Verify against raw git object bytes#802
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wlynch merged 2 commits intosigstore:mainfrom May 6, 2026
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go-git's loose object parser uses last-wins semantics for duplicate singleton headers (tree, author, committer, etc.), while git-core uses first-wins. An attacker can craft a commit or tag whose raw bytes hash to one set of contents under git-core but re-encode through go-git to a different signed payload, letting a legitimate signature verify against attacker-controlled bytes. Replace the go-git decode + EncodeWithoutSignature path with SplitCommit and SplitTag, which operate directly on the object-database bytes (the same bytes git-core feeds its verifier) and reject objects with structural ambiguities — duplicate singleton headers, duplicate gpgsig, malformed gpgsig continuations. ObjectHash now reassembles via JoinCommit/JoinTag so the recorded hash matches git-core. Signed-off-by: Billy Lynch <billy@chainguard.dev>
See https://git-scm.com/docs/hash-function-transition#_signed_commits Signed-off-by: Billy Lynch <billy@chainguard.dev>
cpanato
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May 6, 2026
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Observed review from cpanato+4115580 (@cpanato) |
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Merging w/o E2E because github actions outage has broken the Sigstore beacon token workflow, blocking the tests. |
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Summary
go-git's loose object parser uses last-wins semantics for duplicate singleton headers (tree, author, committer, etc.), while git-core uses first-wins. An attacker can craft a commit or tag whose raw bytes hash to one set of contents under git-core but re-encode through go-git to a different signed payload, letting a legitimate signature verify against attacker-controlled bytes.
Replace the go-git decode + EncodeWithoutSignature path with SplitCommit and SplitTag, which operate directly on the object-database bytes (the same bytes git-core feeds its verifier) and reject objects with structural ambiguities — duplicate singleton headers, duplicate gpgsig, malformed gpgsig continuations. ObjectHash now reassembles via JoinCommit/JoinTag so the recorded hash matches git-core.
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