CVE-2020-11993: Apache HTTP Server DoS Vulnerability

CVE-2020-11993 Overview

CVE-2020-11993 is a race condition vulnerability affecting Apache HTTP Server versions 2.4.20 through 2.4.43. The flaw exists within the mod_http2 module and occurs when trace or debug logging is enabled. Under certain HTTP/2 traffic patterns, logging statements are executed on the wrong connection context, leading to concurrent use of memory pools. This can result in memory corruption and denial of service conditions.

The vulnerability is particularly concerning for organizations that have enabled verbose logging on production servers for troubleshooting purposes. An unauthenticated remote attacker can exploit this issue by sending specially crafted HTTP/2 requests that trigger the edge case traffic patterns, causing the server to crash or become unresponsive.

Critical Impact

Remote attackers can cause denial of service against Apache HTTP servers with HTTP/2 and debug logging enabled, potentially disrupting critical web services.

Affected Products

• Apache HTTP Server versions 2.4.20 to 2.4.43
• NetApp Clustered Data ONTAP
• Canonical Ubuntu Linux 16.04, 18.04, and 20.04 LTS
• openSUSE Leap 15.1 and 15.2
• Debian Linux 10.0
• Fedora 31 and 32
• Oracle Communications Element Manager
• Oracle Communications Session Report Manager
• Oracle Communications Session Route Manager
• Oracle Enterprise Manager Ops Center 12.4.0.0
• Oracle Hyperion Infrastructure Technology 11.1.2.4
• Oracle Instantis EnterpriseTrack 17.1, 17.2, and 17.3
• Oracle ZFS Storage Appliance Kit 8.8

Discovery Timeline

• 2020-08-07 - CVE-2020-11993 published to NVD
• 2025-05-01 - Last updated in NVD database

Technical Details for CVE-2020-11993

Vulnerability Analysis

This vulnerability stems from a race condition in the HTTP/2 module (mod_http2) of Apache HTTP Server. When debug or trace logging is configured for the module, certain traffic edge patterns cause logging operations to be performed on incorrect connection contexts. This results in multiple threads concurrently accessing and modifying the same memory pools without proper synchronization.

The issue is classified as CWE-444 (Inconsistent Interpretation of HTTP Requests), though it fundamentally involves a race condition in memory pool management. The vulnerability can be exploited remotely over the network without requiring authentication, and successful exploitation leads to denial of service through memory corruption or server crashes.

Root Cause

The root cause lies in improper connection context handling within the mod_http2 logging code paths. When HTTP/2 connections experience specific traffic patterns—particularly those involving rapid connection state transitions—the module's logging functions may reference stale or incorrect connection objects. This leads to logging statements being associated with wrong connection contexts, causing memory pool operations intended for one connection to be executed against another connection's memory pool. Since Apache uses per-connection memory pools for efficient resource management, this cross-contamination corrupts pool metadata and triggers undefined behavior.

Attack Vector

The attack leverages the network-accessible HTTP/2 protocol to trigger the vulnerable code path. An attacker can exploit this vulnerability by:

1. Establishing multiple HTTP/2 connections to the target server
2. Sending rapid sequences of requests that create specific traffic edge patterns
3. Causing connection state transitions that trigger the race condition in logging code
4. Inducing memory pool corruption that crashes worker processes or the entire server

The attack does not require authentication and can be performed remotely. However, the vulnerability only manifests when mod_http2 logging is configured above the "info" level (i.e., debug or trace levels). This represents a common configuration for servers undergoing troubleshooting or performance analysis.

Detection Methods for CVE-2020-11993

Indicators of Compromise

• Apache HTTP Server child process crashes or unexpected restarts, particularly under HTTP/2 traffic
• Segmentation faults or memory corruption errors in Apache error logs referencing mod_http2
• Inconsistent or corrupted log entries where logging statements appear associated with wrong connections
• Elevated HTTP/2 connection failures or protocol errors reported by clients

Detection Strategies

• Monitor Apache error logs for segfault messages, particularly those mentioning h2_ prefixed functions or memory pool operations
• Implement process monitoring to detect abnormal httpd worker process termination rates
• Deploy web application firewall rules to detect anomalous HTTP/2 connection patterns characteristic of exploitation attempts
• Use system-level monitoring tools like auditd to track unexpected Apache process crashes

Monitoring Recommendations

• Configure centralized log aggregation to correlate Apache crashes across multiple servers for early detection of exploitation campaigns
• Set up alerts for Apache process restart frequency exceeding baseline thresholds
• Monitor HTTP/2 connection metrics for unusual patterns such as high connection churn or elevated error rates
• Review Apache configuration periodically to identify servers with debug logging enabled in production environments

How to Mitigate CVE-2020-11993

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.44 or later, which contains the security fix for this vulnerability
• As a temporary mitigation, configure the LogLevel of mod_http2 to "info" or lower to prevent triggering the vulnerable code path
• Review all production Apache servers to identify instances with HTTP/2 debug logging enabled and reduce logging verbosity
• Consider temporarily disabling mod_http2 if HTTP/2 support is not critical while waiting for patching

Patch Information

Apache has released version 2.4.44 which addresses CVE-2020-11993 along with other security issues. The fix corrects the connection context handling in the mod_http2 logging code to ensure logging operations are performed against the correct connection's memory pool. Organizations should apply updates through their distribution's package manager or by downloading the patched version from the Apache HTTPD Security Vulnerabilities page.

Multiple Linux distributions have released patches for this vulnerability. Refer to distribution-specific security advisories including Debian Security Advisory DSA-4757, Ubuntu Security Notice USN-4458-1, and the Gentoo GLSA 202008-04 for guidance on applying updates.

Workarounds

• Set LogLevel h2:info or lower in the Apache configuration to prevent the vulnerable debug logging code paths from being executed
• Disable the mod_http2 module entirely by commenting out or removing the LoadModule http2_module directive if HTTP/2 is not required
• Implement network-level rate limiting for HTTP/2 connections to reduce the likelihood of triggering exploitation patterns
• Consider deploying a reverse proxy in front of vulnerable Apache servers that can handle HTTP/2 termination while Apache operates with HTTP/1.1 only

# Mitigate CVE-2020-11993 by reducing mod_http2 log level
# Add to httpd.conf or apache2.conf

# Set HTTP/2 module logging to info level (default is warn)
LogLevel h2:info

# Alternative: Completely disable HTTP/2 module if not needed
# Comment out or remove the following line:
# LoadModule http2_module modules/mod_http2.so

# Restart Apache to apply changes
# systemctl restart httpd   # RHEL/CentOS
# systemctl restart apache2 # Debian/Ubuntu