CVE-2026-87817: GitPython Repository Content Can Impersonate The Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-239g-whfq-7xj9",
  "modified" : "2026-09-30T23:27:59Z",
  "published" : "2026-09-30T23:27:59Z",
  "aliases" : [ "CVE-2026-87817" ],
  "summary" : "GitPython: Repository content can impersonate the git directory, leading to arbitrary code execution",
  "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's repository, GitPython resolves `git_dir` to\nthe **working-tree root** while real git correctly resolves `<root>/.git`. Everything GitPython then\ntreats as \"inside the git directory\" is attacker-authored content — 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` — `isfile(curpath/gitdir)` **and** `isfile(curpath/commondir)` **and** `isfile(curpath/HEAD)`\n2. `git/repo/base.py:320` — `is_git_dir(curpath)`\n3. `git/repo/base.py:341` — `dotgit = osp.join(curpath, \".git\")` ← 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`'s 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 — 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 — 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      <- shadowed\nattacker's 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 — a CI runner building a fork\npull request, a code-scanning/SBOM service, a mirror, a dependency bot, an AI code-review/agent tool —\ncan be made to:\n\n1. **Execute arbitrary commands** via the tracked `<root>/hooks/pre-commit` when the victim calls\n   `index.commit()`.\n2. **Read files outside the repository**: the tracked `<root>/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's 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 — `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 — reintroducing the exec step.\nMeasured at each commit with the healthy-looking layout: `406b98e1` → hook did not fire; `9bc287a2` …\n`3.1.59` → 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)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "gitpython"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.1.60"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.1.59"
    }
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-427", "CWE-94" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-30T23:27:59Z",
    "nvd_published_at" : null
  }
}