PYSEC-2026-2408

## Summary

The `discover_pipeline_files()` function in `src/ciguard/discovery.py` (introduced in v0.8.0 and used by the MCP `scan_repo` tool shipped in v0.8.1) walks a directory tree following symlinks, with cycle protection via tracking visited resolved paths. An attacker who can plant a symlink in a directory the user (or AI agent) scans can cause discovery to walk into the symlink target and return paths to pipeline-shaped files outside the requested root.

## Threat scenario

**MCP confused-deputy.** A user runs Claude Desktop / Claude Code / Cursor with the ciguard MCP server registered. The agent is fed an adversarial prompt to scan a directory containing planted symlinks (e.g. via a malicious clone or extracted tarball). `ciguard.scan_repo` walks the symlinks, returning paths and (via subsequent `scan` calls) file content from `~/.aws/`, `~/.config/`, `/etc/some-pipeline-config/`, etc. Pipeline files often contain hardcoded secrets, internal hostnames, deploy keys.

## Patch

- New `follow_symlinks: bool = False` parameter on `discover_pipeline_files`. Default refuses to descend into symlinked directories OR symlinked files. - Belt-and-braces: results are filtered to those whose `.resolve()` lies under `root.resolve()`, applied even when callers opt in to `follow_symlinks=True`. - 3 regression tests in `tests/test_discovery.py::TestSymlinkSafety`.

## Discovery

Found during ciguard's first self-conducted penetration test cycle (PTES + OWASP TG v4.2 + CREST framing), 2026-04-26.

## CVSS Scoring

- CVSS v3.1: `CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:C/C:L/I:N/A:N` — 4.4 (Medium) - CVSS v4.0: `CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:P/VC:L/VI:N/VA:N/SC:L/SI:N/SA:N` — first.org calc 5.7 (Medium); GitHub's calc returns 2.4 (Low). Vector is correct — calculator profiles differ.

## Reproduction

```python from pathlib import Path from ciguard.discovery import discover_pipeline_files # In a victim dir, plant: trojan -> /etc # (or any other accessible dir containing pipeline-shaped files) for f in discover_pipeline_files(Path('/tmp/victim')): print(f) # pre-fix: includes paths under /etc; post-fix: only /tmp/victim/ ```

## References

- Fix released in [v0.8.2](https://github.com/Jo-Jo98/ciguard/releases/tag/v0.8.2) - CI regression gate added in [v0.8.3](https://github.com/Jo-Jo98/ciguard/releases/tag/v0.8.3) - https://www.cve.org/CVERecord?id=CVE-2026-44220

See also: [GHSA-w828-4qhx-vxx3](https://github.com/advisories/GHSA-w828-4qhx-vxx3) — same conceptual pattern (path-validation flaw in an AI-agent tool) in Claude SDK for Python, CWE-59 + CWE-367