == 0 && i + 1 < argc) {
       destination_root = argv[++i];
     } else if (strcmp(argv[i], "--allow-delete") == 0) {
       allow_delete = true;
+    } else if (strcmp(argv[i], "--allow-unauthenticated") == 0) {
+      allow_unauthenticated = true;
     } else if (strcmp(argv[i], "-p") == 0 && i + 1 < argc) {
       char* end;
       long p = strtol(argv[++i], &end, 10);
@@ -343,10 +379,12 @@ int main(int argc, char* argv[]) {
     return 1;
   }
   if (stdio_mode) {
+    /* SSH authenticates the stdio transport outside of FastSync. */
+    allow_unauthenticated = true;
     io_set_fds(STDIN_FILENO, STDOUT_FILENO);
     handler(STDIN_FILENO);
     file_set_authorized_root(-1, NULL);
-    utils_set_authorized_root_fd(-1);
+    utils_set_authorized_root(-1, NULL);
     close(authorized_root_fd);
     free(authorized_root);
     return 0;
@@ -357,8 +395,8 @@ int main(int argc, char* argv[]) {
     return 1;
   }
   if (use_tls) {
-    if (!tls_cert || !tls_key) {
-      fprintf(stderr, "Error: --tls requires --cert and --key\n");
+    if (!tls_cert || !tls_key || !tls_ca || !required_client_cn) {
+      fprintf(stderr, "Error: --tls requires --cert, --key, --ca, and --client-cn\n");
       server_delete(&g_server);
       return 1;
     }

src/shared/chunk.c

@@ -1,4 +1,6 @@
 #include <stddef.h>
+#include <stdint.h>
+#include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -14,18 +16,29 @@
 
 /* Maximum individual file data size within a chunk (64 MB) */
 #define MAX_FILE_DATA_SIZE (64ULL * 1024 * 1024)
+#define MAX_FILES_PER_CHUNK 65536U
 
 Chunk* chunk_create(File** items, int element_count) {
+  if (element_count < 0 || (element_count > 0 && items == NULL))
+    return NULL;
   Chunk* chunk = (Chunk*)malloc(sizeof(Chunk));
   if (chunk == NULL) {
     perror("ERROR: Could not allocate memory for chunk structure");
     return NULL;
   }
 
-  chunk->items = (File**)malloc(element_count * sizeof(File*));
-  if (chunk->items == NULL) {
-    free(chunk);
-    return NULL;
+  if (element_count == 0) {
+    chunk->items = NULL;
+  } else {
+    if ((size_t)element_count > SIZE_MAX / sizeof(File*)) {
+      free(chunk);
+      return NULL;
+    }
+    chunk->items = (File**)malloc((size_t)element_count * sizeof(File*));
+    if (chunk->items == NULL) {
+      free(chunk);
+      return NULL;
+    }
   }
 
   for (int i = 0; i < element_count; i++) {
@@ -50,15 +63,31 @@ void chunk_destroy(void* item) {
 }
 
 static unsigned long long per_file_serialize_size(File* file, bool use_metadata) {
-  return sizeof(size_t) + strlen(file->path) +
-         (use_metadata ? sizeof(int) + (file->metadata ? FILE_METADATA_WIRE_SIZE : 0) : 0) +
-         sizeof(size_t) + file->data->size;
+  unsigned long long size = sizeof(size_t);
+  size_t path_len = strlen(file->path);
+  unsigned long long metadata_size =
+      use_metadata ? sizeof(int) + (file->metadata ? FILE_METADATA_WIRE_SIZE : 0) : 0;
+  if ((unsigned long long)path_len > ULLONG_MAX - size)
+    return 0;
+  size += path_len;
+  if (metadata_size > ULLONG_MAX - size)
+    return 0;
+  size += metadata_size;
+  if (sizeof(size_t) > ULLONG_MAX - size)
+    return 0;
+  size += sizeof(size_t);
+  if ((unsigned long long)file->data->size > ULLONG_MAX - size)
+    return 0;
+  return size + file->data->size;
 }
 
 Data* chunk_serialize(Chunk* chunk, bool use_metadata) {
   unsigned long long data_size = 0;
   for (int i = 0; i < chunk->element_count; i++) {
-    data_size += per_file_serialize_size(chunk->items[i], use_metadata);
+    unsigned long long file_size = per_file_serialize_size(chunk->items[i], use_metadata);
+    if (file_size == 0 || file_size > ULLONG_MAX - data_size || data_size + file_size > SIZE_MAX)
+      return NULL;
+    data_size += file_size;
   }
   Data* data = data_create_empty(data_size);
   if (data == NULL) {
@@ -88,10 +117,17 @@ Data* chunk_serialize(Chunk* chunk, bool use_metadata) {
 
