FastSync/.opencode/skills/security-audit/SKILL.md

name, description

name	description
security-audit	Performs a security audit of FastSync — checks TLS config, input validation, buffer safety, crypto hygiene, and network attack surface. Use when the user says "security audit", "check security", "harden", or wants a security review.

Security Audit Skill

Read-only security review of the FastSync codebase or specific modules. Produces a report — does NOT edit files.

Workflow

Step 1: Scope the Audit

Determine what to audit:

• Full codebase audit
• Specific module (e.g., transport_tls.c, protocol.c)
• Specific vulnerability class (e.g., buffer overflows, TLS misconfig)

Step 2: Identify Attack Surface

Network input points:

src/server/server.c          — TCP accept, per-connection handling
src/shared/protocol.c        — all wire protocol parsing
src/shared/config.c          — config deserialization
src/shared/chunk.c           — chunk deserialization
src/shared/transport_tls.c   — TLS handshake and data
src/shared/transport_ssh.c   — SSH data via stdio

Step 3: Read All Relevant Files

Read every file in scope completely. Focus on:

• All receive_* calls and their validation
• All malloc/calloc calls and their size calculations
• All string operations (strcpy, sprintf, snprintf)
• All path operations (filename handling, directory creation)
• All TLS/SSL operations and error handling

Step 4: Apply Security Checklist

Input Validation

• All receive_* return values checked
• Received size fields validated against bounds
• Path traversal prevention (.. in filenames)
• Null bytes in filenames handled
• Chunk/file counts validated before allocation

Buffer Safety

• No strcpy — use snprintf
• malloc size calculations don't overflow
• No fixed-size stack buffers for unbounded input
• Off-by-one in path concatenation

TLS/SSL

• TLS 1.2 minimum enforced
• Certificate verification when CA provided
• SSL error codes checked after SSL_read/SSL_write
• No hardcoded certificates/keys
• Strong cipher suites only

Memory Safety in Error Paths

• All error paths free allocated resources
• No use-after-free on error paths
• Partial reads handled

Denial of Service

• Bounded memory allocation
• Timeout on connections
• Malformed messages handled gracefully

Step 5: Check for Common Vulnerabilities

# Grep for dangerous patterns
grep -rn "strcpy\|strcat\|sprintf" src/
grep -rn "malloc.*\*" src/  # potential integer overflow in size calc
grep -rn "receive_n_data" src/  # check all return values
grep -rn "NULL" src/ | grep -v "//"  # check null handling

Step 6: Output Report

=== SECURITY AUDIT SUMMARY ===
Scope: <what was audited>
Files reviewed: <count>

Critical: <count>
High: <count>
Medium: <count>
Low: <count>
Informational: <count>

=== FINDINGS ===

[1] <file:line> — CRITICAL (<category>)
    Description: <what's wrong>
    Exploit scenario: <how it could be triggered>
    Fix: <concrete code change>

...

=== VERDICT ===
[PASS] No critical/high issues found
  — or —
[FAIL] <N> critical/high issues must be fixed

Rules

• Do NOT edit any source files
• Do NOT run builds or tests
• Report ALL issues — don't filter or minimize
• Be specific about line numbers and fix suggestions
• Consider both remote and local attack vectors