When a clone fails, users naturally turn to the output of the git clone command. To assist in such scenarios, the output includes the messages from the remote git pack-objects process, delivered via what Git calls the "sideband channel." Given that the remote server is, by nature, remote, there is no guarantee that it runs an unmodified Git version. This exposes Git to ANSI escape sequence injection (see CWE-150, https://cwe.mitre.org/data/definitions/150.html), which can corrupt terminal state, hide information, and even insert characters into the input buffer (as if the user had typed those characters). This patch series addresses this vulnerability by sanitizing the sideband channel. It is important to note that the lack of sanitization in the sideband channel is already "exploited" by the Git user community, albeit in well-intentioned ways. For instance, certain server-side hooks use ANSI color sequences in error messages to make them more noticeable during intentional failed fetches, e.g. as seen at https://github.com/kikeonline/githook-explode and https://github.com/arosien/bart/blob/HEAD/hooks/post-receive.php To accommodate such use cases, Git will allow ANSI color sequences to pass through by default, while presenting all other ASCII control characters in a common form (e.g., presenting the ESC character as ^[). This vulnerability was reported to the Git security mailing list in early November, along with these fixes, as part of an iteration of the patches that led to the coordinated security release on Tuesday, January 14th, 2025. While Git for Windows included these fixes in v2.47.1(2), the consensus, apart from one reviewer, was not to include them in Git's embargoed versions. The risk was considered too high to disrupt existing scenarios that depend on control characters received via the sideband channel being sent verbatim to the user's terminal emulator. Several reviewers suggested advising terminal emulator writers about these "quality of implementation issues" instead. I was quite surprised by this approach, as it seems overly optimistic to assume that terminal emulators could distinguish between control characters intentionally sent by Git and those unintentionally relayed from the remote server. Please note that this patch series applies cleanly on top of v2.47.2. To apply it cleanly on top of v2.40.4 (the oldest of the most recently serviced security releases), the calls to test_grep need to be replaced with calls to test_i18ngrep, and the calls to git_config_get_string_tmp() need to be replaced with calls to git_config_get_string(). Johannes Schindelin (3): sideband: mask control characters sideband: introduce an "escape hatch" to allow control characters sideband: do allow ANSI color sequences by default Documentation/config.txt | 2 + Documentation/config/sideband.txt | 16 ++++++ sideband.c | 78 ++++++++++++++++++++++++++++- t/t5409-colorize-remote-messages.sh | 30 +++++++++++ 4 files changed, 124 insertions(+), 2 deletions(-) create mode 100644 Documentation/config/sideband.txt base-commit: e1fbebe347426ef7974dc2198f8a277b7c31c8fe Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1853%2Fdscho%2Fsanitize-sideband-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1853/dscho/sanitize-sideband-v1 Pull-Request: https://github.com/gitgitgadget/git/pull/1853 -- gitgitgadget
