Gitsign is a keyless Sigstore to signing tool for Git commits with your a GitHub / OIDC identity. Prior to 0.16.0, gitsign verify and gitsign verify-tag re-encode commit/tag objects through go-git's EncodeWithoutSignature before checking the signature, instead of verifying against the raw git object bytes. For malformed objects with duplicate tree headers, git-core and go-git parse different trees: git-core uses the first, go-git uses the second. A signature crafted over the go-git-normalized form (second tree) passes gitsign verify while git-core resolves the commit to a completely different tree. This breaks the invariant that a verified signature, the commit semantics git-core presents to users, and the object hash logged in Rekor all refer to the same content. This vulnerability is fixed in 0.16.0.

Affected: Go: github.com/sigstore/gitsign

## Summary

`gitsign verify` and `gitsign verify-tag` re-encode commit/tag objects through go-git's `EncodeWithoutSignature` before checking the signature, instead of verifying against the raw git object bytes. For malformed objects with duplicate `tree` headers, git-core and go-git parse different trees: git-core uses the first, go-git uses the second. A signature crafted over the go-git-normalized form (second tree) passes `gitsign verify` while git-core resolves the commit to a completely different tree. This breaks the invariant that a verified signature, the commit semantics git-core presents to users, and the object hash logged in Rekor all refer to the same content.

## Severity

**Medium** (CVSS 3.1: 5.7)

`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N`

- **Attack Vector:** Network — a malformed commit can be distributed via any accessible git remote - **Attack Complexity:** High — exploitation requires crafting malformed objects that also bypass git server fsck checks (not universally enabled) - **Privileges Required:** None — the most impactful form (signature replay) requires no signing key - **User Interaction:** Required — a victim must run `gitsign verify` on the malformed commit - **Scope:** Unchanged — impact is confined to the repository under verification - **Confidentiality Impact:** None - **Integrity Impact:** High — a verified signature appears to endorse content different from what git-core resolves and presents to users - **Availability Impact:** None…

Affected: Ubuntu:25.10: gitsign, Ubuntu:Pro:26.04:LTS: gitsign

Gitsign is a keyless Sigstore to signing tool for Git commits with your a GitHub / OIDC identity. Prior to 0.16.0, gitsign verify and gitsign verify-tag re-encode commit/tag objects through go-git's EncodeWithoutSignature before checking the signature, instead of verifying against the raw git object bytes. For malformed objects with duplicate tree headers, git-core and go-git parse different trees: git-core uses the first, go-git uses the second. A signature crafted over the go-git-normalized form (second tree) passes gitsign verify while git-core resolves the commit to a completely different tree. This breaks the invariant that a verified signature, the commit semantics git-core presents to users, and the object hash logged in Rekor all refer to the same content. This vulnerability is fixed in 0.16.0.