CVE-2026-55766
MediumCVSS 4.8Exploitation Probability (EPSS)
Low risk5th percentile - higher than 5% of all known CVEs
Summary
The guzzlehttp/psr7 library before version 2.12.1 did not reject CR/LF characters in certain HTTP start-line fields (request method, protocol version, response reason phrase). An attacker can inject controlled data into these fields, and after serializing the PSR-7 message to raw HTTP/1.x (e.g., via Message::toString()), it may lead to HTTP header injection.
Risk Assessment
The risk involves potential manipulation of HTTP headers in serialized messages, which can lead to HTTP response splitting, cache poisoning, or other integrity violations in network communication.
Recommendation
Immediately update the guzzlehttp/psr7 library to version 2.12.1 or later. After updating, verify that your application does not serialize PSR-7 messages with user-controlled data.
Other vulnerabilities in guzzlehttp/psr7
See all- CVE-2026-59882Medium
In the guzzlehttp/psr7 library prior to version 2.12.3, the Uri::assertValidHost() function does not reject invalid URI host components that may contain authority delimiters, embedded ports, or malformed IPv6 brackets. This causes Uri::getHost() to potentially return a value different from the actual URI authority, which can lead to errors in security or routing decisions.
- CVE-2026-49214Medium
The guzzlehttp/psr7 library in versions prior to 2.10.2 did not reject ASCII control characters, whitespace, or DEL in URI host components. This can lead to the injection of additional headers by an attacker, posing a risk to applications using user-controlled URLs.
- CVE-2026-48998Medium
The guzzlehttp/psr7 library in versions prior to 2.10.2 contains improper Host header validation, which can lead to incorrect URI processing in HTTP requests. An attacker can provide a malicious Host header, resulting in a discrepancy between the original value and the URI host value.
Original NVD description (English source)
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.