Apache Tomcat has disclosed CVE-2026-24733, a low-severity security constraint bypass involving HTTP/0.9 request handling. The advisory, published on 2026-02-17 by the Apache Tomcat security team, describes a flaw rooted in Tomcat’s acceptance of HTTP/0.9 requests without restricting them to the GET method. HTTP/0.9 is an obsolete and minimal protocol version that predates modern HTTP method and header semantics and is rarely used intentionally in contemporary deployments. The vulnerability arises when specific access-control rules are configured in a particular way. If a security constraint is set to allow HEAD requests to a URI while denying GET requests to the same resource, Tomcat’s handling of HTTP/0.9 can create an enforcement gap. Under normal HTTP versions, such a configuration would prevent retrieval of the response body via GET. However, by sending a specification-invalid HEAD request over HTTP/0.9, an attacker may bypass the intended restriction and access the resource in a manner inconsistent with the configured policy. This issue is situational by design. Exploitation requires both a precise security-constraint configuration—HEAD allowed and GET denied—and a network path that accepts and forwards HTTP/0.9 traffic without normalization. While uncommon in modern architectures, such conditions may still exist in legacy integrations or certain proxy and load balancer topologies where protocol handling is not strictly enforced. Affected versions include supported Tomcat branches as well as older end-of-life releases. Apache recommends upgrading to fixed versions as the primary mitigation. Organizations should also reassess access-control rules governing HEAD versus GET methods and ensure that reverse proxies and load balancers block or normalize deprecated protocol variants to prevent unexpected downgrade scenarios.
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