# CVE-2020-5249: HTTP Response Splitting (Early Hints) in Puma

> Attackers can inject malicious content via HTTP response splitting in Puma's early-hints headers if untrusted input is allowed, enabling XSS or header injection.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 2 Mar 2020
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2020-5249, GHSA-33vf-4xgg-9r58

## What is CVE-2020-5249?

CVE-2020-5249 is a medium-severity vulnerability in Puma, published on 2 Mar 2020. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

In Puma (RubyGem) before 4.3.3 and 3.12.4, if an application using Puma allows untrusted input in an early-hints header, an attacker can use a carriage return character to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting. While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS). This is related to CVE-2020-5247, which fixed this vulnerability but only for regular responses. This has been fixed in 4.3.3 and 3.12.4.

Weakness: CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component), CWE-113 (Improper Neutralization of CRLF Sequences in HTTP Headers)

## Which versions are affected by CVE-2020-5249?

4 open source packages ship an affected version, including puma. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `puma` | RubyGems | All versions | 3.12.4 |
| `puma` | RubyGems | All versions | 3.12.4 |
| `puma` | RubyGems | From 4.0.0 | 4.3.3 |
| `puma` | RubyGems | From 4.0.0 | 4.3.3 |

Products: Puma Puma.

## How do I fix CVE-2020-5249?

Upgrade puma to a fixed version for your release line: 3.12.4 or 4.3.3.

## Is CVE-2020-5249 being exploited?

There is no known exploitation of CVE-2020-5249 in the wild and no public exploit code.

## Other vulnerabilities in Puma

- [CVE-2023-40175](https://insights.sbomify.com/CVE-2023-40175/): Inconsistent Interpretation of HTTP Requests in puma (high 7.3)
- [CVE-2024-21647](https://insights.sbomify.com/CVE-2024-21647/): HTTP Request/Response Smuggling in puma (medium 5.9)
- [CVE-2024-45614](https://insights.sbomify.com/CVE-2024-45614/): Header normalization allows for client to clobber proxy set headers in Puma (medium 5.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-5249 and fix it if it is affected.

What it is: CVE-2020-5249 is a medium-severity vulnerability in Puma, published on 2 Mar 2020. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2020-5249 in the wild and no public exploit code.

Affected packages and fixed versions:
- puma (RubyGems): affected all versions; fixed in 3.12.4
- puma (RubyGems): affected all versions; fixed in 3.12.4
- puma (RubyGems): affected from 4.0.0; fixed in 4.3.3
- puma (RubyGems): affected from 4.0.0; fixed in 4.3.3

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-2020-5249/ (Markdown: https://insights.sbomify.com/CVE-2020-5249/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-5249
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-5249
- OSV: https://osv.dev/vulnerability/CVE-2020-5249

---
From sbomify Insights: https://insights.sbomify.com/CVE-2020-5249/
Summaries are generated and may contain mistakes; the official description above is the record.