Envoy is a cloud-native edge/middle/service proxy. 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 in versions 1.18.3, 1.17.3, 1.16.4, 1.15.5 contain new path normalization option to decode escaped slash characters. As a workaround, if back end servers treat "%2F" and "/" and "%5C" and "\" interchangeably and a URL path based access control is configured, one may reconfigure the back end server to not treat "%2F" and "/" and "%5C" and "\" interchangeably.