Apache HTTP Server CVE-2026-33523

Exploitation requires Apache HTTP Server to be configured as a reverse proxy or load balancer (proxy modules active) forwarding responses from backend servers to clients. The backend server must be controlled by or compromised by an attacker, allowing the attacker to craft HTTP responses with injected CRLF sequences in header fields. This is NOT a vulnerability in origin-server deployments where Apache terminates client connections directly; it is SPECIFIC to proxy configurations. No user interaction is required (CVSS UI:N). Authentication is not required from the client perspective (PR:N), but the attacker must have write access to the backend server or the ability to inject a malicious backend response, which is a prerequisite condition. The vulnerability is limited to scenarios where the backend is untrusted or can be compromised, making it a targeted risk in environments with exposed or insecure upstream services. Organizations running Apache as a reverse proxy to internal, trusted application servers with network segmentation in place face lower exploitation likelihood than those proxying to third-party or internet-facing backends.

CVSS 6.5 reflects moderate severity with network attack vector (AV:N), low complexity (AC:L), no authentication (PR:N), no user interaction (UI:N), and partial impact to confidentiality and integrity (C:L/I:L). CISA SSVC corroborates risk assessment: Exploitation status is 'none' (not yet actively exploited at time of analysis), Automatable is 'yes' (attack requires no human judgment once payload is crafted), and Technical Impact is 'partial' (affects proxy responses but not availability or complete data compromise). The combination of AV:N/AC:L/PR:N/UI:N suggests easy weaponization, but 'Exploitation: none' status and low EPSS inference indicate this is not currently being exploited in the wild despite technical feasibility. Real-world risk depends heavily on deployment: organizations running Apache as a reverse proxy to untrusted backend systems or in environments where backend compromise is plausible face elevated risk; forward proxy or origin-server configurations with trusted backends face lower practical risk. The partial confidentiality and integrity impact (header injection, cache poisoning, XSS via reflected headers) is significant for session security and client safety, warranting priority patching in proxy-heavy architectures.

An attacker compromises or gains control of a backend application server proxied by a vulnerable Apache HTTP Server instance. The attacker crafts an HTTP response containing injected CRLF sequences in header values (e.g., 'Set-Cookie: session=abc Set-Cookie: admin=true'). When a client requests content, Apache forwards the compromised response unfiltered. The injected headers reach the client's browser, poisoning the session cookie or injecting malicious headers that trigger XSS or CSRF attacks. Alternatively, an attacker on the same network as the backend (internal compromised system) sends a crafted response that gets cached by the proxy, affecting all subsequent clients.