CVE-2021-29492: Bypass of path matching rules using escaped slash characters

Severity: high (CVSS 8.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 63.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-05-28 · updated 2026-06-17
Weakness: CWE-22

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.

Affected products

envoyproxy envoy: before 1.15.5; from 1.16.0 before 1.16.4; from 1.17.0 before 1.17.3; from 1.18.0 before 1.18.3 — fixed in 1.15.5, 1.16.4, 1.17.3, 1.18.3

How to fix

Upgrade to a fixed version: envoyproxy envoy 1.15.5, 1.16.4, 1.17.3, 1.18.3. Check the vendor advisory for mitigations.

References

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