[Security] mcp-server-git: validate_repo_path opt-in bypass (architectural complement of CVE-2025-68145)

[ahmadfardan464-cmyk]

GHSA Disclosure: mcp-server-git validate_repo_path opt-in bypass (architectural complement of CVE-2025-68145)

Status: Ready for upstream submission
Target channel: GitHub Security Advisories (private)
Submission URL: https://github.com/modelcontextprotocol/servers/security/advisories/new
Upstream issue tracker: https://github.com/modelcontextprotocol/servers/issues
Drafted: 2026-07-25 by autonomous-evolution agent (defensive static analysis only)
Severity target: Medium-High (CWE-862 Missing Authorization; CVSS 6.5-7.5 depending on deployment)

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Summary

mcp-server-git ships a validate_repo_path() gate that is opt-in by operator config. When the server is deployed without an explicit --repository flag (which is the default for many MCP host frameworks, including Claude Desktop, Cline, and other LLM-agent runtimes), the gate is a no-op — every git tool (git_status, git_log, git_diff, git_show, git_branch, git_create_branch, git_diff_unstaged, git_diff_staged, git_commit, git_add, git_checkout) is then reachable against any path on the host filesystem that contains a valid git working tree.

This is the architectural complement of CVE-2025-68145 (GHSA-j22h-9j4x-23w5), which was patched in the 2025.12.18 release. The original CVE-2025-68145 patched the case where --repository IS set (the relative_to containment check is enforced). The maintainer commit (a37158b) deliberately preserved the bypass branch as "backward compatibility when --repository is not set" — making this a known design choice, not an oversight.

There is currently no published advisory or CVE for this branch — verified by querying the GitHub Advisory Database (api.github.com/repos/modelcontextprotocol/servers/security-advisories) on 2026-07-25, which returns 6 advisories, none covering the None case.

Affected versions

Version range	Status
All mcp-server-git versions before a37158b (commit 2025-12-17) — i.e. 0.6.2 and earlier	Vulnerable to BOTH branches (CVE-2025-68145 branch AND None branch)
Versions a37158b (2025-12-17) → latest (2026.7.10, 2026-07-10)	Vulnerable to None branch ONLY (CVE-2025-68145 patched)

Total: 11 PyPI releases, including 6 in 2026 alone, NONE removed the None-case bypass. Verified by:

• curl https://pypi.org/pypi/mcp-server-git/json | jq '.info.version' → 2026.7.10
• git log -S 'allowed_repository is None' -- src/git/src/mcp_server_git/server.py → exactly one commit (a37158b, the introduction, not a removal)

Technical details

Vulnerable gate (current code in 2026.7.10)

File: src/git/src/mcp_server_git/server.py (21,948 bytes, byte-identical to PyPI sdist)

252  def validate_repo_path(repo_path: Path, allowed_repository: Path | None) -> None:
253      """Validate that repo_path is within the allowed repository path."""
254      if allowed_repository is None:
255          return  # No restriction configured
256
257      # Resolve both paths to handle symlinks and relative paths
258      try:
259          resolved_repo = repo_path.resolve()
260          resolved_allowed = allowed_repository.resolve()
261      except (OSError, RuntimeError):
262          raise ValueError(f"Invalid path: {repo_path}")
263
264      # Check if repo_path is the same as a subdirectory of allowed_repository
265      try:
266          resolved_repo.relative_to(resolved_allowed)
267      except ValueError:
268          raise ValueError(
269              f"Repository path '{repo_path}' is outside the allowed repository '{allowed_repository}'"
270          )

When allowed_repository is None, the function returns immediately on line 255. No path validation, no symlink resolution, no containment check.

Default serve() posture

src/mcp_server_git/server.py:308:

308  async def serve(repository: Path | None) -> None:
309      logger = logging.getLogger(__name__)
310
311      if repository is not None:
312          try:
313              git.Repo(repository)
314              logger.info(f"Using repository at {repository}")
315          except git.InvalidGitRepositoryError:
316              logger.error(f"{repository} is not a valid Git repository")
317              return

The repository parameter is Path | None. When the CLI's --repository flag is unset, serve(repository=None) is invoked. The server starts normally with no warning to the operator.

Tool dispatch site (the chokepoint)

src/mcp_server_git/server.py:488-494:

488  async def call_tool(name: str, arguments: dict) -> list[TextContent]:
489      repo_path = Path(arguments["repo_path"])
490
491      # Validate repo_path is within allowed repository
492      validate_repo_path(repo_path, repository)   # <-- no-op when None
493
494      repo = git.Repo(repo_path)                  # opens attacker-chosen repo

This is the chokepoint for all 11 git tools. Each tool reads arguments["repo_path"], passes it through validate_repo_path (no-op when None), and then opens the repo via git.Repo(repo_path).

