CVE-2026-48913 Overview
CVE-2026-48913 is a Use After Free vulnerability [CWE-416] in the mod_http2 module of Apache HTTP Server. The flaw triggers when file handles are already exhausted on the server. Affected versions span from 2.4.55 through 2.4.67. An attacker can reach the vulnerable code path over the network without authentication or user interaction. Successful exploitation can affect confidentiality, integrity, and availability of the HTTP server process.
Critical Impact
A remote, unauthenticated attacker can exploit an exhausted file handle condition in mod_http2 to trigger use-after-free behavior in Apache HTTP Server, potentially impacting service stability and data integrity.
Affected Products
- Apache HTTP Server 2.4.55
- Apache HTTP Server versions 2.4.56 through 2.4.66
- Apache HTTP Server 2.4.67
Discovery Timeline
- 2026-06-08 - CVE-2026-48913 published to the National Vulnerability Database (NVD)
- 2026-06-10 - Last updated in NVD database
Technical Details for CVE-2026-48913
Vulnerability Analysis
The vulnerability is a use-after-free condition in Apache HTTP Server's mod_http2 module. The module implements the HTTP/2 protocol and manages request streams, response buffers, and associated file handles. When the server process exhausts its available file handles, error-handling paths in mod_http2 reference memory that has already been released. The freed memory can later be reallocated or overwritten before the dangling pointer is dereferenced. This produces undefined behavior in the worker process handling HTTP/2 traffic. The issue affects the 2.4.55 through 2.4.67 release range, where the relevant mod_http2 code paths share the same lifecycle handling logic.
Root Cause
The root cause is improper memory lifecycle management in mod_http2 when an allocation or file-handle acquisition fails due to resource exhaustion. The cleanup logic releases an object while another code path retains a pointer to it. When file descriptor limits are reached, the failure branch executes without invalidating the retained reference, leaving a stale pointer that is later dereferenced.
Attack Vector
An attacker reaches the vulnerable condition over the network by sending HTTP/2 requests that drive the server toward file handle exhaustion. Once descriptors are exhausted, subsequent requests exercise the error path containing the dangling reference. No authentication or user interaction is required. The attack surface is any Apache HTTP Server instance with mod_http2 enabled and reachable on the network.
No verified proof-of-concept code is publicly available. Refer to the Apache HTTP Server Security Advisory and the Openwall OSS Security Mailing List for technical details.
Detection Methods for CVE-2026-48913
Indicators of Compromise
- Apache error_log entries reporting file descriptor exhaustion, EMFILE, or accept() failed messages on instances running mod_http2.
- Unexpected Apache child process crashes, segmentation faults, or core dumps tied to HTTP/2 request handling.
- Sudden increases in HTTP/2 connection counts or stream concurrency from a small number of source addresses preceding worker process instability.
Detection Strategies
- Inventory all Apache HTTP Server installations and identify versions in the 2.4.55 through 2.4.67 range with mod_http2 loaded.
- Enable verbose logging on mod_http2 (LogLevel http2:info) and correlate HTTP/2 stream errors with worker restarts.
- Monitor process-level file descriptor usage on Apache workers using lsof or /proc/<pid>/fd and alert on counts approaching the configured ulimit -n.
Monitoring Recommendations
- Track Apache worker crash rates and segmentation fault counts in centralized logging pipelines.
- Alert on bursts of HTTP/2 RST_STREAM frames, abnormal stream multiplexing rates, or connections exceeding H2MaxSessionStreams thresholds.
- Correlate file descriptor exhaustion events with HTTP/2 client identifiers, source IP addresses, and user-agent strings to identify abusive sources.
How to Mitigate CVE-2026-48913
Immediate Actions Required
- Upgrade Apache HTTP Server to a fixed release published after version 2.4.67, as listed on the Apache HTTP Server Vulnerabilities page.
- If patching is not immediately possible, disable mod_http2 by commenting out the LoadModule http2_module directive and removing Protocols h2 h2c from the configuration.
- Raise the process file descriptor limit (ulimit -n) to reduce the likelihood of reaching the exhaustion condition that triggers the use-after-free.
Patch Information
The Apache HTTP Server project addresses this vulnerability in releases following 2.4.67. Consult the Apache HTTP Server 2.4 vulnerabilities advisory for the exact fixed version and changelog entry. Apply the patched binary or rebuild from source, then restart the httpd service to load the corrected mod_http2 module.
Workarounds
- Disable HTTP/2 by removing h2 and h2c from the Protocols directive while keeping HTTP/1.1 enabled for client compatibility.
- Place a reverse proxy or load balancer in front of Apache that terminates HTTP/2 and forwards HTTP/1.1 to the backend.
- Tune H2MaxSessionStreams, H2MaxWorkers, and connection limits to reduce concurrent stream pressure and lower the chance of file handle exhaustion.
# Configuration example: disable mod_http2 as a temporary workaround
# In httpd.conf or the relevant Apache configuration file:
# Comment out the module load directive
#LoadModule http2_module modules/mod_http2.so
# Restrict protocols to HTTP/1.1 only
Protocols http/1.1
# Validate configuration and reload
apachectl configtest
systemctl reload httpd
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
