New Apache HTTP Server Vulnerability Allows DoS and Potential RCE
A critical vulnerability has been identified in Apache HTTP Server version 2.4.66 that affects the HTTP/2 module. The flaw, catalogued as CVE-2026-23918, receives a CVSS base score of 8.8, placing it firmly in the high severity range. It impacts any deployment that enables the mod_http2 component and runs a multi‑threaded MPM, which includes the majority of default Linux distributions and container images. Systems using the APR memory allocator with mmap, such as Debian‑based platforms, are additionally exposed to a remote code execution (RCE) risk. The vulnerability does not affect the prefork MPM, but the widespread default configuration makes the potential attack surface large.
The bug originates from a double‑free condition in the stream cleanup routine of the HTTP/2 handler. An attacker sends a crafted HEADERS frame followed immediately by an RST_STREAM frame containing a non‑zero error code on the same stream before the server registers the stream. This sequence triggers two callbacks that each attempt to free the same internal stream structure, placing a dangling pointer into a cleanup array. When the server later iterates this array, the second free call operates on already released memory, causing either a crash (denial of service) or, under specific memory‑allocation conditions, the reuse of the freed address to execute arbitrary code. Exploitation requires only a single TCP connection, no authentication, and can be automated to repeatedly crash worker processes, while the RCE path relies on predictable mmap reuse and the server's scoreboard memory as a stable data store.
The presence of this flaw has immediate strategic implications for any organization that relies on Apache HTTP Server for public‑facing services. Although no public exploits have been observed yet, proof‑of‑concept code demonstrates that an attacker can achieve full system compromise in a laboratory environment within minutes, suggesting a low barrier to weaponization once the necessary information leaks are obtained. The dual impact, service disruption and potential remote code execution, means that both availability and confidentiality can be jeopardized. Organizations should prioritize remediation by upgrading, applying mitigations, and enhancing detection to limit exposure while the vulnerability remains actively investigated.
Attack Surface
Server OS, Web Application, Infrastructure
Tactics
Execution, Impact, Initial Access
Techniques
- T1190 – Exploit Public Facing Application
- T1499 – Endpoint Denial of Service
- T1203 – Exploitation for Client Execution
- T1106 – Native API
SuperPRO's Threat Countermeasures Procedures
- Upgrade Apache HTTP Server to version 2.4.67 or later, which patches CVE-2026-23918.
- If immediate upgrade is not possible, disable
- Switch the Multi‑Processing Module (MPM) from a threaded model (event or worker) to prefork to avoid the vulnerable code path.
- Apply the latest Apache Portable Runtime (APR) package that includes the mmap allocator fix for Debian based systems.
- Deploy an IDS/IPS rule that alerts on HTTP/2 HEADERS frames immediately followed by RST_STREAM frames with non‑zero error codes on the same stream.
- Configure a Web Application Firewall (WAF) to block malformed HTTP/2 frame sequences matching the exploit pattern.
- Monitor Apache error logs for repeated worker process crashes and implement automatic process respawn limits to mitigate denial‑of‑service impact.