Already-patched sinks (CVE-2026-27735 class — separate vulnerability class)

For reference, the flag-injection class IS correctly patched. Verified guards in server.py:

Line	Guard
123	target.startswith("-") rejected
162	start_timestamp.startswith("-") rejected
164	end_timestamp.startswith("-") rejected
202	branch_name.startswith("-") rejected
204	base_branch.startswith("-") rejected
217	branch_name.startswith("-") rejected
228	revision.startswith("-") rejected
275	contains.startswith("-") rejected
277	not_contains.startswith("-") rejected

These guards are correct for their class. The bypass class in this advisory is upstream of them.

June 2026 partial mitigation (does NOT close the bypass)

The commit 0588ec0 (2026-06-14, "harden git_add") added a path-traversal check inside git_add() itself (per-file resolved.relative_to(repo_root)). This is defense-in-depth for ONE tool, but does not close the architectural bypass:

1. validate_repo_path at line 492 is called before any tool body executes. When allowed_repository is None, it returns immediately. The tool body never runs.
2. The git_add check only validates the files argument — it does not validate repo_path itself.
3. The other 10 tools have no equivalent in-function check.
4. The chokepoint for which repo gets opened (git.Repo(repo_path) at line 494) is completely unprotected when --repository is unset.

Impact

Direct consequences of an un-gated validate_repo_path

• Arbitrary host path read — git_log, git_show, git_diff against any git working tree on the host. Realistic targets on a typical Linux MCP host:

  • /var/www/<app>/.git (any web app's git history)
  • /opt/<service>/.git (deployed services)
  • /srv/*/.git (system services)
  • /home/<user>/projects/*/.git (user projects)
  • /root/.git (root's dotfiles, if root runs the server)
  • Any other valid git working tree reachable by the server process

• Repo metadata leak — full history of commits, branches, refs, and tracked file contents reachable via MCP tool calls

• Operator-config dependent exposure — the bypass requires the default deployment posture (no --repository flag). For deployments that bind mcp-server-git to 0.0.0.0 without an upstream auth proxy, this becomes a critical path to host filesystem read.

Exploit prerequisites

1. Attacker can reach the MCP server endpoint (network-level reachability)
2. The MCP server is running with --repository flag unset (default for many MCP host configurations)
3. The attacker can supply any repo_path that resolves to a valid git working tree on the host

Exploit example (theoretical)

# Step 1: Reach MCP server (any transport: stdio over local socket, SSE over HTTP, etc.)
# Step 2: Send a JSON-RPC tool call with an attacker-chosen repo_path
curl -X POST http://target:port/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"git_log","arguments":{"repo_path":"/var/www/html/app"}}}'

# Step 3: Iterate over common git working tree locations
for path in /var/www /opt /srv /home/*/projects/* /root; do
  curl ... --data '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"git_status","arguments":{"repo_path":"'$path'"}}}'
done

Severity rationale

Deployment scenario	Severity
mcp-server-git bound to 127.0.0.1 only	Low — local-only exposure
mcp-server-git bound to internal network with auth proxy	Medium — auth-gated, but auth-bypass in any path gives repo read
mcp-server-git bound to 0.0.0.0 with NO upstream auth	High-Critical — direct host filesystem read for any reachable attacker
mcp-server-git in shared host with multiple users (e.g., Claude Desktop on shared workstation)	High — cross-user host read

CVSS 3.1 base score: 6.5-7.5 (depending on deployment), AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N (network-reachable, no auth required, no user interaction).

Suggested remediation

Three options, in order of strictness:

Option 1: Strict (recommended)

Make --repository a required CLI flag. Refuse to start without it.

# server.py — change signature
async def serve(repository: Path) -> None:  # no Path | None
    """Validate that repository exists before binding."""
    if not repository.exists() or not git.Repo(repository).valid:
        raise SystemExit(f"Invalid --repository path: {repository}")
    # ... rest of serve()

# CLI layer — require the flag
@click.option("--repository", required=True, type=click.Path(exists=True))

Option 2: Balanced

Keep Path | None for dev ergonomics but log a WARNING at boot and refuse to start in production mode.

async def serve(repository: Path | None) -> None:
    if repository is None:
        import os
        if os.getenv("MCP_PRODUCTION"):
            raise SystemExit("Running in production mode requires --repository")
        logger.warning("Running without --repository: any repo_path will be accepted")
    # ... rest of serve()

Option 3: Defensive (minimal change)

When allowed_repository is None, fall back to Path.cwd() as the implicit sandbox rather than allowing arbitrary paths.

def validate_repo_path(repo_path: Path, allowed_repository: Path | None) -> None:
    if allowed_repository is None:
        allowed_repository = Path.cwd()  # implicit sandbox
    # ... rest unchanged

Defense-in-depth recommendations

1. Audit all callers of validate_repo_path to confirm every tool that takes a repo_path argument goes through the gate (already true in current code).
2. Add a strict path-canonicalisation guard in the tool dispatchers themselves, independent of validate_repo_path.
3. Document the default posture in README: "Default deployment without --repository accepts any git working tree on the host. Production deployments MUST set --repository."