 Chunk* chunk_deserialize(Data* data, bool use_metadata) {
   ArrayList* files = array_list_create(file_destroy);
+  if (files == NULL)
+    return NULL;
   char* data_pointer = data->data;
   size_t remaining_size = data->size;
 
   while (remaining_size > 0) {
+    if ((unsigned int)files->size >= MAX_FILES_PER_CHUNK) {
+      log_message(LOG_LEVEL_ERROR, "Chunk contains too many files");
+      array_list_delete(files);
+      return NULL;
+    }
     if (remaining_size < sizeof(size_t)) {
       log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for path length");
       array_list_delete(files);
@@ -109,6 +145,10 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
       return NULL;
     }
 
+    if (path_len == SIZE_MAX) {
+      array_list_delete(files);
+      return NULL;
+    }
     char* path = malloc(path_len + 1);
     if (path == NULL) {
       perror("Could not allocate memory for file path");
@@ -117,34 +157,53 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
     }
     memcpy(path, data_pointer, path_len);
     path[path_len] = '\0';
+    if (memchr(path, '\0', path_len) != NULL) {
+      free(path);
+      array_list_delete(files);
+      return NULL;
+    }
     data_pointer += path_len;
     remaining_size -= path_len;
 
     File* file = file_create(path);
     free(path);
+    if (file == NULL) {
+      array_list_delete(files);
+      return NULL;
+    }
 
     if (use_metadata) {
       if (remaining_size < sizeof(int)) {
         log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for metadata");
+        file_destroy(file);
         array_list_delete(files);
         return NULL;
       }
       // Peek at present flag to determine total size needed before reading
       int present_flag;
       memcpy(&present_flag, data_pointer, sizeof(int));
-      if (present_flag && remaining_size < sizeof(int) + FILE_METADATA_WIRE_SIZE) {
+      if ((present_flag != 0 && present_flag != 1) ||
+          (present_flag == 1 && remaining_size < sizeof(int) + FILE_METADATA_WIRE_SIZE)) {
         log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for metadata body");
+        file_destroy(file);
         array_list_delete(files);
         return NULL;
       }
       file->metadata = metadata_from_buf(&data_pointer);
       remaining_size -= sizeof(int);
-      if (file->metadata)
+      if (present_flag == 1) {
+        if (file->metadata == NULL) {
+          file_destroy(file);
+          array_list_delete(files);
+          return NULL;
+        }
         remaining_size -= FILE_METADATA_WIRE_SIZE;
+      }
     }
 
     if (remaining_size < sizeof(size_t)) {
       log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for data size");
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
@@ -156,6 +215,7 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
 
     if (remaining_size < file_data_size) {
       log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for file content");
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
@@ -164,30 +224,48 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
     if (file_data_size > MAX_FILE_DATA_SIZE) {
       log_message(LOG_LEVEL_ERROR, "File data size %zu exceeds maximum %llu", file_data_size,
                   (unsigned long long)MAX_FILE_DATA_SIZE);
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
 
-    void* file_data = malloc(file_data_size);
+    size_t allocation_size = file_data_size > 0 ? file_data_size : 1;
+    void* file_data = malloc(allocation_size);
     if (file_data == NULL) {
       perror("Could not allocate memory for file data");
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
     memcpy(file_data, data_pointer, file_data_size);
+    Data* replacement = data_create(file_data, file_data_size);
+    if (replacement == NULL) {
+      file_destroy(file);
+      array_list_delete(files);
+      return NULL;
+    }
     data_destroy(file->data);
-    file->data = data_create(file_data, file_data_size);
+    file->data = replacement;
     data_pointer += file_data_size;
     remaining_size -= file_data_size;
 
-    array_list_add(files, file);
+    if (!array_list_add(files, file)) {
+      file_destroy(file);
+      array_list_delete(files);
+      return NULL;
+    }
   }
 
   File** file_array = (File**)array_list_to_array(files);
+  if (files->size > 0 && file_array == NULL) {
+    array_list_delete(files);
+    return NULL;
+  }
   Chunk* chunk = chunk_create(file_array, files->size);
 
   free(file_array);
-  files->item_destroyer = NULL;
+  if (chunk != NULL)
+    files->item_destroyer = NULL;
   array_list_delete(files);
 
   return chunk;

src/shared/config.c

@@ -100,6 +100,35 @@ static void config_set_defaults(Config* config) {
   config->compress_choice = NULL;
 }
 
