CVE-2021-29492
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`.
Does this matter?
EPSS puts the probability of exploitation in the next 30 days at 63.1%, higher than 99% of all known CVEs. Patch or mitigate before the next change window.
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
- 8.3 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L
- EPSS
- 63.10% probability · 99th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-22
- Affected
- envoyproxy/envoy
- Source
- security-advisories@github.com
EPSS moved from 63.1% on 25 October 2026 to 63.1% on 3 November 2026. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”.
References
- https://github.com/envoyproxy/envoy/security/advisories/GHSA-4987-27fx-x6cfMitigation, Third Party Advisory
- https://github.com/envoyproxy/envoy/security/advisories/GHSA-4987-27fx-x6cfMitigation, Third Party Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.