Security Fix Generation

Generate concrete, production-ready code fixes for security findings. This is not an advisory skill -- it produces actual code changes that resolve vulnerabilities and offers to apply them via the Edit tool.

Supported Flags

Read ../../shared/schemas/flags.md for the full flag specification.

Flag	Fix Behavior
--scope	Identifies which findings to fix. file:<path> fixes findings in that file. Default: all unfixed findings in --scope changed.
--depth quick	Generate minimal fix (single-line change, no refactoring).
--depth standard	Fix with surrounding improvements (add validation, improve error handling).
--depth deep	Standard + refactor surrounding code to prevent similar issues, add defensive checks.
--depth expert	Deep + generate regression test, add security comments, update related code paths.
--severity	Only fix findings at or above this severity.
--format	Default text. Use json to output fix objects matching findings schema.

Workflow

Step 1: Identify Target Finding

Resolve what to fix from user input. Accept any of these forms:

1. Finding ID: e.g., INJ-001. Read from .appsec/findings.json to load the finding.
2. File and line: e.g., src/db/queries.ts:45. Scan findings for a match, or analyze the location directly.
3. Description: e.g., "the SQL injection in the user lookup". Search findings by title/description.
4. Batch mode: No specific target means fix all findings in scope, ordered by severity (critical first).

If no findings exist in .appsec/findings.json, analyze the target location directly to identify the vulnerability before generating a fix.

Step 2: Understand the Vulnerability

For each finding to fix:

1. Read the finding record (if it exists): severity, CWE, description, location, snippet.
2. Read the vulnerable code: Use the Read tool to load the file. Read at least 30 lines of surrounding context.
3. Identify the root cause: What specific coding pattern causes the vulnerability?
4. Identify constraints: What does the code need to do? What are the inputs/outputs? What framework/library is in use?
5. Check for existing mitigations: Is there partial validation? A security library already imported? Framework-level protection available?

Step 3: Select Fix Strategy

Choose the most appropriate fix strategy based on the vulnerability type:

Vulnerability	Preferred Fix Strategy
SQL Injection (CWE-89)	Parameterized queries / prepared statements
XSS (CWE-79)	Context-aware output encoding, CSP headers
Command Injection (CWE-78)	Allowlist validation, avoid shell execution, use library APIs
Path Traversal (CWE-22)	Canonicalize + validate against base directory
SSRF (CWE-918)	URL allowlist, disable redirects, validate scheme/host
Insecure Deserialization (CWE-502)	Type-safe deserialization, allowlisted classes
Hardcoded Secrets (CWE-798)	Environment variables or secret manager references
Missing Auth (CWE-306)	Add authentication middleware/decorator
Broken Access Control (CWE-862)	Add authorization check before resource access
Weak Crypto (CWE-327)	Replace with current recommended algorithm
Open Redirect (CWE-601)	Validate redirect target against allowlist
Race Condition (CWE-362)	Add locking, use atomic operations

Step 4: Generate the Fix

Produce a concrete code change:

1. Write the actual fixed code. Not pseudocode, not advice -- real code that compiles/runs.
2. Match the existing code style: indentation, naming conventions, import style, error handling patterns.
3. Use framework-idiomatic solutions: If Express, use Express middleware. If Django, use Django's ORM parameterization. If React, use React's built-in XSS protection.
4. Minimize blast radius: Change only what is necessary. Do not refactor unrelated code (unless --depth deep or expert).
5. Add imports if the fix requires new dependencies. Note if a package install is needed.
6. Preserve functionality: The fix must not break the code's intended behavior.

Step 5: Validate the Fix

Before presenting:

1. Syntax check: Ensure the generated code is syntactically valid.
2. Completeness check: Does the fix fully resolve the finding, or is it partial?
3. Side effect check: Could the fix break other functionality? Flag if so.
4. Regression check: Could the fix introduce a new vulnerability? (e.g., overly permissive allowlist).

Step 6: Present and Apply

Present the fix to the user:

## Fix: <Finding ID> - <Title>

**Severity**: <severity> | **CWE**: <CWE-ID> | **File**: <path>

### Root Cause
<1-2 sentence explanation>

### Fix
<description of what the fix does>

```diff
- <old code>
+ <new code>

Additional Changes (if any)

• New import: <import statement>
• New dependency: <package> (run <install command>)

Then ask: "Apply this fix?" If the user confirms (or `--fix` flag was passed from a parent skill), use the Edit tool to apply the change.

### Step 7: Update Finding Record

After applying a fix:

1. Update the finding in `.appsec/findings.json` with status `fix-applied`.
2. Add `fix.applied_at` timestamp and `fix.diff` with the actual change made.
3. Inform the user to run `/appsec:verify` to confirm the fix resolves the issue.

## Output Format

Findings follow `../../shared/schemas/findings.md`. When outputting fixes:

- `fix.summary`: One-line description of the fix.
- `fix.diff`: Unified diff of the change.
- `metadata.tool`: `"fix"`

Finding ID prefix: **FIX** (e.g., `FIX-001`) for new findings discovered during fix analysis. Fixes to existing findings retain the original finding ID.

## Pragmatism Notes

- Prefer the simplest correct fix. A one-line parameterized query beats a custom sanitization function.
- If a framework provides a built-in security mechanism, use it rather than hand-rolling.
- When multiple fix strategies exist, prefer the one already used elsewhere in the codebase for consistency.
- If the fix requires an architectural change beyond a single file, describe the full change but only apply the immediate file-level fix. Note the broader change needed.
- Never generate fixes that simply suppress warnings or disable security features.
- If unsure about a fix's correctness, present it with caveats rather than applying silently.