+static bool valid_wire_bool(int value) {
+  return value == 0 || value == 1;
+}
+
+static bool validate_received_config(const Config* config) {
+  return valid_wire_bool(config->save_to_disk) && valid_wire_bool(config->use_multithreading) &&
+         valid_wire_bool(config->use_chunk_serialization) &&
+         valid_wire_bool(config->use_compression) && valid_wire_bool(config->use_metadata) &&
+         valid_wire_bool(config->use_sendfile) && valid_wire_bool(config->use_delete) &&
+         valid_wire_bool(config->use_incremental) && valid_wire_bool(config->use_delta) &&
+         valid_wire_bool(config->backup) && valid_wire_bool(config->follow_symlinks) &&
+         valid_wire_bool(config->copy_links) && valid_wire_bool(config->safe_links) &&
+         valid_wire_bool(config->copy_unsafe_links) &&
+         valid_wire_bool(config->preserve_hard_links) && valid_wire_bool(config->preserve_acls) &&
+         valid_wire_bool(config->preserve_xattrs) && valid_wire_bool(config->preserve_devices) &&
+         valid_wire_bool(config->preserve_sparse) && valid_wire_bool(config->update) &&
+         valid_wire_bool(config->inplace) && valid_wire_bool(config->append) &&
+         valid_wire_bool(config->append_verify) && valid_wire_bool(config->delete_excluded) &&
+         valid_wire_bool(config->delete_after) && valid_wire_bool(config->relative) &&
+         valid_wire_bool(config->prune_empty_dirs) && valid_wire_bool(config->partial) &&
+         valid_wire_bool(config->delete_before) && valid_wire_bool(config->checksum) &&
+         (!config->use_compression ||
+          (config->compression_level >= 1 && config->compression_level <= 22)) &&
+         config->chunk_size > 0 && config->chunk_size <= MAX_CHUNK_SIZE &&
+         config->delta_block_size >= DELTA_BLOCK_SIZE_MIN &&
+         config->delta_block_size <= DELTA_BLOCK_SIZE_MAX &&
+         config->delta_max_file_size <= DELTA_MAX_FILE_SIZE && config->max_delete >= 0;
+}
+
 Config* config_create(void) {
   Config* config = malloc(sizeof(Config));
   if (!config)
@@ -320,18 +349,26 @@ Config* config_receive(int file_descriptor) {
   int tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->save_to_disk = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->use_multithreading = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
   config->use_chunk_serialization = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->use_compression = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->use_metadata = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
@@ -340,15 +377,23 @@ Config* config_receive(int file_descriptor) {
     goto error;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->use_sendfile = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->use_delete = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->use_incremental = tmp;
   if (!receive_int(file_descriptor, &tmp))
     goto error;
+  if (!valid_wire_bool(tmp))
+    goto error;
   config->use_delta = tmp;
   if (!receive_n_data(file_descriptor, &config->delta_block_size, sizeof(config->delta_block_size)))
     goto error;
@@ -440,6 +485,11 @@ Config* config_receive(int file_descriptor) {
     send_status(file_descriptor, STATUS_ERROR);
     goto error;
   }
+  if (!validate_received_config(config)) {
+    fprintf(stderr, "Invalid configuration received from client\n");
+    send_status(file_descriptor, STATUS_ERROR);
+    goto error;
+  }
   if (!send_status(file_descriptor, STATUS_OK))
     goto error;
   return config;

src/shared/delta.c

@@ -36,6 +36,9 @@ DeltaSignature* delta_signature_create(const void* old_file_data, uint64_t old_f
   if (old_file_data == NULL || old_file_size == 0 || block_size == 0)
     return NULL;
 
+  if (old_file_size > DELTA_MAX_FILE_SIZE || old_file_size > UINT32_MAX * (uint64_t)block_size)
+    return NULL;
+
   uint32_t block_count = (uint32_t)((old_file_size + block_size - 1) / block_size);
 
   DeltaSignature* sig = malloc(sizeof(DeltaSignature));
@@ -118,6 +121,13 @@ DeltaSignature* delta_signature_deserialize(const Data* data) {
     return NULL;
   }
 
