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The vulnerability arises when kas relies exclusively on a Git commit ID (SHA-1 or SHA-256) to determine whether a repository checkout matches the state validated at configuration time. Git, up to version 2.47.3, does not warn or block checkout of a branch whose name is formatted as a 40‑character (SHA-1) or 64‑character (SHA-256) hexadecimal string.
If an attacker gains control of the referenced repository and creates a branch with a name identical to the expected commit ID, `git checkout` will fetch that branch instead of the intended commit. Kas, lacking any additional verification, treats this branch as the legitimate commit, allowing the attacker to substitute arbitrary content.
The risk is particularly high for SHA‑256 commit IDs, which are otherwise considered cryptographically strong. While major forges like GitHub and GitLab block the creation of such branch names, many self‑hosted Git instances do not, making the attack practical in those environments.
The patch introduces a check for branch names that match the SHA-1 (40 hex) or SHA-256 (64 hex) pattern. When such a name is requested, kas appends `^{commit}` to force Git to interpret the reference as a commit rather than a branch, causing the checkout to fail if a branch with that name exists.
DailyCVE Form:
Platform: kas
Version: <5.3
Vulnerability : Improper Verification
Severity: Low
date: 2026-06-01
Prediction: 2026-06-01
What Undercode Say:
Create a branch that looks like a SHA-256 commit ID git checkout -b 4cb4a3d01122ffaec9feaae768a5814092f6f9b5 Demonstrate the vulnerable kas configuration (excerpt) In kas config file: repos: example: url: https://github.com/attacker/malicious-repo commit: 4cb4a3d01122ffaec9feaae768a5814092f6f9b5 When kas <5.3 processes the above, git checks out the malicious branch instead of the intended commit.
Exploit:
1. Compromise a repository that is referenced in a kas configuration.
2. Push a branch whose name exactly matches the expected commit ID (40/64 hex chars).
3. Wait for the kas build process to fetch the repository – it will check out the attacker’s branch, replacing the intended content.
Protection:
- Update to kas version 5.3 (or later) which includes commit
4cb4a3d. - Do not rely solely on commit IDs; use cryptographic signatures (signed commits/tags) for integrity validation.
- Mirror the repository to a trusted location and validate its contents manually before referencing it in kas.
Impact:
Successful exploitation enables supply‑chain attacks, arbitrary code injection, and data tampering within builds that depend on the vulnerable repository. The attack can degrade build reliability and introduce unauthorised modifications without the user’s knowledge.
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Sources:
Reported By: github.com
Extra Source Hub:
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