HTTP Smuggling via Transfer-Encoding Header in Puma (CVE-2020-11077)

HTTP Smuggling via Transfer-Encoding Header in Puma (CVE-2020-11077) | HOL Guard CVE

Why this deserves its current priority

CVSS is 6.8. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps puma (rubygems), puma (rubygems). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
pumarubygems	>=0 <3.12.6	3.12.6
pumarubygems	>=4.0.0 <4.3.5	4.3.5

Recommended response

Check inventory. Check lockfiles and deployed manifests for puma, puma.
Review the reported fix. Update puma to 3.12.6; puma to 4.3.5 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
May 22, 2020
Evidence: source:osv:source_dates:source-dates:record
Source modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:record
First seen by HOL
Aug 7, 2026

Sources and claim methodology

Upstream source description

### Impact This is a similar but different vulnerability to the one patched in 3.12.5 and 4.3.4. A client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. 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. ### Patches The problem has been fixed in Puma 3.12.6 and Puma 4.3.5. ### For more information If you have any questions or comments about this advisory: * Open an issue in [Puma](https://github.com/puma/puma) * See our [security policy](https://github.com/puma/puma/security/policy)

Quoted source text, attributed separately from HOL analysis.