CVE-2026-55766
guzzlehttp/psr7: CRLF Injection in HTTP Start-Line Serialization
Description
guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP. Prior to 2.12.1, guzzlehttp/psr7 did not reject CR/LF characters in certain first-party HTTP start-line fields: the request method, protocol version, and response reason phrase. If an application placed attacker-controlled data into one of those fields and later serialized the PSR-7 message as raw HTTP/1.x, for example with Message::toString() or an equivalent serializer, the serialized message could contain attacker-controlled header lines. The issue can also be reached through Message::parseRequest() or Message::parseResponse() when malformed raw messages are parsed into first-party PSR-7 objects and then serialized again. Creating or modifying a Request, Response, or other PSR-7 object alone is not sufficient. The issue requires the malformed message to be serialized and written to the network, forwarded, replayed, or otherwise processed by software that does not independently reject the malformed start line. This vulnerability is fixed in 2.12.1.
INFO
Published Date :
June 23, 2026, 4:17 p.m.
Last Modified :
Oct. 25, 2026, 11:10 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | [email protected] |
Solution
- Update guzzlehttp/psr7 to version 2.12.1 or later.
- Ensure applications do not place attacker-controlled data in start-line fields.
- Validate raw HTTP messages before parsing and serialization.
Public PoC/Exploit Available at Github
CVE-2026-55766 has a 3 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-55766.
| URL | Resource |
|---|---|
| https://github.com/guzzle/psr7/security/advisories/GHSA-vm85-hxw5-5432 | Mitigation Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-55766 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-55766
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Demo project with vulnerabilities to test tools
PHP
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Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-55766 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-55766 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Oct. 25, 2026
Action Type Old Value New Value Added Translation Title: psr7 de guzzle, Description: guzzlehttp/psr7 es una implementación de biblioteca de mensajes HTTP PSR-7 en PHP. Antes de la versión 2.12.1, guzzlehttp/psr7 no rechazaba los caracteres CR/LF en ciertos campos de línea de inicio HTTP de primera parte: el método de solicitud, la versión del protocolo y la frase de motivo de respuesta. Si una aplicación colocaba datos controlados por el atacante en uno de esos campos y luego serializaba el mensaje PSR-7 como HTTP/1.x sin procesar, por ejemplo, con Message::toString() o un serializador equivalente, el mensaje serializado podría contener líneas de encabezado controladas por el atacante. El problema también puede alcanzarse a través de Message::parseRequest() o Message::parseResponse() cuando los mensajes sin procesar malformados se analizan en objetos PSR-7 de primera parte y luego se serializan de nuevo. Crear o modificar un objeto Request, Response u otro objeto PSR-7 por sí solo no es suficiente. El problema requiere que el mensaje malformado sea serializado y escrito a la red, reenviado, reproducido o procesado de otra manera por software que no rechace de forma independiente la línea de inicio malformada. Esta vulnerabilidad está corregida en la versión 2.12.1. -
Initial Analysis by [email protected]
Jun. 30, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:guzzlephp:psr-7:*:*:*:*:*:*:*:* versions up to (excluding) 2.12.1 Added Reference Type GitHub, Inc.: https://github.com/guzzle/psr7/security/advisories/GHSA-vm85-hxw5-5432 Types: Mitigation, Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jun. 23, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-55766', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-06-23T15:49:11.244663Z'} -
New CVE Received by [email protected]
Jun. 23, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'guzzle', 'product': 'psr7', 'versions': [{'status': 'affected', 'version': '< 2.12.1'}]}] Added Description guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP. Prior to 2.12.1, guzzlehttp/psr7 did not reject CR/LF characters in certain first-party HTTP start-line fields: the request method, protocol version, and response reason phrase. If an application placed attacker-controlled data into one of those fields and later serialized the PSR-7 message as raw HTTP/1.x, for example with Message::toString() or an equivalent serializer, the serialized message could contain attacker-controlled header lines. The issue can also be reached through Message::parseRequest() or Message::parseResponse() when malformed raw messages are parsed into first-party PSR-7 objects and then serialized again. Creating or modifying a Request, Response, or other PSR-7 object alone is not sufficient. The issue requires the malformed message to be serialized and written to the network, forwarded, replayed, or otherwise processed by software that does not independently reject the malformed start line. This vulnerability is fixed in 2.12.1. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N Added CWE CWE-93 Added CWE CWE-113 Added Reference https://github.com/guzzle/psr7/security/advisories/GHSA-vm85-hxw5-5432