# CVE-2021-31920: Security feature bypass in n/a

> Attackers can bypass path-based authorization policies in Istio by using multiple slashes or escaped characters in HTTP requests.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 May 2021
- **Updated:** 3 Aug 2024

## What is CVE-2021-31920?

CVE-2021-31920 is a medium-severity vulnerability in n/a, published on 27 May 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Istio before 1.8.6 and 1.9.x before 1.9.5 has a remotely exploitable vulnerability where an HTTP request path with multiple slashes or escaped slash characters (%2F or %5C) could potentially bypass an Istio authorization policy when path based authorization rules are used.

Weakness: CWE-706 (Use of Incorrectly-Resolved Name or Reference)

## Which versions are affected by CVE-2021-31920?

It affects 1 product.


Products: Istio Istio.

## How do I fix CVE-2021-31920?

Upgrade to a fixed version.

## Is CVE-2021-31920 being exploited?

There is no known exploitation of CVE-2021-31920 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2021-31921](https://insights.sbomify.com/CVE-2021-31921/): Missing authorization in n/a (critical 9.8)
- [CVE-2021-34824](https://insights.sbomify.com/CVE-2021-34824/): Information disclosure in n/a (high 8.8)
- [CVE-2026-31837](https://insights.sbomify.com/CVE-2026-31837/): Istio JWKS resolver to prevent private key material from being exposed when JWKS fetch fails. (high 8.7)
- [CVE-2021-39155](https://insights.sbomify.com/CVE-2021-39155/): Authorization Policy Bypass Due to Case Insensitive Host Comparison (high 8.3)
- [CVE-2021-39156](https://insights.sbomify.com/CVE-2021-39156/): Fragments in Path May Lead to Authorization Policy Bypass (high 8.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-31920 and fix it if it is affected.

What it is: CVE-2021-31920 is a medium-severity vulnerability in n/a, published on 27 May 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-31920 in the wild and no public exploit code.

Affected product: n/a. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2021-31920/ (Markdown: https://insights.sbomify.com/CVE-2021-31920/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-31920
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-31920
- OSV: https://osv.dev/vulnerability/CVE-2021-31920

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-31920/
Summaries are generated and may contain mistakes; the official description above is the record.