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

> Attackers can bypass URL-based access controls in Envoy by using escaped slashes, potentially gaining unauthorized access to protected backend resources.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 28 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** BIT-envoy-2021-29492, GHSA-4987-27fx-x6cf

## What is CVE-2021-29492?

CVE-2021-29492 is a high-severity path traversal vulnerability in Envoyproxy Envoy, published on 28 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### 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.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

## Which versions are affected by CVE-2021-29492?

1 open source package ship an affected version, including envoy. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `envoy` | Bitnami | All versions | 1.15.5, 1.16.4, 1.17.3, 1.18.3 |

Products: Envoyproxy Envoy.

## How do I fix CVE-2021-29492?

Upgrade envoy to a fixed version for your release line: 1.15.5, 1.16.4, 1.17.3 or 1.18.3.

## Is CVE-2021-29492 being exploited?

There is no known exploitation of CVE-2021-29492 in the wild and no public exploit code.

## Other vulnerabilities in Envoyproxy Envoy

- [CVE-2022-29226](https://insights.sbomify.com/CVE-2022-29226/): Trivial authentication bypass in Envoy (critical 10.0)
- [CVE-2019-18801](https://insights.sbomify.com/CVE-2019-18801/): Memory corruption in n/a (critical 9.8)
- [CVE-2019-18802](https://insights.sbomify.com/CVE-2019-18802/): Security feature bypass in n/a (critical 9.8)
- [CVE-2020-35470](https://insights.sbomify.com/CVE-2020-35470/): Information disclosure in n/a (high 8.8)
- [CVE-2024-23324](https://insights.sbomify.com/CVE-2024-23324/): Envoy ext auth can be bypassed when Proxy protocol filter sets invalid UTF-8 metadata (high 8.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-29492 and fix it if it is affected.

What it is: CVE-2021-29492 is a high-severity path traversal vulnerability in Envoyproxy Envoy, published on 28 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-29492 in the wild and no public exploit code.

Affected packages and fixed versions:
- envoy (Bitnami): affected all versions; fixed in 1.15.5, 1.16.4, 1.17.3, 1.18.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-29492/ (Markdown: https://insights.sbomify.com/CVE-2021-29492/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29492
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29492
- OSV: https://osv.dev/vulnerability/CVE-2021-29492

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-29492/
Summaries are generated and may contain mistakes; the official description above is the record.