Git Hash Chain Malleability

Abstract

Git commit signing is widely entrusted to serve as evidence that a commit hash uniquely and immutably identifies a specific piece of signed content. We show this invariant does not hold. Given any signed commit, an attacker without access to the signing key, and without breaking SHA2, can produce a second, distinct commit with an identical tree, identical metadata, a valid signature, and a ``Verified'' badge from a Git Forge such as Github, differing only in its commit hash. The modified commit cascades to modify the values of all the subsequent, dependent commit hashes; hence we introduce the terminology ``hash chain malleability'' to describe this phenomenon. The malleability in signed Git hashes is feasible due to the inherent malleability present in many of the data representations that make up a commit. In this paper we show three such malleation routes: (i) algebraic inversion s -> n-s for ECDSA; (ii) structural insertion of an unhashed OpenPGP subpacket (RFC4880 5.2.3) for RSA and EdDSA; and (iii) non-canonical DER length re-encoding (X.690 10.1) inside the CMS envelope for S/MIME. Algebraic inversion for ECDSA signatures and subpacket insertion were found to pass local verification (git verify-commit), and all three methods yield a persistent, independent ``Verified'' record on Github. We discuss the consequences of Git hash chain malleation for hash-based commit blocking, dependency pinning (Nixpkgs, Go modules, Github Actions), and reproducible-build systems that treat the commit hash as a content-addressable primary key, and we provide proof-of-concept tooling that automates all three routes.