+  if (sig->block_size == 0 || sig->block_size > DELTA_BLOCK_SIZE_MAX ||
+      sig->file_size > DELTA_MAX_FILE_SIZE || sig->file_size == 0 ||
+      (sig->file_size + sig->block_size - 1) / sig->block_size != sig->block_count) {
+    free(sig);
+    return NULL;
+  }
+
   uint64_t expected = sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t) +
                       (uint64_t)sig->block_count * (sizeof(uint32_t) + sizeof(uint32_t));
   if (data->size < expected) {
@@ -463,7 +473,8 @@ Delta* delta_deserialize(const Data* data) {
 void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
                   uint32_t block_size) {
   if (!old_data || !delta || (delta->new_file_size > 0 && delta->instructions == NULL) ||
-      (delta->instruction_count > 0 && block_size == 0))
+      (delta->instruction_count > 0 && block_size == 0) ||
+      delta->new_file_size > DELTA_MAX_FILE_SIZE || delta->new_file_size > SIZE_MAX)
     return NULL;
 
   void* output = malloc((size_t)delta->new_file_size);

src/shared/file.c

@@ -23,6 +23,9 @@
 #include "protocol.h"
 #include "utils.h"
 
+#define MAX_SERVER_DELETE_COUNT 100000U
+#define MAX_FILE_DATA_SIZE (64ULL * 1024 * 1024)
+
 bool file_checksum(File* file, uint64_t* checksum) {
   if (!file || !checksum || !file->data)
     return false;
@@ -155,7 +158,28 @@ bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
 
 static bool to_disk_secure(const char* path, const void* data, unsigned long long data_size,
                            bool inplace, bool sparse, const FileMetadata* metadata);
-static int open_secure_parent(const char* path, char** leaf_out);
+static int open_secure_parent(const char* path, char** leaf_out, bool create_dirs);
+
+bool file_path_exists_secure(const char* path) {
+  struct stat st;
+  return file_stat_secure(path, &st);
+}
+
+bool file_stat_secure(const char* path, struct stat* st) {
+  if (!path || !st)
+    return false;
+  char* leaf = NULL;
+  int parent_fd = open_secure_parent(path, &leaf, false);
+  if (parent_fd < 0)
+    return false;
+  int fd = openat(parent_fd, leaf, O_RDONLY | O_NONBLOCK | O_CLOEXEC | O_NOFOLLOW);
+  bool exists = fd >= 0 && fstat(fd, st) == 0 && S_ISREG(st->st_mode);
+  if (fd >= 0)
+    close(fd);
+  close(parent_fd);
+  free(leaf);
+  return exists;
+}
 static bool rename_secure(const char* old_path, const char* new_path);
 static int authorized_root_fd = -1;
 static char* authorized_root_path;
@@ -440,8 +464,16 @@ static File* receive_delta_file(int fd, const Config* config, const char* check_
       }
     }
 
+    Data* replacement = data_create(new_data, (size_t)new_size);
+    if (replacement == NULL) {
+      file_destroy(file);
+      free(old_data);
+      delta_signature_destroy(sig);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
     data_destroy(file->data);
-    file->data = data_create(new_data, (size_t)new_size);
+    file->data = replacement;
 
     free(old_data);
     delta_signature_destroy(sig);
@@ -483,6 +515,12 @@ static File* receive_delta_file(int fd, const Config* config, const char* check_
         send_status(fd, STATUS_ERROR);
         return NULL;
       }
+      if (uncompressed->size > MAX_FILE_DATA_SIZE) {
+        data_destroy(uncompressed);
+        file_destroy(file);
+        send_status(fd, STATUS_ERROR);
+        return NULL;
+      }
       file_data = uncompressed;
     }
 
@@ -493,6 +531,7 @@ static File* receive_delta_file(int fd, const Config* config, const char* check_
 
   delta_signature_destroy(sig);
   free(old_data);
+  send_status(fd, STATUS_ERROR);
   return NULL;
 }
 
@@ -532,7 +571,7 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   int old_fd = -1;
   if (full_path) {
     char* leaf = NULL;
-    int parent_fd = open_secure_parent(full_path, &leaf);
+    int parent_fd = open_secure_parent(full_path, &leaf, false);
     if (parent_fd >= 0) {
       old_fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
       free(leaf);
@@ -542,7 +581,7 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   }
   unsigned long long old_size = has_old_file ? (unsigned long long)st.st_size : 0;
   void* old_data = NULL;
-  if (has_old_file && old_size > 0) {
+  if (has_old_file && old_size > 0 && old_size <= DELTA_MAX_FILE_SIZE && old_size <= SIZE_MAX) {
     old_data = malloc((size_t)old_size);
     if (old_data) {
       size_t got = 0;
@@ -643,6 +682,12 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
       send_status(fd, STATUS_ERROR);
       return NULL;
     }
+    if (uncompressed->size > MAX_FILE_DATA_SIZE) {
+      data_destroy(uncompressed);
+      file_destroy(file);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
     file_data = uncompressed;
   }
 
@@ -651,7 +696,7 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   return file;
 }
 
