A significant security flaw, identified as CVE-2026-49975 and known as the ‘HTTP/2 Bomb,’ has been exposed in the Apache HTTP Server. This vulnerability enables remote attackers to significantly disrupt server operations by exhausting memory resources without requiring authentication.
Understanding the HTTP/2 Bomb Vulnerability
The vulnerability arises from the way Apache HTTP Server handles HTTP/2 requests. Specifically, the flaw occurs during the merging of multiple cookie header fields, which are not correctly accounted for against the LimitRequestFields directive. This oversight allows attackers to bypass vital resource protections.
By crafting a small, HPACK-encoded HTTP/2 request, attackers can cause the server to expand these requests into numerous cookie header fields. This process forces the server to allocate substantial memory repeatedly, leading to potential service disruptions.
Exploitation Tactics and Impact
Attackers exploit this vulnerability by manipulating HTTP/2 flow control. By setting the initial window size to zero, they can stall data transmission, keeping streams open indefinitely and preventing the release of allocated memory. This tactic creates a persistent state of memory exhaustion.
All versions of Apache HTTP Server from 2.4.17 to 2.4.67 are susceptible to this attack. The vulnerability has been addressed in version 2.4.68 and later. An exploit script demonstrating the attack is available on GitHub, allowing attackers to reproduce the scenario in a controlled Docker environment with an 8 GB memory cap.
Mitigation and Future Outlook
Organizations using vulnerable versions of Apache HTTP Server are urged to upgrade to version 2.4.68 or beyond immediately. For those unable to patch immediately, disabling HTTP/2 can serve as a temporary safeguard. Additionally, monitoring for unusual memory usage patterns can help identify attacks early.
The release of the exploit underscores the importance of timely updates and vigilant monitoring in maintaining server security. As cyber threats evolve, staying informed and proactive is crucial for safeguarding critical infrastructures.
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