From: Phillip Wood Date: Tue, 25 Aug 2026 09:04:36 GMT Subject: Re: [RFC PATCH 6/6] hex: allow only lowercase object IDs in breaking changes mode Message-ID: In-Reply-To: <20260729233215.398654-7-sandals@crustytoothpaste.net> 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 > --- > 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 >