-static int open_secure_parent(const char* path, char** leaf_out) {
+static int open_secure_parent(const char* path, char** leaf_out, bool create_dirs) {
   char* copy = str_dup(path);
   if (!copy)
     return -1;
@@ -691,7 +736,7 @@ static int open_secure_parent(const char* path, char** leaf_out) {
   while (component) {
     if (strcmp(component, ".") != 0 && strcmp(component, "..") != 0) {
       int next = openat(fd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-      if (next < 0 && errno == ENOENT && mkdirat(fd, component, 0755) == 0)
+      if (create_dirs && next < 0 && errno == ENOENT && mkdirat(fd, component, 0755) == 0)
         next = openat(fd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
       if (next < 0) {
         close(fd);
@@ -711,8 +756,8 @@ static int open_secure_parent(const char* path, char** leaf_out) {
 
 static bool rename_secure(const char* old_path, const char* new_path) {
   char *old_leaf = NULL, *new_leaf = NULL;
-  int old_parent = open_secure_parent(old_path, &old_leaf);
-  int new_parent = open_secure_parent(new_path, &new_leaf);
+  int old_parent = open_secure_parent(old_path, &old_leaf, true);
+  int new_parent = open_secure_parent(new_path, &new_leaf, true);
   bool ok = old_parent >= 0 && new_parent >= 0 &&
             renameat(old_parent, old_leaf, new_parent, new_leaf) == 0;
   if (old_parent >= 0)
@@ -741,7 +786,7 @@ static bool write_all(int fd, const void* data, unsigned long long size) {
 static bool to_disk_secure(const char* path, const void* data, unsigned long long data_size,
                            bool inplace, bool sparse, const FileMetadata* metadata) {
   char* leaf = NULL;
-  int dirfd = open_secure_parent(path, &leaf);
+  int dirfd = open_secure_parent(path, &leaf, true);
   if (dirfd < 0)
     return false;
   int fd = -1;
@@ -1011,6 +1056,11 @@ File* file_receive(const Config* config, int file_descriptor) {
       file_destroy(file);
       return NULL;
     }
+    if (file_data_uncompressed->size > MAX_FILE_DATA_SIZE) {
+      data_destroy(file_data_uncompressed);
+      file_destroy(file);
+      return NULL;
+    }
     file_data = file_data_uncompressed;
   }
   data_destroy(file->data);
@@ -1068,7 +1118,8 @@ int receive_manifest(int fd, const Config* config, int* next_status) {
     return *status_out == STATUS_FINISHED ? 0 : -1;
   }
   fprintf(stderr, "Deleting files not in manifest...\n");
-  bool deletion_ok = delete_extras(config->receive_root_directory, manifest);
+  bool deletion_ok =
+      delete_extras_limited(config->receive_root_directory, manifest, MAX_SERVER_DELETE_COUNT);
   array_list_delete(manifest);
   return deletion_ok ? 0 : -1;
 }

src/shared/file.h

@@ -40,6 +40,8 @@ bool to_disk(const char* path, const void* data, unsigned long long data_size, b
 bool file_save_to_disk(const char* root_directory, const File* file, const Config* config);
 void file_set_authorized_root(int fd, const char* canonical_path);
 File* receive_incremental_check(int fd, const Config* config, bool* skipped);
+bool file_path_exists_secure(const char* path);
+bool file_stat_secure(const char* path, struct stat* st);
 int receive_manifest(int fd, const Config* config, int* next_status);
 
 #endif

src/shared/metadata.c

@@ -76,6 +76,11 @@ FileMetadata* metadata_from_buf(char** buf) {
   memcpy(&mtime_nsec, *buf, sizeof(mtime_nsec));
   *buf += sizeof(mtime_nsec);
   m->mtime_nsec = (long)mtime_nsec;
+  if (present != 1 || mtime_nsec < 0 || mtime_nsec >= 1000000000LL || mode < 0 || uid < 0 ||
+      gid < 0) {
+    free(m);
+    return NULL;
+  }
   return m;
 }
 
