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CVE-2026-42563 jelmer CVE debrief

CVE-2026-42563 is a HIGH severity vulnerability in Dulwich, a pure-Python implementation of Git file formats and protocols. The vulnerability affects versions 0.24.0 to 1.2.4 of Dulwich. An attacker who can cause a victim to merge an untrusted branch can achieve arbitrary command execution by crafting malicious file paths. The vulnerability is due to the `ProcessMergeDriver` substituting the file path from the git tree into the merge driver command via the `%P` placeholder and executing it with `subprocess.run(..., shell=True)`. Version 1.2.5 of Dulwich fixes the issue.

Vendor
jelmer
Product
dulwich
CVSS
HIGH 7.7
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-10
Original CVE updated
2026-07-21
Advisory published
2026-06-10
Advisory updated
2026-07-21

Who should care

Users of Dulwich, especially those who merge untrusted branches, should be aware of this vulnerability and take steps to mitigate it.

Technical summary

The `ProcessMergeDriver` in Dulwich substitutes the file path from the git tree into the merge driver command via the `%P` placeholder and executes it with `subprocess.run(..., shell=True)`. This allows an attacker to execute arbitrary commands by crafting malicious file paths.

Defensive priority

HIGH

Recommended defensive actions

  • Upgrade to Dulwich version 1.2.5 or later.
  • Avoid merging untrusted branches.

Evidence notes

The vulnerability is due to the insecure use of `subprocess.run(..., shell=True)` in the `ProcessMergeDriver`.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-42563 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-42563

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-42563 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-42563

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.