Vitest is a Vite-native testing framework for JavaScript and TypeScript projects. From 4.1.11 until 4.1.12 and 5.0.3, isFileLoadingAllowed and the filesystem read interpret backslashes differently on Windows in packages/mocker/src/node/interceptorPlugin.ts. When a client can reach the Vite dev server on Windows and register a redirect mock containing backslash traversal, POSIX joining preserves a Windows backslash traversal that the later filesystem read interprets as a path separator, so the client can read a file outside server.fs.allow that the Vitest process can access. This issue is fixed in versions 4.1.12 and 5.0.3.
Affected: npm: @vitest/mocker
## Summary
The path traversal fix released for [GHSA-82fw-gwwq-j7x9](https://github.com/vitest-dev/vitest/security/advisories/GHSA-82fw-gwwq-j7x9) can be bypassed on Windows by using a backslash in the redirect URL pathname.
The fixed code joins an attacker-controlled URL pathname with the Vite root using `node:path/posix`, then passes that string to Vite's file allowlist check. On Windows, a value such as `C:/project/root/..\secret.txt` retains `..\` during the POSIX join and begins with the allowed root, so the allowlist accepts it. The later `readFile()` call uses Windows filesystem semantics, where the backslash is a path separator, and reads the sibling file outside the configured root.
I reproduced the issue with the published `@vitest/mocker` packages in both fixed release lines:
- `@vitest/mocker@4.1.11` - `@vitest/mocker@5.0.0`
The current `main` source at commit `2ce29d5fa758046e5453bd92b8ed6c9da9709bb5` has the same behavior.
## Patches
Fixed in `@vitest/mocker` 5.0.3, released on 2026-09-30 (PR #11377), and backported to 4.1.12. The redirect registration now converts backslashes in the URL pathname to forward slashes before it joins the pathname with the root, so `..\` resolves the same as `../` and the allowlist check rejects it.
## Relationship to GHSA-82fw-gwwq-j7x9
This is a bypass of the accepted fix, not a separate unrelated path. It uses the same `vitest:interceptor:register` redirect registration and the same file-reading sink as the original adv…