@@ -175,7 +180,8 @@ FileMetadata* metadata_receive(int file_descriptor, int* ok) {
 void file_restore_metadata(const char* path, const FileMetadata* metadata) {
   if (metadata == NULL)
     return;
-  if (chmod(path, metadata->mode & 07777 & ~(S_ISUID | S_ISGID)) != 0)
+  mode_t safe_mode = metadata->mode & 0777 & ~(S_IWGRP | S_IWOTH);
+  if (chmod(path, safe_mode) != 0)
     log_message(LOG_LEVEL_WARNING, "Failed to chmod %s: %s", path, strerror(errno));
   /* Never apply client-supplied ownership.  The descriptor API below is the
      receiver write path; retain this legacy API only for compatibility. */
@@ -192,7 +198,8 @@ bool file_restore_metadata_fd(int fd, const FileMetadata* metadata) {
   if (fd < 0 || metadata == NULL)
     return metadata == NULL;
   bool ok = true;
-  if (fchmod(fd, metadata->mode & 07777 & ~(S_ISUID | S_ISGID)) != 0)
+  mode_t safe_mode = metadata->mode & 0777 & ~(S_IWGRP | S_IWOTH);
+  if (fchmod(fd, safe_mode) != 0)
     ok = false;
   /* Client uid/gid values are deliberately not authoritative. */
   struct timespec times[2] = {{.tv_sec = 0, .tv_nsec = UTIME_OMIT},

src/shared/multiprocessing.c

@@ -234,9 +234,9 @@ int receive_thread(void* pipeline_context) {
         }
         char* full_path = path_cat(config->receive_root_directory, check_path);
         struct stat st;
-        bool has_old = full_path && lstat(full_path, &st) == 0;
+        bool has_old = full_path && file_stat_secure(full_path, &st);
         bool match = has_old && (unsigned long long)st.st_size == check_size &&
-                     (long long)st.st_mtime == check_mtime;
+                     (long long)st.st_mtime == check_mtime && S_ISREG(st.st_mode);
         if (!send_status(file_descriptor, match ? STATUS_OK : STATUS_NEXT))
           RECEIVE_THREAD_FAIL();
         free(full_path);

src/shared/protocol.c

@@ -13,7 +13,7 @@
 
 #define RECEIVE_TIMEOUT_SEC 60 /* 60 second per-message timeout */
 #define SEND_TIMEOUT_SEC 60
-#define MAX_CONNECTION_MEMORY (1024ULL * 1024 * 1024) /* 1 GB total per connection */
+#define MAX_CONNECTION_MEMORY (256ULL * 1024 * 1024) /* bounded cumulative receive budget */
 
 static __thread int io_read_fd = -1;
 static __thread int io_write_fd = -1;
@@ -25,7 +25,7 @@ static struct timespec bw_last_refill = {0, 0};
 static mtx_t bw_mutex;
 static once_flag bw_mutex_once = ONCE_FLAG_INIT;
 
-static __thread unsigned long long total_allocated_bytes = 0;
+static __thread unsigned long long total_allocated_bytes;
 
 void io_set_fds(int read_fd, int write_fd) {
   io_read_fd = read_fd;
@@ -260,6 +260,11 @@ char* receive_str(int file_descriptor) {
     free(data);
     return NULL;
   }
+  if (memchr(data, '\0', size) != NULL) {
+    free(data);
+    log_message(LOG_LEVEL_ERROR, "Received string contains an embedded NUL");
+    return NULL;
+  }
   data[size] = '\0';
   total_allocated_bytes += size + 1;
   log_message(LOG_LEVEL_DEBUG, "Received String: %s", data);
@@ -287,9 +292,7 @@ Data* receive_data(int file_descriptor) {
   }
   size_t allocation_size = size == 0 ? 1 : (size_t)size;
   if (allocation_size > MAX_CONNECTION_MEMORY - total_allocated_bytes) {
-    log_message(LOG_LEVEL_ERROR, "Per-connection memory limit exceeded (%llu + %llu > %llu)",
-                (unsigned long long)total_allocated_bytes, size,
-                (unsigned long long)MAX_CONNECTION_MEMORY);
+    log_message(LOG_LEVEL_ERROR, "Per-connection memory limit exceeded");
     return NULL;
   }
   void* data = malloc(allocation_size);

src/shared/transport_tls.c

@@ -11,6 +11,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/socket.h>
+#include <sys/stat.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
@@ -34,6 +35,10 @@ static void log_ssl_errors(void) {
 
 static SSL_CTX* create_ssl_ctx(bool is_server, const char* cert, const char* key,
                                const char* ca_path) {
+  if (!is_server && !ca_path) {
+    log_message(LOG_LEVEL_ERROR, "TLS clients require a CA certificate path");
+    return NULL;
+  }
   const SSL_METHOD* method = is_server ? TLS_server_method() : TLS_client_method();
   SSL_CTX* ctx = SSL_CTX_new(method);
   if (!ctx) {
@@ -42,9 +47,23 @@ static SSL_CTX* create_ssl_ctx(bool is_server, const char* cert, const char* key
     return NULL;
   }
 
