CVE-2026-42305: Dulwich has an arbitrary file write via NTFS-hostile tree entries on Windows

Synopsis

An arbitrary file write vulnerability exists in Dulwich, a pure-Python Git implementation, affecting versions 0.10.0 through 1.2.5. The flaw is reachable over the network without authentication, but requires a victim to clone or check out a malicious repository; on Windows, specially crafted tree entries containing NTFS-hostile filename bytes are written outside the intended working directory, leading to remote code execution. No fix version has been published yet; HarborGuard is tracking the advisory and will make a patched-image rebuild available as soon as an upstream fix is released.

HarborGuard Coverage

Exploit Conditions

• Network reachabilityRequired

  The malicious repository is served over a network-accessible Git endpoint; the victim's Dulwich client must be able to reach it to trigger the path traversal during clone, fetch, or checkout.

• AuthenticationNot required

  No account or credentials are needed; any unauthenticated Git repository the victim is persuaded to clone is sufficient to deliver the malicious tree.

• Victim interactionRequired

  A user on Windows must actively clone, fetch, or check out the attacker-controlled repository, making this a social-engineering scenario where the victim is directed to a hostile Git URL.

• Attack complexityDetail

  Exploit reliability is high and condition-free once the victim interacts: the NTFS-hostile filename bytes are processed deterministically by Dulwich's broken validator with no race condition or memory-layout dependency required.

Blast Radius

• An attacker writes arbitrary files to any path the running process can reach on the victim's Windows filesystem, including startup folders, shell init scripts, or application config directories.
• Arbitrary file placement at attacker-chosen locations enables remote code execution in the context of the user running Dulwich, giving the attacker full control of that user session.
• Confidential files accessible to the victim user (source code, credentials, tokens stored on disk) are at risk of being overwritten or shadowed by attacker-supplied content.
• A POSIX user who clones and re-publishes an affected repository can unknowingly act as a relay, propagating the malicious tree to any Windows consumer who later fetches from that mirror.