CVE-2026-55766Network attack vector

Header injection via unvalidated HTTP start-line fields

Published Jun 23, 2026 · Updated Jun 23, 2026

CRLF injection in Guzzle PSR-7 before 2.12.1 allows remote attackers to inject header lines through serialized HTTP/1.x messages. Request methods, protocol versions, and response reason phrases accept CR or LF, and Message serialization writes those values into raw start lines without rejecting injected line breaks. Exploitation requires attacker-controlled data to enter one of those fields and the malformed message to be serialized and processed by an HTTP/1.x component; standard Guzzle client use is not affected.

CVSS severity4.8
Medium
EPSS probability0.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CRLF injection in Guzzle PSR-7 before 2.12.1 allows remote attackers to inject header lines through serialized HTTP/1.x messages. Request methods, protocol versions, and response reason phrases accept CR or LF, and Message serialization writes those values into raw start lines without rejecting injected line breaks. Exploitation requires attacker-controlled data to enter one of those fields and the malformed message to be serialized and processed by an HTTP/1.x component; standard Guzzle client use is not affected.

The record

CVE
CVE-2026-55766
Published
Jun 23, 2026
Updated
Jun 23, 2026
Vendor
Guzzle
Product
Guzzle PSR-7
Classifications
CWE-93, CWE-113, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 23, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 23, 2026Record 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=< 2.12.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Guzzle · Guzzle PSR-7version=< 2.12.1

Published CVSS scores

4.8GitHubCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/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.

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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-93Improper Neutralization of CRLF Sequences ('CRLF Injection')
CWE-113Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
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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