CVE-2026-46729 Overview
CVE-2026-46729 is a NULL pointer dereference vulnerability in the mod_heartmonitor module of Apache HTTP Server when configured with a unicast listener. The flaw affects Apache HTTP Server versions 2.4.0 through 2.4.68. A remote attacker can trigger the dereference over the network without authentication, causing the server process to crash and producing a denial-of-service condition. The weakness is classified under CWE-476.
Critical Impact
Unauthenticated remote attackers can crash Apache HTTP Server instances that load mod_heartmonitor with a unicast listener, disrupting availability for every virtual host served by the process.
Affected Products
- Apache HTTP Server 2.4.0 through 2.4.68
- Deployments loading mod_heartmonitor with a unicast listener
- Downstream distributions bundling vulnerable httpd 2.4.x builds
Discovery Timeline
- 2026-10-01 - CVE-2026-46729 published to NVD
- 2026-10-08 - Last updated in NVD database
Technical Details for CVE-2026-46729
Vulnerability Analysis
The defect lives in mod_heartmonitor, the Apache HTTP Server module used by cluster-aware load balancers to collect heartbeat data from backend workers. When the module is configured to listen for heartbeats over unicast transport, specific inbound packets cause the handler to operate on a pointer that was never initialized or was returned as NULL by an upstream call. Dereferencing that pointer crashes the worker process. Because the input path is reachable over the network and requires no authentication, a single crafted packet is sufficient to disrupt service.
The impact is limited to availability. The vulnerability does not leak memory, modify data, or enable code execution, which matches the CVSS vector showing no confidentiality or integrity impact and high availability impact.
Root Cause
The underlying weakness is a missing validation check on a pointer before it is dereferenced inside the unicast listener code path of mod_heartmonitor. The module assumes the structure returned when parsing or processing an incoming heartbeat frame is always valid. Malformed or unexpected input causes that assumption to fail, resulting in the CWE-476 condition.
Attack Vector
Exploitation requires network reachability to the port where mod_heartmonitor accepts unicast heartbeat traffic. An attacker sends a crafted packet that drives the module through the unvalidated code path. The server process terminates, and clients served by that process receive connection failures until the worker is restarted. No user interaction, credentials, or prior foothold are required.
No public proof-of-concept exploit has been published at the time of writing. Technical discussion is available on the Openwall OSS Security Mailing List and the Apache HTTP Server Vulnerabilities page.
Detection Methods for CVE-2026-46729
Indicators of Compromise
- Unexpected segmentation faults or SIGSEGV entries for httpd worker processes in system logs
- Repeated worker restarts recorded in error_log without a corresponding configuration change
- Inbound traffic to the mod_heartmonitor unicast listener port from untrusted networks
Detection Strategies
- Inventory running Apache HTTP Server builds and flag any version in the 2.4.0 through 2.4.68 range that loads mod_heartmonitor
- Alert on httpd process crash patterns using host telemetry such as coredumpctl, systemd-coredump, or EDR process-exit events
- Correlate network sensor data for anomalous unicast heartbeat packets reaching reverse proxy or load balancer hosts
Monitoring Recommendations
- Forward httpderror_log and operating system crash logs to a centralized analytics platform for longitudinal review
- Track worker respawn rate as a service-health metric and alert on sudden increases
- Monitor firewall and flow logs for connections to the heartbeat listener originating outside the expected backend network
How to Mitigate CVE-2026-46729
Immediate Actions Required
- Identify every host running Apache HTTP Server 2.4.0 through 2.4.68 with mod_heartmonitor enabled
- Restrict network access to the unicast heartbeat listener so it is reachable only from trusted cluster peers
- Upgrade affected servers to the fixed Apache HTTP Server release listed on the vendor advisory as soon as it is available in your distribution channel
Patch Information
The Apache HTTP Server project tracks fixed versions on the Apache HTTP Server Vulnerabilities page. Administrators should upgrade to the first httpd 2.4.x release that lists CVE-2026-46729 as resolved. Operators using distribution packages should apply the vendor backport once published rather than running mixed builds.
Workarounds
- Disable mod_heartmonitor on servers that do not require cluster heartbeat collection by removing the LoadModule heartmonitor_module directive
- Bind the unicast listener to an internal management interface and block external access with host-based firewall rules
- Place the affected server behind a network access control list that only permits heartbeat traffic from known backend workers
# Example: restrict unicast heartbeat traffic to a trusted backend subnet using iptables
iptables -A INPUT -p udp --dport 27999 -s 10.0.10.0/24 -j ACCEPT
iptables -A INPUT -p udp --dport 27999 -j DROP
# Example: disable mod_heartmonitor if cluster heartbeat collection is not required
# In httpd.conf, comment out the module load directive:
# LoadModule heartmonitor_module modules/mod_heartmonitor.so
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.