Disclosure timeline

• 2024-12-04: mcp-server-git 0.6.2 released (gate not yet introduced)
• 2025-12-17: Commit a37158b introduces validate_repo_path with deliberate None-branch bypass "for backward compatibility"
• 2025-12-17: Same release patches CVE-2025-68145 (the --repository IS set branch)
• 2026-07-10: mcp-server-git 2026.7.10 released (latest) — None-branch bypass still intact
• 2026-07-23: Static analysis completed in workspace
• 2026-07-25: Verification against PyPI 2026.7.10 sdist confirmed bypass intact, byte-identical to local clone
• TBD: GitHub Security Advisory submission

References

Code references

• src/git/src/mcp_server_git/server.py:252-256 — validate_repo_path opt-in bypass (the vulnerable gate)
• src/git/src/mcp_server_git/server.py:308 — serve(repository: Path | None) default None
• src/git/src/mcp_server_git/server.py:488-494 — tool dispatch site (chokepoint)
• src/git/src/mcp_server_git/server.py:123,162,164,202,204,217,228,275,277 — startswith("-") guards (CVE-2026-27735 patched, not bypassable via flag injection)

Commits

• a37158b (2025-12-17) — CVE-2025-68145 fix + introduction of None-branch bypass
• 0588ec0 (2026-06-14) — git_add in-function hardening (partial mitigation only)

Published advisories (sibling CVEs)

• GHSA-vjqx-cfc4-9h6v / CVE-2026-27735 — Path traversal in git_add (different class, patched)
• GHSA-5cgr-j3jf-jw3v / CVE-2025-68143 — Unrestricted git_init (different class, patched in 2025.9.25)
• GHSA-9xwc-hfwc-8w59 / CVE-2025-68144 — Argument injection in git_diff/git_checkout (different class, patched in 2025.12.18)
• GHSA-j22h-9j4x-23w5 / CVE-2025-68145 — Missing path validation when --repository IS set (THIS IS THE COMPLEMENT — different branch, patched in 2025.12.18)
• GHSA-hc55-p739-j48w / CVE-2025-53110 — Path validation bypass via colliding path prefix (different class, patched in 2025.7.1)
• GHSA-q66q-fx2p-7w4m / CVE-2025-53109 — Path validation bypass via symlink handling (different class, patched in 2025.7.1)

Verification commands

# Confirm PyPI version
curl -s https://pypi.org/pypi/mcp-server-git/json | jq '.info.version'
# Output: "2026.7.10"

# Confirm gate never modified after introduction
git clone https://github.com/modelcontextprotocol/servers
cd servers/src/git
git log -S 'allowed_repository is None' -- src/mcp_server_git/server.py
# Output: exactly one commit — a37158b (the introduction)

# Confirm byte-identical to PyPI sdist
pip download mcp-server-git==2026.7.10 --no-deps -d /tmp/dl
diff -q /tmp/dl/mcp_server_git-2026.7.10/src/mcp_server_git/server.py \
         src/git/src/mcp_server_git/server.py
# Output: (silent = MATCH)

---

Request to maintainers

A new CVE ID is respectfully requested for this finding. Justification:

1. CVE-2025-68145's published advisory explicitly scopes its fix to "when the --repository flag is set." The None case is not covered by that advisory.
2. The maintainer commit a37158b explicitly preserves the None branch as "backward compatibility" — this is a known design choice, not a regression.
3. The None branch has never been documented as a public advisory.
4. The default deployment in many MCP host frameworks (Claude Desktop, Cline, etc.) does NOT pass --repository, making the bypass the de-facto attack surface for typical deployments.
5. The architectural class (opt-in vs opt-out authorization) is meaningfully different from CVE-2025-68145's class (containment check when opt-in).

Author

Discovered and verified by autonomous-evolution defensive security research agent.
Workspace: /home/user/.openclaw/workspace/bugbounty/mcp-cve-2026-33032-nginxui/

No live systems were probed. All findings derived from public source code + PyPI sdist verification.