Florianbuetow Claude Code Race Conditions

Race Conditions (RACE)

Analyze source code for race condition vulnerabilities including time-of-check to time-of-use (TOCTOU), double-spend, check-then-act without locking, file system race conditions, shared state across async boundaries, and non-atomic counter operations. Race conditions are among the hardest bugs to detect through testing because they depend on timing, making static analysis essential.

Supported Flags

Read ../../shared/schemas/flags.md for the full flag specification. This skill supports all cross-cutting flags. Key flags for this skill:

• --scope determines which files to analyze (default: changed)
• --depth standard reads code and checks for common race patterns
• --depth deep traces shared state across call graphs and async boundaries
• --severity filters output (race conditions are often high or critical)

Framework Context

Key CWEs in scope:

• CWE-362: Concurrent Execution Using Shared Resource with Improper Synchronization
• CWE-367: Time-of-Check Time-of-Use (TOCTOU) Race Condition
• CWE-820: Missing Synchronization
• CWE-821: Incorrect Synchronization
• CWE-829: Inclusion of Functionality from Untrusted Control Sphere

Detection Patterns

Read references/detection-patterns.md for the full catalog of code patterns, search heuristics, language-specific examples, and false positive guidance.

Workflow

1. Determine Scope

Parse flags and resolve the file list per ../../shared/schemas/flags.md. Filter to files likely to contain race-prone logic:

• Database transaction handlers (**/services/**, **/handlers/**, **/models/**)
• Payment and financial logic (**/payments/**, **/billing/**, **/wallet/**)
• File system operations (**/storage/**, **/upload/**, **/fs/**)
• Async/concurrent code (**/workers/**, **/tasks/**, **/jobs/**)
• Counter and state management (**/counters/**, **/state/**, **/cache/**)

2. Check for Available Scanners

Detect scanners per ../../shared/schemas/scanners.md:

1. semgrep -- primary scanner for concurrency patterns
2. go vet -- Go-specific race detection heuristics
3. bandit -- Python threading and synchronization issues

Record which scanners are available and which are missing.

3. Run Scanners (If Available)

If semgrep is available, run with rules targeting concurrency:

semgrep scan --config auto --json --quiet <target>

Filter results to rules matching race condition, TOCTOU, and concurrency patterns. Normalize output to the findings schema.

4. Claude Code Analysis

Regardless of scanner availability, perform manual code analysis:

1. Check-then-act audit: Find patterns where a condition is checked and then acted on without atomic guarantees (e.g., check balance then debit).
2. File TOCTOU: Find file existence checks followed by file operations without locking or atomic alternatives.
3. Shared state across await: Identify mutable state read before an await point and used after it without re-validation.
4. Non-atomic counters: Find counter increments (read-modify-write) without locking or atomic operations.
5. Database transactions: Verify financial and state-changing operations use proper isolation levels and row-level locking.
6. Async/parallel iteration: Find shared mutable state accessed in parallel loops, goroutines, or thread pools.

When --depth deep, additionally trace:

• Full data flow of shared variables across async boundaries
• Database isolation level configuration
• Lock acquisition ordering (deadlock potential)

5. Report Findings

Format output per ../../shared/schemas/findings.md using the RACE prefix (e.g., RACE-001, RACE-002).

Include for each finding:

• Severity and confidence
• Exact file location with code snippet
• The race window (what happens between check and act)
• Exploit scenario describing how timing can be abused
• Concrete fix with diff when possible
• CWE references

What to Look For

These are the high-signal patterns specific to race conditions. Each maps to a detection pattern in references/detection-patterns.md.

1. TOCTOU in file operations -- Checking file existence or permissions then operating on the file in a separate call.

2. Double-spend / check-then-debit -- Reading a balance, comparing it, then debiting in separate non-atomic steps.

3. Check-then-act without lock -- Any pattern where a condition is checked and the result is assumed to hold when the action executes.

4. Shared state across await -- Reading mutable state, yielding execution (await/yield), then using the stale value.

5. Non-atomic read-modify-write -- Counter increments, sequence generators, or flag toggles without synchronization.

6. Missing database transaction isolation -- Financial operations using default (READ COMMITTED) isolation when SERIALIZABLE is needed.

7. Parallel iteration over shared collection -- Modifying a shared list, map, or set from concurrent goroutines, threads, or async tasks.

Scanner Integration

Fallback (no scanner): Use Grep with patterns from references/detection-patterns.md to find check-then-act patterns, file stat calls, counter operations, and async state access. Report findings with confidence: medium.

Output Format

Use the findings schema from ../../shared/schemas/findings.md.

• ID prefix: RACE (e.g., RACE-001)
• metadata.tool: race-conditions
• metadata.framework: specialized
• metadata.category: RACE
• references.cwe: CWE-362, CWE-367
• references.stride: T (Tampering) or E (Elevation of Privilege)

Severity guidance for this category:

• critical: Double-spend in financial operations, authentication bypass via race
• high: TOCTOU in security-sensitive file operations, check-then-act on authorization
• medium: Non-atomic counters affecting business logic, shared state across await
• low: Theoretical races with no clear exploit path, cosmetic counter inaccuracies

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