Re: [RFC PATCH 6/6] hex: allow only lowercase object IDs in breaking changes mode

[Phillip Wood <phillip.wood123@gmail.com>]

Hi brian

On 30/07/2026 00:32, brian m. carlson wrote:
> Git has historically allowed either lowercase or uppercase hex for
> object IDs, but it has always emitted only lowercase.  This has caused
> people to expect only lowercase and not handle uppercase.
> 
> As an example, Git's own example hooks look for "[0-9a-f]" in several
> places, but there are many other Git-adjacent pieces of software,
> including Gitolite, which make the assumption that object IDs are always
> lowercase.  This is not to criticize the authors of these projects, but
> rather to point out how common this assumption is.  In fact, it's so
> common that we have only one test in our codebase that fails when we
> reject uppercase object IDs.
> 
> More critically, it leads people to make security-based assumptions that
> an object ID either does not contain uppercase characters or that an
> object ID can be expressed uniquely in hex form, neither of which are
> currently true.  Git itself normally uses binary object IDs, which
> avoids many of these problems, but most other projects deal primarily in
> hex object IDs, so they are more affected.

Can you say a bit more about the security problems please - I'm trying 
to understand why ABCDEF is a security risk when abcdef^0 isn't.

Thanks

Phillip

> In preparation for Git 3.0, only allow lowercase hex object IDs in
> breaking changes mode and document this as well.  Update the single
> failing test and add a new one to verify we reject new uppercase object
> IDs.  Note that in t5324, we change the hex character from "A" to "b"
> because in SHA-256 mode, "a" is the correct value, so our test_must_fail
> assertion will unexpectedly succeed in that case.
> 
> Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
> ---
>   Documentation/BreakingChanges.adoc | 5 +++++
>   hex-ll.h                           | 4 ++++
>   t/t1503-rev-parse-verify.sh        | 5 +++++
>   t/t5324-split-commit-graph.sh      | 4 ++--
>   4 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/BreakingChanges.adoc b/Documentation/BreakingChanges.adoc
> index 73bb939359..dbc46d14e3 100644
> --- a/Documentation/BreakingChanges.adoc
> +++ b/Documentation/BreakingChanges.adoc
> @@ -171,6 +171,11 @@ JGit, libgit2 and Gitoxide need to support it.
>     matches the default branch name used in new repositories by many of the
>     big Git forges.
>   
> +* Git will accept hex object IDs only in lowercase. The fact that Git has
> +	historically allowed uppercase characters in hex object IDs has been the
> +	source of a variety of bugs and security problems in software using Git. We
> +	don't expect most users to notice any change.
> +
>   * Git will require Rust as a mandatory part of the build process. While Git
>     already started to adopt Rust in Git 2.49, all parts written in Rust are
>     optional for the time being. This includes:
> diff --git a/hex-ll.h b/hex-ll.h
> index 9da76f17e8..2f9c8d7c25 100644
> --- a/hex-ll.h
> +++ b/hex-ll.h
> @@ -6,7 +6,11 @@ enum hexkind {
>   	HEX_KIND_LOWER = 1,
>   };
>   
> +#ifdef WITH_BREAKING_CHANGES
> +#define HEX_KIND_OID HEX_KIND_LOWER
> +#else
>   #define HEX_KIND_OID HEX_KIND_MIXED
> +#endif
>   
>   extern const signed char hexval_table[256];
>   extern const signed char hexval_lc_table[256];
> diff --git a/t/t1503-rev-parse-verify.sh b/t/t1503-rev-parse-verify.sh
> index 87638a4a2c..f07b45de5a 100755
> --- a/t/t1503-rev-parse-verify.sh
> +++ b/t/t1503-rev-parse-verify.sh
> @@ -60,6 +60,11 @@ test_expect_success 'works with one good rev' '
>   	test "$rev_head" = "$HASH4"
>   '
>   
> +test_expect_success WITH_BREAKING_CHANGES 'rejects uppercase revs' '
> +	UC_HASH=$(echo "$HASH1" | tr a-f A-F) &&
> +	test_must_fail git rev-parse --verify "$UC_HASH"
> +'
> +
>   test_expect_success 'fails with any bad rev or many good revs' '
>   	test_must_fail git rev-parse --verify 2>error &&
>   	test_grep "single revision" error &&
> diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
> index bf7ba0e558..29db815c77 100755
> --- a/t/t5324-split-commit-graph.sh
> +++ b/t/t5324-split-commit-graph.sh
> @@ -349,7 +349,7 @@ test_expect_success 'verify after commit-graph-chain corruption (base)' '
>   		test_must_fail git commit-graph verify 2>test_err &&
>   		grep -v "^+" test_err >err &&
>   		test_grep "invalid commit-graph chain" err &&
> -		corrupt_file "$graphdir/commit-graph-chain" 30 "A" &&
> +		corrupt_file "$graphdir/commit-graph-chain" 30 "a" &&
>   		test_must_fail git commit-graph verify 2>test_err &&
>   		grep -v "^+" test_err >err &&
>   		test_grep "unable to find all commit-graph files" err
> @@ -364,7 +364,7 @@ test_expect_success 'verify after commit-graph-chain corruption (tip)' '
>   		test_must_fail git commit-graph verify 2>test_err &&
>   		grep -v "^+" test_err >err &&
>   		test_grep "invalid commit-graph chain" err &&
> -		corrupt_file "$graphdir/commit-graph-chain" 70 "A" &&
> +		corrupt_file "$graphdir/commit-graph-chain" 70 "b" &&
>   		test_must_fail git commit-graph verify 2>test_err &&
>   		grep -v "^+" test_err >err &&
>   		test_grep "unable to find all commit-graph files" err
>