BREW-OTTERDOG-CVE-2026-87817 (PYSEC-2026-3982)

Vulnerability from osv_homebrew – Published: 2026-09-20 11:09 – Updated: 2026-10-01 09:31 – Source website
VLAI
Summary
GitPython: Repository content can impersonate the git directory, leading to arbitrary code execution
Details

Summary

Repo.__init__ decides which directory is the git directory by testing candidate paths in an order that considers the real .git last. Two earlier tests can be satisfied by ordinary tracked files. Git reserves only the literal name .git, so HEAD, objects/, refs/, config, gitdir, commondir and hooks/ at a repository root are all legal tracked content.

Consequently, after a victim opens or clones an attacker's repository, GitPython resolves git_dir to the working-tree root while real git correctly resolves <root>/.git. Everything GitPython then treats as "inside the git directory" is attacker-authored content — including hooks/, which it executes.

CVE-2026-87817

Affected code (3.1.59)

The discovery loop in git/repo/base.py tests, in order:

  1. git/repo/base.py:299 — isfile(curpath/gitdir) and isfile(curpath/commondir) and isfile(curpath/HEAD)
  2. git/repo/base.py:320 — is_git_dir(curpath)
  3. git/repo/base.py:341 — dotgit = osp.join(curpath, ".git") ← the real git dir, considered last

is_git_dir (git/repo/fun.py:60) requires only that objects/ and refs/ are directories and that HEAD is a file; HEAD's contents are never parsed. The hook path is resolved from index.repo.git_dir (git/index/fun.py:73), i.e. the mis-resolved directory.

Proof of concept

Requires only pip install GitPython==3.1.59. Full script attached as poc1_rce.py; it runs entirely in a temp directory and the payload only writes a marker file.

Attacker repository — four ordinary tracked files at the root:

path mode content
gitdir 100644 .git\n
commondir 100644 .git\n
HEAD 100644 ref: refs/heads/master\n
hooks/pre-commit 100755 #!/bin/sh + payload

Victim — two ordinary calls:

repo = git.Repo.clone_from(url, dst)     # or git.Repo(dst)
repo.index.commit("automated commit")    # code execution happens here

Observed on the PyPI release 3.1.59 (Linux and Windows):

real git says the git dir is : /tmp/.../victim/.git
GitPython says it is         : /tmp/.../victim      <- shadowed
attacker's hook executed     : True
git fsck                     : (clean)

The pre-commit hook runs at git/index/base.py:1201, before write_tree(), so it fires even though the commit later fails.

Impact

A service that opens or clones an untrusted repository with GitPython — a CI runner building a fork pull request, a code-scanning/SBOM service, a mirror, a dependency bot, an AI code-review/agent tool — can be made to:

  1. Execute arbitrary commands via the tracked <root>/hooks/pre-commit when the victim calls index.commit().
  2. Read files outside the repository: the tracked <root>/config becomes the repository config and is parsed with merge_includes=True (git/repo/base.py:765), so [include] path = ~/.aws/credentials discloses the file. (Repo._config_reader still defaults merge_includes=True, so the hardening added in 3.1.59 for .gitmodules/GHSA-7833 does not cover this path.)
  3. Write a config file to an attacker-chosen directory via an absolute tracked commondir.

Delivery is silent: git clone exits 0, git fsck (including --strict, and with transfer.fsckObjects/fetch.fsckObjects=true) reports nothing, and the clone passes every read-only probe (head.commit, branches, is_dirty(), untracked_files, iter_commits) because a tracked commondir of .git pins common_dir to the real .git.

Threat model / preconditions

  • Attacker controls the content of a repository the victim opens or clones with GitPython (public repo, fork PR, mirrored dependency).
  • For code execution, the victim performs a commit via GitPython's native index.commit(). For the file-read impact, opening the repo and reading config is enough.
  • GIT_WORK_TREE is not a mitigation — git_dir is assigned and the discovery loop breaks before the environment is consulted.
  • Real git is unaffected; only GitPython mis-resolves the directory.

Remediation

  1. Test curpath/.git before the gitdir/commondir/HEAD triple and before is_git_dir(curpath).
  2. Resolve hook_path / _commit_hook_path / _get_validated_reflog_path through repo.common_dir.
  3. Containment-check commondir/gitdir contents before joining (reuse SymbolicReference._get_validated_path).
  4. Validate HEAD in is_git_dir (require ref: refs/... or a 40-hex sha).
  5. Pass merge_includes=False in Repo._config_reader (git/repo/base.py:765).

