CVE-2020-9490: Apache HTTP Server DoS Vulnerability

CVE-2020-9490 Overview

CVE-2020-9490 is a denial of service vulnerability affecting Apache HTTP Server versions 2.4.20 through 2.4.43. The vulnerability exists in the HTTP/2 protocol implementation, specifically in how the server processes the Cache-Digest header. When a malicious client sends a specially crafted value for this header in an HTTP/2 request, it causes the server to crash when it subsequently attempts to perform an HTTP/2 PUSH operation for a resource.

Critical Impact
This vulnerability enables remote attackers to cause a complete denial of service by crashing the Apache HTTP Server through specially crafted HTTP/2 requests, potentially taking down web services and applications relying on affected servers.

Affected Products

Apache HTTP Server versions 2.4.20 to 2.4.43
Oracle Communications Element Manager, Communications Session Report Manager, 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, 17.3
Oracle ZFS Storage Appliance Kit 8.8
Red Hat Enterprise Linux 6.0, 7.0, 7.6, 7.7, 8.0 and various EUS releases
Red Hat OpenStack 16.1
Debian Linux 10.0
Canonical Ubuntu Linux 16.04 ESM, 18.04 LTS, 20.04 LTS
openSUSE Leap 15.1, 15.2
Fedora 31, 32

Discovery Timeline

August 7, 2020 - CVE-2020-9490 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-9490

Vulnerability Analysis

This vulnerability resides in the mod_http2 module of Apache HTTP Server, which implements the HTTP/2 protocol. The flaw is classified as CWE-444 (Inconsistent Interpretation of HTTP Requests, also known as HTTP Request Smuggling). The root issue lies in how the server parses and validates the Cache-Digest header values before using them in HTTP/2 PUSH operations.

When HTTP/2 Server Push is enabled, the server can proactively send resources to clients before they explicitly request them. The Cache-Digest header mechanism allows clients to communicate what resources they already have cached. However, the vulnerable code does not properly handle malformed or specially crafted values in this header, leading to memory corruption when the server attempts to process a PUSH operation.

Root Cause

The vulnerability stems from improper input validation in the HTTP/2 implementation within mod_http2. Specifically, when processing the Cache-Digest header, the server fails to adequately sanitize or validate the input data. This leads to memory corruption when malformed data is subsequently used during HTTP/2 PUSH resource operations. The lack of proper bounds checking or data validation allows attackers to trigger a crash condition.

Attack Vector

The attack can be executed remotely over the network without requiring authentication or user interaction. An attacker can craft a malicious HTTP/2 request containing a specially formed Cache-Digest header value. When the target Apache server has HTTP/2 and Server Push enabled, the malicious request triggers a crash when the server attempts to push a resource to the client. This can be repeatedly exploited to maintain a persistent denial of service condition.

The attack flow typically involves:

Attacker establishes an HTTP/2 connection to the vulnerable Apache server
Attacker sends a request with a maliciously crafted Cache-Digest header
Server attempts to perform an HTTP/2 PUSH operation for a resource
Memory corruption occurs during PUSH processing, causing the server process to crash

Detection Methods for CVE-2020-9490

Indicators of Compromise

Unexpected Apache HTTP Server process crashes or restarts in system logs
High frequency of HTTP/2 connection establishment from single or multiple sources followed by server crashes
Error log entries indicating memory corruption or segmentation faults in the httpd process
Unusual Cache-Digest header values in HTTP/2 access logs (if detailed header logging is enabled)

Detection Strategies

Monitor Apache error logs for crash events, segmentation faults, or child process termination patterns
Implement network-based detection rules to identify anomalous or malformed Cache-Digest header values in HTTP/2 traffic
Deploy endpoint detection solutions that can identify unexpected process termination patterns for the httpd service
Utilize web application firewalls (WAF) configured to inspect HTTP/2 headers for malicious patterns

Monitoring Recommendations

Enable detailed logging for HTTP/2 connections and header information in Apache configuration
Set up alerting for abnormal server restart frequencies or crash patterns
Monitor system stability metrics including process uptime and restart counts for Apache services
Implement log aggregation to correlate multiple crash events that may indicate an active exploitation attempt

How to Mitigate CVE-2020-9490

Immediate Actions Required

Upgrade Apache HTTP Server to version 2.4.44 or later, which contains the fix for this vulnerability
If immediate patching is not possible, disable HTTP/2 Server Push by adding H2Push off directive to the Apache configuration
Review and restrict access to the web server from untrusted networks where feasible
Implement rate limiting for incoming connections to reduce the impact of potential DoS attacks

Patch Information

Apache has released version 2.4.44 which addresses this vulnerability. Organizations should upgrade to this version or later as the primary remediation. Detailed information about the fix is available in the Apache HTTP Server Security Vulnerabilities documentation. Multiple Linux distributions have also released patched packages, including:

Workarounds

Configure Apache with H2Push off directive to disable HTTP/2 Server Push functionality and mitigate the vulnerability without patching
Consider temporarily disabling HTTP/2 entirely using Protocols http/1.1 if Server Push cannot be disabled independently
Deploy a reverse proxy or WAF in front of vulnerable Apache servers to filter malicious HTTP/2 requests
Implement network segmentation to limit exposure of vulnerable servers to untrusted networks

bash

# Apache configuration to mitigate CVE-2020-9490
# Add to httpd.conf or appropriate virtual host configuration

# Option 1: Disable HTTP/2 Server Push (recommended workaround)
H2Push off

# Option 2: Disable HTTP/2 entirely (more restrictive)
# Protocols http/1.1

# Verify configuration syntax before restart
apachectl configtest

# Restart Apache to apply changes
systemctl restart httpd
# or
systemctl restart apache2