CVE-2020-9494: Apache Traffic Server DoS Vulnerability

CVE-2020-9494 Overview

Apache Traffic Server versions 6.0.0 to 6.2.3, 7.0.0 to 7.1.10, and 8.0.0 to 8.0.7 contain a resource exhaustion vulnerability in the HTTP/2 protocol handler. Specially crafted HTTP/2 HEADERS frames can cause the server to allocate excessive memory and consume CPU cycles by spinning the processing thread, leading to denial of service conditions.

Critical Impact

This vulnerability allows remote attackers to cause denial of service by sending malicious HTTP/2 HEADERS frames, potentially exhausting server memory and CPU resources without requiring authentication.

Affected Products

• Apache Traffic Server 6.0.0 to 6.2.3
• Apache Traffic Server 7.0.0 to 7.1.10
• Apache Traffic Server 8.0.0 to 8.0.7
• Debian Linux 10.0

Discovery Timeline

• 2020-06-24 - CVE-2020-9494 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-9494

Vulnerability Analysis

This vulnerability is classified as CWE-770 (Allocation of Resources Without Limits or Throttling). The flaw exists in Apache Traffic Server's HTTP/2 implementation, specifically in how it processes HEADERS frames. When the server receives certain types of malformed or specially crafted HTTP/2 HEADERS frames, it fails to properly limit resource allocation during frame processing.

The attack can be executed remotely over the network without any authentication or user interaction. The vulnerability affects only the availability of the system, with no impact on confidentiality or integrity. An attacker can exploit this flaw to cause significant memory consumption and CPU thread spinning, effectively rendering the proxy server unresponsive to legitimate traffic.

Root Cause

The root cause stems from improper resource allocation controls in the HTTP/2 HEADERS frame processing logic. Apache Traffic Server does not adequately validate or limit the resources allocated when parsing certain HTTP/2 HEADERS frame structures, allowing attackers to trigger excessive memory allocation and prolonged thread execution cycles.

Attack Vector

The attack is network-based and requires no privileges or user interaction. An attacker sends specially crafted HTTP/2 HEADERS frames to a vulnerable Apache Traffic Server instance. Upon processing these malformed frames, the server allocates large amounts of memory and enters a processing loop that spins the handling thread, consuming CPU resources.

The vulnerability can be exploited by any remote attacker who can establish an HTTP/2 connection to the target server. The attack leverages the inherent complexity of HTTP/2 frame processing, where insufficient bounds checking allows for resource exhaustion attacks. See the Apache Traffic Server Mailing List Post for technical details on the vulnerability mechanism.

Detection Methods for CVE-2020-9494

Indicators of Compromise

• Sudden and unexplained memory consumption spikes on Apache Traffic Server processes
• HTTP/2 connection attempts with abnormal or malformed HEADERS frame structures in network traffic logs
• Server threads showing extended execution times or appearing to hang during HTTP/2 request processing
• Service degradation or unresponsiveness correlated with HTTP/2 traffic patterns

Detection Strategies

• Monitor Apache Traffic Server process memory usage and set alerts for abnormal allocation patterns
• Implement network-level HTTP/2 traffic analysis to detect malformed HEADERS frames
• Configure logging to capture detailed HTTP/2 frame information for forensic analysis
• Deploy intrusion detection rules targeting known HTTP/2 denial of service attack patterns

Monitoring Recommendations

• Establish baseline metrics for Traffic Server memory and CPU utilization under normal operations
• Implement real-time alerting when resource consumption exceeds established thresholds
• Monitor HTTP/2 connection statistics for anomalies in HEADERS frame processing
• Review access logs for patterns of repeated connection attempts from single sources

How to Mitigate CVE-2020-9494

Immediate Actions Required

• Upgrade Apache Traffic Server to a patched version (6.2.4+, 7.1.11+, or 8.0.8+)
• If immediate patching is not possible, consider temporarily disabling HTTP/2 support
• Implement network-level rate limiting for incoming HTTP/2 connections
• Monitor server resources closely for signs of exploitation attempts

Patch Information

Apache has released security updates to address this vulnerability. Organizations should upgrade to the latest available version of Apache Traffic Server. For Debian systems, the fix is available through Debian Security Advisory DSA-4710. Administrators should consult the Apache Traffic Server Mailing List Post for detailed upgrade guidance.

Workarounds

• Disable HTTP/2 protocol support in Apache Traffic Server configuration until patching is possible
• Implement upstream network filtering to rate-limit or inspect HTTP/2 traffic
• Deploy a web application firewall (WAF) with HTTP/2 inspection capabilities in front of Traffic Server
• Configure resource limits (memory, CPU) for Traffic Server processes to contain potential exploitation impact

# Example: Disable HTTP/2 in Apache Traffic Server configuration
# Edit records.config to disable HTTP/2 support as a temporary mitigation
CONFIG proxy.config.http2.enabled INT 0

# Apply configuration changes
traffic_ctl config reload