Headline
CVE-2022-39260: Heap overflow in `git shell` leading to RCE
Git is an open source, scalable, distributed revision control system. git shell is a restricted login shell that can be used to implement Git’s push/pull functionality via SSH. In versions prior to 2.30.6, 2.31.5, 2.32.4, 2.33.5, 2.34.5, 2.35.5, 2.36.3, and 2.37.4, the function that splits the command arguments into an array improperly uses an int to represent the number of entries in the array, allowing a malicious actor to intentionally overflow the return value, leading to arbitrary heap writes. Because the resulting array is then passed to execv(), it is possible to leverage this attack to gain remote code execution on a victim machine. Note that a victim must first allow access to git shell as a login shell in order to be vulnerable to this attack. This problem is patched in versions 2.30.6, 2.31.5, 2.32.4, 2.33.5, 2.34.5, 2.35.5, 2.36.3, and 2.37.4 and users are advised to upgrade to the latest version. Disabling git shell access via remote logins is a viable short-term workaround.
Impact
git shell is a restricted login shell that can be used to implement Git’s push/pull functionality via SSH. Typically git shell only allows executing receive-pack, upload-pack, or upload-archive, each followed by an arbitrary <argv> specified by the caller:
If $HOME/git-shell-commands exists and is a directory, however, then any script in that directory can be executed by name with user-provided arguments.
Before executing commands, git shell splits the given <argv> string into an array with split_cmdline(). This function improperly uses an int to represent the number of entries (argc) in the argv array, allowing a malicious actor to intentionally overflow the return value, leading to arbitrary heap writes. Because the resulting argv array is then passed to execv(), it is possible to leverage this attack to gain remote code execution on a victim machine.
Note that a victim must first allow access to git shell as a login shell, and allow interactive mode by having the $HOME/git-shell-commands directory present in order to be vulnerable to this attack.
Patches
The problem has been patched in the versions published on 2022-10-18, going back to v2.30.x.
Workarounds
The most complete workaround is upgrading to the most recent patched version published.
If doing so is impractical, then disabling git shell access via remote logins is encouraged in the short-term. If git shell cannot be disabled as a login shell, then disabling its interactive mode by removing the git-shell-commands directory is encouraged.
Credit
Credit for finding the vulnerability goes to Kevin Backhouse of the GitHub Security Lab.
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