CVE-2021-29492: Envoyproxy Envoy

High severity, CVSS 8.3. EPSS: 66.2% chance of exploitation in the next 30 days.

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.

Affected products

• Envoyproxy Envoy: before 1.15.5 (fixed in 1.15.5); from 1.16.0, before 1.16.4 (fixed in 1.16.4); from 1.17.0, before 1.17.3 (fixed in 1.17.3); from 1.18.0, before 1.18.3 (fixed in 1.18.3)

Published 2021-05-28. Last modified 2026-06-17.