CVE-2026-49975 Overview
CVE-2026-49975 is a memory allocation vulnerability in the mod_http module of Apache HTTP Server. The flaw allows a remote, unauthenticated attacker to trigger a denial of service by sending malicious HTTP requests that cause the server to allocate memory using an excessive size value. The issue affects Apache HTTP Server versions 2.4.17 through 2.4.67. The vulnerability is tracked under CWE-789: Memory Allocation with Excessive Size Value.
Critical Impact
Remote attackers can exhaust server memory resources without authentication, leading to denial of service for legitimate users accessing Apache HTTP Server instances.
Affected Products
- Apache HTTP Server versions 2.4.17 through 2.4.67
- F5 NGINX (affected per NVD CPE data)
- Debian Linux 11.0
Discovery Timeline
- 2026-06-08 - CVE-2026-49975 published to the National Vulnerability Database
- 2026-06-10 - Last updated in NVD database
Technical Details for CVE-2026-49975
Vulnerability Analysis
The vulnerability resides in the mod_http module that handles HTTP request processing in Apache HTTP Server. When the module parses certain malicious HTTP requests, it computes a memory allocation size from attacker-controlled fields without enforcing an upper bound. The server then attempts to allocate a buffer matching that excessive value.
Because the request requires no authentication and exploits a low-complexity network-facing code path, any internet-reachable instance running an affected version is exposed. Repeated requests amplify memory pressure on the worker processes, eventually causing the server to stop responding to legitimate traffic.
Root Cause
The root cause is insufficient validation of size values used in memory allocation calls inside mod_http. The module trusts request-derived length fields when sizing internal buffers, allowing an attacker to influence allocation requests that exceed reasonable bounds. This matches the pattern described by CWE-789, where a product allocates memory based on an untrusted size value.
Attack Vector
An attacker sends crafted HTTP requests to a vulnerable Apache HTTP Server endpoint. Each request triggers mod_http to allocate large memory blocks based on values embedded in the request. Sustained delivery of such requests exhausts available memory, causing worker process termination, swap thrashing, or operating system out-of-memory conditions. The attack requires no credentials, no user interaction, and only network reachability to the affected service.
No public proof-of-concept exploit has been published. Technical details are tracked in the Apache HTTP Server Vulnerabilities advisory and the Openwall OSS-Security discussion.
Detection Methods for CVE-2026-49975
Indicators of Compromise
- Sudden spikes in resident memory consumption by httpd or apache2 worker processes without a corresponding increase in legitimate request volume.
- Repeated worker process restarts, segmentation faults, or out-of-memory kills logged in /var/log/apache2/error.log or /var/log/httpd/error_log.
- Inbound HTTP requests from a small set of source addresses containing oversized headers, content-length values, or malformed framing.
Detection Strategies
- Monitor Apache error logs for AH00052 worker exit codes and kernel oom-killer messages targeting httpd processes.
- Inspect HTTP request telemetry for anomalous header sizes, abnormal Content-Length values, or repeated requests sharing identical malformed patterns.
- Correlate sudden drops in HTTP 200 response rates with rising memory utilization to identify in-progress resource exhaustion.
Monitoring Recommendations
- Enable verbose logging in mod_http and forward logs to a centralized analytics platform for anomaly detection.
- Track per-source-IP request rates and request-size distributions at the load balancer or web application firewall layer.
- Alert on Apache worker memory growth exceeding baseline thresholds within short observation windows.
How to Mitigate CVE-2026-49975
Immediate Actions Required
- Identify all Apache HTTP Server instances running versions 2.4.17 through 2.4.67 and prioritize internet-exposed hosts.
- Apply the vendor-supplied security update referenced in the Apache HTTP Server Vulnerabilities advisory.
- Restart Apache services after patching to ensure the vulnerable mod_http code is no longer loaded.
Patch Information
Apache has published fix information in the Apache HTTP Server 2.4 Vulnerabilities page. Debian users should follow the Debian LTS announcement for distribution-specific package updates. Upgrade to a fixed Apache HTTP Server release beyond 2.4.67 as published by the Apache Software Foundation.
Workarounds
- Deploy a web application firewall rule that rejects HTTP requests with abnormally large headers or content-length values until patching completes.
- Set conservative LimitRequestBody, LimitRequestFields, LimitRequestFieldSize, and LimitRequestLine directives in the Apache configuration to constrain attacker-controlled allocation inputs.
- Place affected servers behind a reverse proxy that normalizes and size-limits HTTP requests before they reach mod_http.
# Example hardening directives in httpd.conf or apache2.conf
LimitRequestBody 10485760
LimitRequestFields 100
LimitRequestFieldSize 8190
LimitRequestLine 8190
# Reload configuration after changes
sudo apachectl configtest && sudo systemctl reload apache2
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
