When packing objects into a single so-called packfile, objects may not get stored as complete copies but instead as deltas against another object "base". A specially crafted delta object could trigger an integer overflow and thus bypass our input validation, which may result in copying memory before or after the base object into the final deflated object. This may lead to objects containing copies of system memory being written into the object database. As the hash of those objects cannot be easily controlled by the attacker, it is unlikely that any of those objects will be valid and referenced by the commit graph.
Note that the error could also be triggered by the function git_apply__patch. But as this function is not in use outside of our test suite, it is not a possible attack vector.