CVE-2026-50015: pnpm: Arbitrary File Write/Delete via Malicious Patch File (Path Traversal)

Summary

pnpm's patch application pipeline (@pnpm/patch-package) performs no path validation on file paths extracted from .patch files. An attacker who contributes a malicious patch file via a pull request can write attacker-controlled content to or delete arbitrary files on the filesystem during pnpm install, as the user running the install. The diff --git header paths containing ../../ sequences traverse out of the package directory, and the traversal is difficult to catch in code review because patch file diff headers are opaque to most reviewers.

Vulnerability Details

During pnpm install, when a patchedDependencies entry is present in pnpm-workspace.yaml, pnpm reads the referenced .patch file and applies it via the embedded @pnpm/patch-package library. The applyPatchToDir function at patching/apply-patch/src/index.ts:12-13 calls process.chdir(opts.patchedDir), setting the working directory to the installed package location deep inside nodemodules/.pnpm/.

The patch parser at @pnpm/patch-package/dist/patch/parse.js:88 extracts file paths from diff --git a/(.?) b/(.?) headers using a regex with no path sanitization. The executeEffects function in apply.js then operates on these unsanitized paths:

File write (apply.js:35-49): javascript case 'file creation': { const eff = effect fs.ensureDirSync(dirname(eff.path)) fs.writeFileSync(eff.path, fileContents, { mode: eff.mode }) break }

File delete (apply.js:13-22): javascript case 'file deletion': { const eff = effect // TODO: integrity checks if (!opts.dryRun) { fs.unlinkSync(eff.path) } break }

A path like ../../../../../../../../../../home/user/.ssh/authorizedkeys in the patch header traverses out of the package directory to an arbitrary location.

Proof of Concept

bash Write variant: bash autofynaudit/exploits/vuln6patchtraversalwrite/exploit.sh Result: PASS -- /tmp/vuln6pwned created with attacker-controlled content

Delete variant: bash autofynaudit/exploits/vuln7patchtraversaldelete/exploit.sh Result: PASS -- /tmp/vuln7target deleted by malicious patch

Combined chain (delete + replace SSH authorizedkeys): bash autofynaudit/exploits/chain2patchsshbackdoor/exploit.sh Result: PASS -- authorizedkeys replaced with attacker's public key

Impact

Arbitrary file write and delete as the user running pnpm install, limited to paths writable by that user. An attacker who submits a PR adding a .patch file and patchedDependencies config can target SSH authorizedkeys, shell configuration, CI/CD files, or other writable files. Patch files may receive less review scrutiny than package.json changes because the ../ traversal sequences are in diff --git headers that look like patch metadata.

Suggested Remediation

Validate parsed patch file paths against the package root directory. Reject any path that resolves outside the patched package directory via path.resolve + prefix check. Alternatively, sanitize at parse time by rejecting paths containing .. components in parse.js.

---

> Discovered by AutoFyn > Full audit report: auditreport.md > Exploit script: exploit.sh