-  SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION);
+  if (SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION) != 1) {
+    SSL_CTX_free(ctx);
+    return NULL;
+  }
+  if (SSL_CTX_set_cipher_list(ctx, "HIGH:!aNULL:!eNULL:!MD5:!RC4:!3DES") != 1) {
+    SSL_CTX_free(ctx);
+    return NULL;
+  }
 
   if (cert && key) {
+    struct stat key_stat;
+    if (stat(key, &key_stat) != 0 || !S_ISREG(key_stat.st_mode) || key_stat.st_uid != geteuid() ||
+        (key_stat.st_mode & (S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH))) {
+      log_message(LOG_LEVEL_ERROR, "TLS private key must be owned by the current user and private");
+      SSL_CTX_free(ctx);
+      return NULL;
+    }
     if (SSL_CTX_use_certificate_file(ctx, cert, SSL_FILETYPE_PEM) <= 0) {
       log_message(LOG_LEVEL_ERROR, "Failed to load certificate: %s", cert);
       log_ssl_errors();
@@ -91,7 +110,10 @@ static SSL* wrap_fd_with_ssl(int fd, SSL_CTX* ctx, bool is_server, const char* h
   // Enable hostname verification for client connections when a hostname is provided.
   // Must be done before SSL_connect to take effect during the handshake.
   if (!is_server && hostname) {
-    SSL_set1_host(ssl, hostname);
+    if (SSL_set1_host(ssl, hostname) != 1) {
+      SSL_free(ssl);
+      return NULL;
+    }
   }
 
   // Retry SSL_accept/SSL_connect on WANT_READ/WANT_WRITE (non-blocking handshake)

src/shared/utils.c

@@ -7,13 +7,67 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdint.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
 static int authorized_root_fd = -1;
+static char* authorized_root_path;
+
+void utils_set_authorized_root(int fd, const char* canonical_path) {
+  authorized_root_fd = fd;
+  free(authorized_root_path);
+  authorized_root_path = canonical_path ? str_dup(canonical_path) : NULL;
+}
 
 void utils_set_authorized_root_fd(int fd) {
-  authorized_root_fd = fd;
+  utils_set_authorized_root(fd, NULL);
+}
+
+static bool path_is_within_root(const char* root, const char* path) {
+  size_t root_len = strlen(root);
+  return strncmp(root, path, root_len) == 0 && (path[root_len] == '\0' || path[root_len] == '/');
+}
+
+static int open_authorized_destination(const char* dest_root) {
+  if (authorized_root_fd < 0 || !authorized_root_path || !dest_root ||
+      !path_is_within_root(authorized_root_path, dest_root))
+    return -1;
+
+  int dirfd = dup(authorized_root_fd);
+  if (dirfd < 0)
+    return -1;
+
+  const char* relative_path = dest_root + strlen(authorized_root_path);
+  while (*relative_path == '/')
+    relative_path++;
+  char* relative = str_dup(*relative_path ? relative_path : ".");
+  if (!relative) {
+    close(dirfd);
+    return -1;
+  }
+
+  char* saveptr = NULL;
+  char* component = strtok_r(relative, "/", &saveptr);
+  while (component) {
+    if (strcmp(component, ".") == 0 || strcmp(component, "..") == 0) {
+      free(relative);
+      close(dirfd);
+      return -1;
+    }
+    int next = openat(dirfd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+    if (next < 0) {
+      free(relative);
+      close(dirfd);
+      return -1;
+    }
+    close(dirfd);
+    dirfd = next;
+    component = strtok_r(NULL, "/", &saveptr);
+  }
+
+  free(relative);
+  return dirfd;
 }
 
 bool mkdir_r(const char* path) {
@@ -143,7 +197,8 @@ static bool is_dir_in_manifest(const char* rel_path, ArrayList* manifest) {
   return false;
 }
 
-static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifest) {
+static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifest,
+                             size_t max_delete, size_t* deleted_count) {
   int scanfd = dup(dirfd);
   if (scanfd < 0)
     return false;
@@ -173,12 +228,17 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
       int childfd = openat(dirfd, entry->d_name, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
       bool child_removed = false;
       if (childfd >= 0) {
-        child_removed = delete_extras_fd(childfd, child_rel, manifest);
+        child_removed = delete_extras_fd(childfd, child_rel, manifest, max_delete, deleted_count);
         close(childfd);
       }
-      if (child_removed && !is_dir_in_manifest(child_rel, manifest) &&
-          unlinkat(dirfd, entry->d_name, AT_REMOVEDIR) != 0 && errno != ENOENT) {
-        operation_ok = false;
+      if (child_removed && !is_dir_in_manifest(child_rel, manifest)) {
+        if (*deleted_count >= max_delete) {
+          operation_ok = false;
+        } else if (unlinkat(dirfd, entry->d_name, AT_REMOVEDIR) != 0 && errno != ENOENT) {
+          operation_ok = false;
+        } else {
+          (*deleted_count)++;
+        }
       } else if (!child_removed) {
         all_removed = false;
       }
@@ -192,8 +252,15 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
         }
       }
       if (!found) {
+        if (*deleted_count >= max_delete) {
+          operation_ok = false;
+          free(child_rel);
+          continue;
+        }
         if (unlinkat(dirfd, entry->d_name, 0) != 0 && errno != ENOENT)
           operation_ok = false;
+        else
+          (*deleted_count)++;
         fprintf(stderr, "  Deleted: %s\n", child_rel);
       } else {
         all_removed = false;
@@ -206,18 +273,27 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
   return operation_ok;
 }
 
