## Summary
`isValidReturnToPath` validates the post-login `returnTo` path and
already rejects protocol-relative `//` paths — but not the backslash
variant. Browsers normalize `\` to `/`, so `/\evil.com` resolves like
`//evil.com` (a protocol-relative, external URL) while still passing the
existing `//` check:
```ts
isValidReturnToPath("/\\evil.com"); // returns true today; should be false
```
This hardens the open-redirect guard by rejecting any path containing a
backslash, so a `returnTo` can only ever be a same-site absolute path.
## Changes
- `isValidReturnToPath`: reject paths containing `\`.
- Added tests for backslash-tricked paths.
Framed as defense-in-depth — the validator should reject this class
regardless of how each consumer performs the redirect.
---------
Co-authored-by: Charles Bochet <charles@twenty.com>
joeltco
7894ae39f0