Nx: OS command injection via git revisions and remote refs

Summary

Nx core builds several git invocations as shell command strings with untrusted values interpolated into them, so a value that should be a git revision or ref is parsed by /bin/sh instead. Two entry points are reachable by an attacker: affected commands, where defaultBase / affected.defaultBase from nx.json (and the NX_BASE / NX_HEAD environment variables) reach git merge-base and git diff; and nx import, where a branch name advertised by a remote repository reaches git fetch, git checkout, and git config. In both cases an attacker who controls a repository — or who opens a pull request against one — gets arbitrary command execution on the machine of anyone who runs an ordinary Nx command against it, including CI runners.

The affected path is the more serious of the two. nx affected and nx show projects --affected run constantly in CI, so a pull request that changes nothing but nx.json is enough to execute code on the runner with whatever credentials that job holds.

Severity

Exploitable by anyone who controls repository content — a fork's pull request, or a repository the victim clones — that the victim then runs an ordinary nx affected or nx import against; no access to the victim's machine is required. We have no evidence of exploitation in the wild.

Affected & Patched Versions

Treat every version below the patched ones as affected.

Remediation

Upgrade to 22.7.8 (22.x line) or 23.1.1 (23.x line) or later:

nx migrate 23.1.1

The fix is a drop-in — no configuration changes are required. If you cannot upgrade, treat nx.json from untrusted sources as executable content, do not run affected commands against pull requests you have not reviewed, and do not run nx import against repositories you do not trust.

Details

affected commands

Nx computes the merge base and the changed-file set by building git merge-base and git diff command lines as strings and running them through a shell. The base and head revisions in those strings come from nx.json's defaultBase / affected.defaultBase or from the NX_BASE / NX_HEAD environment variables, and a related code path reads file contents with git show <revision>:<path> the same way. Because a shell parses the whole line, a revision value containing shell syntax is executed rather than passed to git.

The revisions are wrapped in double quotes, which looks protective but is not: POSIX shells still perform command substitution inside double quotes, so a value of $(…) runs without needing to break out of the quotes.

nx import

The GitRepository helper runs every git operation — fetch, checkout, reset, config, and others — by interpolating its arguments into a shell command string. The untrusted argument is a branch name: nx import lists the branches a remote advertises, offers them to the user to choose from, and feeds the chosen name back into those commands. A hostile repository controls the names of its own branches, so it controls the command that runs when one is selected.

Credits

• Arkadiusz Marta (RE:SOURCE) — Reporter