CVE-2021-41136Network attack vector

Response misrouting via bare-LF request smuggling

Published Oct 12, 2021 · Updated May 27, 2025

HTTP request smuggling in Puma before 4.3.9 and 5.0.0 through 5.5.0 allows remote authenticated users to misroute responses via LF headers. The HTTP/1.1 parser accepts bare LF as a header line ending, so Puma splits a byte sequence that the proxy treats as one request into two requests. Exploitation requires an LF-forwarding proxy, persistent upstream connections, pipelining, and connection reuse; a response can reach another client.

CVSS severity3.7
Low
EPSS probability1.20%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in Puma before 4.3.9 and 5.0.0 through 5.5.0 allows remote authenticated users to misroute responses via LF headers. The HTTP/1.1 parser accepts bare LF as a header line ending, so Puma splits a byte sequence that the proxy treats as one request into two requests. Exploitation requires an LF-forwarding proxy, persistent upstream connections, pipelining, and connection reuse; a response can reach another client.

The record

CVE
CVE-2021-41136
Published
Oct 12, 2021
Updated
May 27, 2025
Vendor
Puma
Product
Puma
Classifications
CWE-444, T1190
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 12, 2021CVE publishedPublication date reported by the CVE source.
  2. May 27, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 4.3.9
  2. Affected versionversion=>= 5.0.0, < 5.5.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Puma · Pumaversion=< 4.3.9; version=>= 5.0.0, < 5.5.1

Published CVSS scores

3.7GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:N
3.6NIST NVDAV:N/AC:H/Au:S/C:P/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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