CVE-2021-29492
Bypass of path matching rules using escaped slash characters
In short
Envoy proxy fails to decode escaped slashes (%2F, %5C) in URLs, allowing attackers to bypass access control rules by crafting paths like /something%2F..%2Fadmin that backend servers decode differently, potentially gaining unauthorized access to restricted areas.
Technical detail
CWE-22 path traversal: Envoy does not normalize escaped slash sequences in HTTP URL paths before applying access control policies (RBAC, JWT). An attacker sends crafted URLs with %2F or %5C bypassing proxy-level filtering; backend servers that decode these sequences can normalize paths and access resources beyond the policy scope. Affects versions ≤1.18.2.
Summary generated and translated by AI from the official description.
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.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Affected products
envoyproxy · envoyWant to know if your infrastructure is exposed to this?
Talk to TrueHacking →