-bool delete_extras(const char* dest_root, ArrayList* manifest) {
-  int rootfd = authorized_root_fd >= 0
-                   ? dup(authorized_root_fd)
-                   : open(dest_root, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete) {
+  int rootfd;
+  if (authorized_root_fd >= 0) {
+    rootfd =
+        authorized_root_path ? open_authorized_destination(dest_root) : dup(authorized_root_fd);
+  } else {
+    rootfd = open(dest_root, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  }
   if (rootfd < 0)
     return false;
-  bool ok = delete_extras_fd(rootfd, "", manifest);
+  size_t deleted_count = 0;
+  bool ok = delete_extras_fd(rootfd, "", manifest, max_delete, &deleted_count);
   if (close(rootfd) != 0)
     ok = false;
   return ok;
 }
 
+bool delete_extras(const char* dest_root, ArrayList* manifest) {
+  return delete_extras_limited(dest_root, manifest, SIZE_MAX);
+}
+
 bool has_path_traversal(const char* path) {
   if (!path)
     return false;

src/shared/utils.h

@@ -2,6 +2,7 @@
 #define UTILS_H
 
 #include "array_list.h"
+#include <stddef.h>
 #include <stdbool.h>
 
 bool mkdir_r(const char* path);
@@ -9,6 +10,8 @@ char* str_dup(const char* string);
 char* path_cat(const char* path1, const char* path2);
 bool glob_match(const char* pattern, const char* str);
 bool delete_extras(const char* dest_root, ArrayList* manifest);
+bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete);
+void utils_set_authorized_root(int fd, const char* canonical_path);
 void utils_set_authorized_root_fd(int fd);
 bool has_path_traversal(const char* path);
 

tests/integration/common.py

@@ -25,7 +25,9 @@ class ServerManager:
     def start(self, extra_args=None):
         self.stop()
         self._port = _find_free_port()
-        cmd = SERVER_CMD + ["-p", str(self._port)]
+        # Plain TCP is intentionally explicit in the server; integration tests
+        # exercise that opt-in mode rather than relying on the secure default.
+        cmd = SERVER_CMD + ["-p", str(self._port), "--allow-unauthenticated"]
         if extra_args:
             cmd += extra_args
         self._proc = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)

tests/integration/test_tls.py

@@ -102,6 +102,7 @@ class TestTLSBasic:
         with ServerManager() as server:
             server.start(extra_args=[
                 "--tls", "--cert", certs["server_cert"], "--key", certs["server_key"],
+                "--ca", certs["ca"], "--client-cn", "fastsync-client",
             ])
             result, dur = run_client(
                 SOURCE_DIR, DEST_DIR,
@@ -125,6 +126,7 @@ class TestTLSBasic:
         with ServerManager() as server:
             server.start(extra_args=[
                 "--tls", "--cert", certs["server_cert"], "--key", certs["server_key"],
+                "--ca", certs["ca"], "--client-cn", "fastsync-client",
             ])
             result, dur = run_client(
                 SOURCE_DIR, DEST_DIR,
@@ -149,6 +151,7 @@ class TestTLSBasic:
         with ServerManager() as server:
             server.start(extra_args=[
                 "--tls", "--cert", certs["server_cert"], "--key", certs["server_key"],
+                "--ca", certs["ca"], "--client-cn", "fastsync-client",
             ])
             result, dur = run_client(
                 SOURCE_DIR, DEST_DIR,

tests/test_client_cli.c

@@ -256,8 +256,8 @@ static void test_parse_args_rejects_unimplemented_options() {
                                         "--ipv4",
                                         "--daemon",
                                         "--config",
-                                         "--server",
-                                         "--compress-choice"};
+                                        "--server",
+                                        "--compress-choice"};
 
   for (size_t i = 0; i < sizeof(options) / sizeof(options[0]); i++) {
     Config* cfg = config_create();