Note for the maintainers

Commit 406b98e1 (2026-05-31, "respect core.hooksPath for commit hooks") resolved the hook path via git rev-parse --git-path, which is immune because git rediscovers the true .git. Commit 9bc287a2 (2026-07-20) reverted it to avoid an unconditional rev-parse dependency — reintroducing the exec step. Measured at each commit with the healthy-looking layout: 406b98e1 → hook did not fire; 9bc287a2 … 3.1.59 → hook fired.

Scope note

Only the repository-root case is reported: where a real .git exists, git prefers it, and GitPython does not. A fake git dir in a subdirectory fools real git too, so that variant is out of scope as a general ecosystem hazard rather than a GitPython defect.

POC Files :

poc1_rce.py poc2_file_read.py

Severity
8.8 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
References
To clipboard 

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "resource": "gitpython",
        "resource_purl": "pkg:pypi/gitpython@3.1.62",
        "upstream_fixed_in": "3.1.60"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "otterdog",
        "purl": "pkg:brew/otterdog"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.0.1"
            },
            {
              "fixed": "1.4.0_3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/gitpython@3.1.62",
        "name": "gitpython",
        "resource": "gitpython",
        "strategy": "registry",
        "subject_version": "3.1.62"
      }
    ]
  },
  "details": "### Summary\n\n`Repo.__init__` decides which directory is the git directory by testing candidate paths in an order that\nconsiders the real `.git` **last**. Two earlier tests can be satisfied by ordinary tracked files. Git\nreserves only the literal name `.git`, so `HEAD`, `objects/`, `refs/`, `config`, `gitdir`, `commondir`\nand `hooks/` at a repository root are all legal tracked content.\n\nConsequently, after a victim opens or clones an attacker\u0027s repository, GitPython resolves `git_dir` to\nthe **working-tree root** while real git correctly resolves `\u003croot\u003e/.git`. Everything GitPython then\ntreats as \"inside the git directory\" is attacker-authored content \u2014 including `hooks/`, which it\nexecutes.\n\n### CVE-2026-87817\n\n### Affected code (3.1.59)\n\nThe discovery loop in `git/repo/base.py` tests, in order:\n\n1. `git/repo/base.py:299` \u2014 `isfile(curpath/gitdir)` **and** `isfile(curpath/commondir)` **and** `isfile(curpath/HEAD)`\n2. `git/repo/base.py:320` \u2014 `is_git_dir(curpath)`\n3. `git/repo/base.py:341` \u2014 `dotgit = osp.join(curpath, \".git\")` \u2190 the real git dir, considered last\n\n`is_git_dir` (`git/repo/fun.py:60`) requires only that `objects/` and `refs/` are directories and that\n`HEAD` is a file; **`HEAD`\u0027s contents are never parsed.** The hook path is resolved from\n`index.repo.git_dir` (`git/index/fun.py:73`), i.e. the mis-resolved directory.\n\n### Proof of concept\n\nRequires only `pip install GitPython==3.1.59`. Full script attached as `poc1_rce.py`; it runs entirely\nin a temp directory and the payload only writes a marker file.\n\nAttacker repository \u2014 four ordinary tracked files at the root:\n\n| path | mode | content |\n|---|---|---|\n| `gitdir` | 100644 | `.git\\n` |\n| `commondir` | 100644 | `.git\\n` |\n| `HEAD` | 100644 | `ref: refs/heads/master\\n` |\n| `hooks/pre-commit` | **100755** | `#!/bin/sh` + payload |\n\nVictim \u2014 two ordinary calls:\n\n```python\nrepo = git.Repo.clone_from(url, dst)     # or git.Repo(dst)\nrepo.index.commit(\"automated commit\")    # code execution happens here\n```\n\nObserved on the PyPI release 3.1.59 (Linux and Windows):\n\n```\nreal git says the git dir is : /tmp/.../victim/.git\nGitPython says it is         : /tmp/.../victim      \u003c- shadowed\nattacker\u0027s hook executed     : True\ngit fsck                     : (clean)\n```\n\nThe `pre-commit` hook runs at `git/index/base.py:1201`, before `write_tree()`, so it fires even though\nthe commit later fails.\n\n### Impact\n\nA service that opens or clones an untrusted repository with GitPython \u2014 a CI runner building a fork\npull request, a code-scanning/SBOM service, a mirror, a dependency bot, an AI code-review/agent tool \u2014\ncan be made to:\n\n1. **Execute arbitrary commands** via the tracked `\u003croot\u003e/hooks/pre-commit` when the victim calls\n   `index.commit()`.