[security-fix] Fix incomplete multi-character sanitization in removeXmlComments (Alert #90)

[github-actions]

Security Fix: Incomplete Multi-Character Sanitization in removeXmlComments

Alert Number: #90
Severity: High (Warning level)
Rule: js/incomplete-multi-character-sanitization
CWE: CWE-20, CWE-80, CWE-116
Location: actions/setup/js/sanitize_content_core.cjs:285

Vulnerability Description

CodeQL detected that the removeXmlComments() function in sanitize_content_core.cjs still had an incomplete multi-character sanitization vulnerability despite the iterative fix applied in PR #7574.

The issue was on line 285 where two separate regex replacements were chained:

s = s.replace(//g, "").replace(/` patterns
2. The removal could expose or create new `/g, "").replace(/ OR `
- Malformed comments: `` | `` (empty) | `` (empty) |
| `outer-->` | `` (empty) | `` (empty) |
| `z--!>` | `` (empty) | `` (empty) |

## Why This Fix Completes PR #7574

PR #7574 added the iterative do-while loop which was essential for handling nested patterns. However, CodeQL continued to flag line 285 because:

1. The chained replacements create an intermediate state between the two operations
2. Even though the loop re-processes, the chaining itself is a code pattern that CodeQL detects as problematic
3. The fix was incomplete - the CORRECT approach is a single atomic regex replacement

This fix completes the security hardening by eliminating the chained operations entirely, following the same best practice as the CodeQL documentation recommends.

## Impact Assessment

**Risk**: Low  
**Breaking Changes**: None  
**Backwards Compatibility**: Full  
**Performance**: Slightly improved (one regex pass instead of two)

The fix only changes how the regex patterns are applied - from two chained operations to one atomic operation. The functional output remains identical for all inputs, with enhanced security against injection attacks.

## Files Modified

- `actions/setup/js/sanitize_content_core.cjs`:
  - Combined two chained `.replace()` calls into a single atomic regex
  - Updated comments to explain the consolidation
  - Maintained the iterative do-while loop from PR #7574

## References

- **CodeQL Alert**: https://github.com/githubnext/gh-aw/security/code-scanning/90
- **Related PR**: #7574 (Fixed alerts #85, #86, #87 with iterative approach)
- **CWE-20**: Improper Input Validation
- **CWE-80**: Improper Neutralization of Script-Related HTML Tags
- **CWE-116**: Improper Encoding or Escaping of Output
- **OWASP**: [A03:2021 – Injection]((redacted)
- **CodeQL**: [Incomplete multi-character sanitization](https://codeql.github.com/codeql-query-help/javascript/js-incomplete-multi-character-sanitization/)

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[pelikhan]

@copilot merge main, recompile

[Copilot]

@pelikhan I've opened a new pull request, #7608, to work on those changes. Once the pull request is ready, I'll request review from you.