CVE-2021-29492

### Description
Envoy does not decode escaped slash sequences `%2F` and `%5C` in HTTP URL paths in versions 1.18.2 and before. A remote attacker may craft a path with escaped slashes, e.g. `/something%2F..%2Fadmin`, to bypass access control, e.g. a block on `/admin`. A backend server could then decode slash sequences and normalize path and provide an attacker access beyond the scope provided for by the access control policy.

### Impact
Escalation of Privileges when using RBAC or JWT filters with enforcement based on URL path. Users with back end servers that interpret `%2F` and `/` and `%5C` and “ interchangeably are impacted.

### Attack Vector
URL paths containing escaped slash characters delivered by untrusted client.

### Patches
Envoy versions 1.18.3, 1.17.3, 1.16.4, 1.15.5 contain new path normalization option to decode escaped slash characters.

### Workarounds
If back end servers treat `%2F` and `/` and `%5C` and “ interchangeably and a URL path based access control is configured, we recommend reconfiguring back end server to not treat `%2F` and `/` and `%5C` and “ interchangeably if feasible.

### Credit
Ruilin Yang ([email protected])

### References
https://blog.envoyproxy.io
https://github.com/envoyproxy/envoy/releases

### For more information
If you have any questions or comments about this advisory:
* Open an issue in [Envoy repo](https://github.com/envoyproxy/envoy/issues)
* Email us at [envoy-security](mailto:[email protected])

Source: CVE-2021-29492