Bug 2491769 (CVE-2026-55766)

Summary: CVE-2026-55766 guzzlehttp/psr7: guzzlehttp/psr7: Information disclosure due to improper handling of CR/LF characters in HTTP start-line fields
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in guzzlehttp/psr7, a PHP library for HTTP messages. This vulnerability allows a remote attacker to inject arbitrary header lines into serialized HTTP/1.x messages. The flaw stems from the library's failure to reject carriage return (CR) and line feed (LF) characters in HTTP start-line fields. This could lead to information disclosure or manipulation of HTTP headers if an application processes attacker-controlled data in these fields and then serializes the message.
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Bug Depends On: 2491784, 2491781, 2491782, 2491783, 2491785    
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Description OSIDB Bzimport 2026-06-23 16:01:29 UTC 
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.