CVE-2026-23918: Apache HTTP/2 Bug Allows DoS and RCE

A serious security flaw has been discovered in the widely used Apache HTTP Server, raising concerns across the cybersecurity community. The vulnerability, identified as CVE-2026-23918, could allow attackers to crash servers or even execute malicious code remotely under certain conditions. This makes it a high-risk issue for organizations running affected versions of Apache.

The vulnerability has been assigned a CVSS score of 8.8, which classifies it as critical. It specifically impacts Apache HTTP Server version 2.4.66 and has been fixed in version 2.4.67. Users and system administrators are strongly advised to upgrade immediately to avoid potential exploitation.

CVE-2026-23918 is a double-free memory vulnerability found in the HTTP/2 implementation of Apache, particularly in the mod_http2 module. In simple terms, a double-free occurs when a program mistakenly frees the same memory space twice. This can lead to crashes, unpredictable behavior, or even allow attackers to manipulate memory for malicious purposes.

The flaw exists in how Apache handles HTTP/2 streams. When a specially crafted sequence of HTTP/2 frames is sent by an attacker, the server mishandles memory cleanup operations. This results in the same memory being released twice, opening the door to exploitation.

The attack method is surprisingly simple for denial-of-service (DoS):

• An attacker opens a single TCP connection
• Sends a HEADERS frame, immediately followed by a RST_STREAM frame
• This sequence triggers the vulnerability before the server fully registers the stream

Due to improper handling in internal callbacks, the same stream is cleaned up twice. When Apache later tries to free that memory again, it crashes the worker process.

• No authentication required
• No special configuration needed
• Works on default Apache setups with HTTP/2 enabled

This makes the vulnerability extremely easy to exploit for DoS attacks.

The DoS attack is the most immediate and practical threat. Attackers can repeatedly send malicious HTTP/2 requests, causing Apache worker processes to crash continuously.

Although Apache automatically restarts crashed workers, ongoing attacks can:

• Interrupt legitimate user requests
• Reduce server availability
• Degrade performance significantly

In high-traffic environments, this could lead to service outages and business disruption.

More concerning is the potential for Remote Code Execution (RCE). While more complex than DoS, researchers have demonstrated a working proof-of-concept under controlled conditions.

Attackers exploit the freed memory by:

• Replacing it with a fake structure using memory mapping (mmap)
• Redirecting function pointers to execute system-level commands
• Using Apache’s internal memory structures (like the scoreboard) as a stable anchor

The scoreboard memory remains at a predictable location during runtime, which increases the feasibility of exploitation even with security protections like ASLR.

However, successful RCE requires:

• Additional information leaks
• Precise memory manipulation
• Favorable conditions in the server environment

Even so, the possibility of RCE makes this vulnerability especially dangerous.