CVE-2026-55767 Guzzlephp CVE debrief

Who should care

Developers and administrators using Guzzle, especially in applications that handle sensitive data or user sessions, should be aware of this vulnerability. This vulnerability may allow attackers to inject cookies or fixate sessions, potentially leading to unauthorized access or data manipulation. Users of Guzzle should ensure they are running version 7.12.1 or later to mitigate this risk.

Technical summary

The vulnerability in Guzzle's CookieJar component stems from improper handling of cookies with dot-only Domain attributes and whitespace-padded variants. The SetCookie::matchesDomain() method removes leading dots from the cookie domain, normalizing dot-only values to an empty string. Meanwhile, SetCookie::validate() only rejects strictly empty domains. This discrepancy allows cookies with dot-only domains to be stored and treated as matching any request host. An attacker can exploit this by setting a cookie on an attacker-controlled origin, which Guzzle will then send to unrelated hosts using the same cookie jar. This could enable cookie injection or session fixation attacks against downstream services.

Defensive priority

Medium priority should be given to updating Guzzle to version 7.12.1 or later. In the meantime, defenders should monitor for suspicious cookie activity and consider implementing additional security measures to protect against potential cookie injection or session fixation attacks.

Recommended defensive actions

• Update Guzzle to version 7.12.1 or later
• Monitor for suspicious cookie activity
• Implement additional security measures to protect against cookie injection or session fixation attacks
• Review and update downstream services to handle injected cookies securely
• Consider using a Web Application Firewall (WAF) to detect and prevent suspicious cookie activity

Evidence notes

The CVE-2026-55767 vulnerability was publicly disclosed on June 23, 2026, and the NVD record was last modified on June 26, 2026. The vulnerability has a CVSS score of 5.8 and is classified as MEDIUM severity. The fix for this vulnerability is included in Guzzle version 7.12.1.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.