CVE-2020-11076Network attack vector

Response smuggling via misparsed Transfer-Encoding list

Published May 22, 2020 · Updated Aug 4, 2024

HTTP response smuggling in Puma 3.12.4 and earlier and 4.0.0 through 4.3.3 allows remote attackers to poison downstream responses. Puma::Client#setup_body treats Transfer-Encoding as chunked only when the entire value equals chunked, so a comma-separated value containing chunked is handled as an unframed body. Unauthenticated network access is sufficient; conflicting message boundaries can inject a response into another client connection.

CVSS severity7.5
High
EPSS probability4.09%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP response smuggling in Puma 3.12.4 and earlier and 4.0.0 through 4.3.3 allows remote attackers to poison downstream responses. Puma::Client#setup_body treats Transfer-Encoding as chunked only when the entire value equals chunked, so a comma-separated value containing chunked is handled as an unframed body. Unauthenticated network access is sufficient; conflicting message boundaries can inject a response into another client connection.

The record

CVE
CVE-2020-11076
Published
May 22, 2020
Updated
Aug 4, 2024
Vendor
Puma
Product
Puma
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 22, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record 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=< 3.12.5
  2. Affected versionversion=>= 4.0.0, < 4.3.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Puma · Pumaversion=< 3.12.5; version=>= 4.0.0, < 4.3.4

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:P/A:N
7.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/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
Privilegesunauthenticated
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.

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Technologies

Your stack

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Your stack

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