CVE-2026-55767 is a medium-severity security vulnerability in guzzlehttp/guzzle (composer), affecting versions < 7.12.1. It is fixed in 7.12.1.
Impact CookieJar incorrectly accepts cookies with a dot-only Domain attribute, such as Domain=., Domain=.., Domain=..., and whitespace-padded variants such as Domain= . . In affected versions, SetCookie::matchesDomain() removes leading dots from the cookie domain, normalizing dot-only values to the empty string; SetCookie::validate() only rejected a strictly empty domain, so these cookies could be stored and the empty normalized domain was treated as matching any request host. An attacker-controlled origin that an application requests with a shared cookie jar can therefore set a cookie that Guzzle later sends to unrelated hosts using the same jar. This may allow cookie injection or session fixation against downstream services, depending on how those services interpret the injected cookie. Applications are affected when they use Guzzle's cookie support, for example new Client(['cookies' => true]) or an explicit shared CookieJar, and reuse the same jar across attacker-controlled and trusted origins. Applications that do not use Guzzle's cookie support, or that use separate cookie jars per origin or trust boundary, are not affected. This issue is distinct from public suffix list validation: dot-only domains contain no domain label and should not match unrelated hosts. Patches The issue is patched in 7.12.1 and later. Starting in that release, Guzzle rejects dot-only cookie Domain attributes and prevents an empty normalized cookie domain from matching any request host. Workarounds If you cannot upgrade immediately, do not reuse the same CookieJar instance across untrusted and trusted origins. Use separate cookie jars per origin or trust boundary, or disable cookie handling for requests to untrusted hosts. Avoid using new Client(['cookies' => true]) for clients that may contact unrelated hosts with different trust levels, because that option creates one shared jar for the client.
CVE-2026-55767 has a CVSS score of 5.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment.
A fixed version is available (7.12.1). Upgrading removes the vulnerable code path.
composer
guzzlehttp/guzzle (< 7.12.1)guzzlehttp/guzzle → 7.12.1 (composer)Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter instead of chasing every advisory.
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Upgrade guzzlehttp/guzzle to 7.12.1 or later to resolve this vulnerability.
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
CVE-2026-55767 is a medium-severity security vulnerability in guzzlehttp/guzzle (composer), affecting versions < 7.12.1. It is fixed in 7.12.1.
CVE-2026-55767 has a CVSS score of 5.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
guzzlehttp/guzzle (composer) versions < 7.12.1 is affected.
Yes. CVE-2026-55767 is fixed in 7.12.1. Upgrade to this version or later.
Whether CVE-2026-55767 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
Upgrade guzzlehttp/guzzle to 7.12.1 or later.