\n2. **Read files outside the repository**: the tracked `\u003croot\u003e/config` becomes the repository config and\n   is parsed with `merge_includes=True` (`git/repo/base.py:765`), so `[include] path = ~/.aws/credentials`\n   discloses the file. (`Repo._config_reader` still defaults `merge_includes=True`, so the hardening\n   added in 3.1.59 for `.gitmodules`/GHSA-7833 does not cover this path.)\n3. **Write a config file to an attacker-chosen directory** via an absolute tracked `commondir`.\n\nDelivery is silent: `git clone` exits 0, `git fsck` (including `--strict`, and with\n`transfer.fsckObjects`/`fetch.fsckObjects=true`) reports nothing, and the clone passes every read-only\nprobe (`head.commit`, `branches`, `is_dirty()`, `untracked_files`, `iter_commits`) because a tracked\n`commondir` of `.git` pins `common_dir` to the real `.git`.\n\n### Threat model / preconditions\n\n- Attacker controls the content of a repository the victim opens or clones with GitPython (public repo,\n  fork PR, mirrored dependency).\n- For code execution, the victim performs a commit via GitPython\u0027s native `index.commit()`. For the\n  file-read impact, opening the repo and reading config is enough.\n- `GIT_WORK_TREE` is **not** a mitigation \u2014 `git_dir` is assigned and the discovery loop breaks before\n  the environment is consulted.\n- Real git is unaffected; only GitPython mis-resolves the directory.\n\n### Remediation\n\n1. Test `curpath/.git` **before** the `gitdir`/`commondir`/`HEAD` triple and before `is_git_dir(curpath)`.\n2. Resolve `hook_path` / `_commit_hook_path` / `_get_validated_reflog_path` through `repo.common_dir`.\n3. Containment-check `commondir`/`gitdir` contents before joining (reuse\n   `SymbolicReference._get_validated_path`).\n4. Validate `HEAD` in `is_git_dir` (require `ref: refs/...` or a 40-hex sha).\n5. Pass `merge_includes=False` in `Repo._config_reader` (`git/repo/base.py:765`).\n\n### Note for the maintainers\n\nCommit `406b98e1` (2026-05-31, \"respect core.hooksPath for commit hooks\") resolved the hook path via\n`git rev-parse --git-path`, which is immune because git rediscovers the true `.git`. Commit `9bc287a2`\n(2026-07-20) reverted it to avoid an unconditional `rev-parse` dependency \u2014 reintroducing the exec step.\nMeasured at each commit with the healthy-looking layout: `406b98e1` \u2192 hook did not fire; `9bc287a2` \u2026\n`3.1.59` \u2192 hook fired.\n\n### Scope note\n\nOnly the **repository-root** case is reported: where a real `.git` exists, git prefers it, and GitPython\ndoes not. A fake git dir in a subdirectory fools real git too, so that variant is out of scope as a\ngeneral ecosystem hazard rather than a GitPython defect.\n\n###POC Files :\n[poc1_rce.py](https://github.com/user-attachments/files/31388464/poc1_rce.py)\n[poc2_file_read.py](https://github.com/user-attachments/files/31388465/poc2_file_read.py)",
  "id": "BREW-otterdog-CVE-2026-87817",
  "modified": "2026-10-01T09:31:27Z",
  "published": "2026-09-20T11:09:09Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/gitpython-developers/GitPython/security/advisories/GHSA-239g-whfq-7xj9"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-87817"
    },
    {
      "type": "WEB",
      "url": "https://github.com/gitpython-developers/GitPython/pull/2218"
    },
    {
      "type": "WEB",
      "url": "https://github.com/gitpython-developers/GitPython/commit/c7cf4d13b1ed0a2e70f2a1f3c6b4fc6c2652cf0b"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/gitpython-developers/GitPython"
    },
    {
      "type": "WEB",
      "url": "https://github.com/gitpython-developers/GitPython/releases/tag/3.1.60"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/gitpython/PYSEC-2026-3982.yaml"
    },
    {
      "type": "WEB",
      "url": "https://www.vulncheck.com/advisories/gitpython-before-3.1.60-remote-code-execution-via-git-directory-impersonation"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "GitPython: Repository content can impersonate the git directory, leading to arbitrary code execution",
  "upstream": [
    "PYSEC-2026-3982",
    "CVE-2026-87817",
    "GHSA-239g-whfq-7xj9"
  ]
}



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