CVE-2024-38809: ETag Parsing DoS Vulnerability

CVE-2024-38809 Overview

CVE-2024-38809 is a Denial of Service (DoS) vulnerability affecting applications that parse ETags from If-Match or If-None-Match HTTP request headers. This vulnerability allows remote attackers to cause resource exhaustion by sending specially crafted ETag headers, potentially rendering affected applications unresponsive. The vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption).

Critical Impact

Applications parsing ETag headers are susceptible to resource exhaustion attacks that can lead to service unavailability. No authentication is required to exploit this vulnerability.

Affected Products

• Spring Framework applications parsing ETag headers
• Applications using If-Match request header parsing
• Applications using If-None-Match request header parsing

Discovery Timeline

• 2024-09-27 - CVE-2024-38809 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-38809

Vulnerability Analysis

This vulnerability stems from improper handling of ETag header parsing in applications processing HTTP conditional requests. When an application parses If-Match or If-None-Match headers, maliciously crafted headers with excessive length or complexity can trigger resource exhaustion conditions. The attack is network-accessible and requires no authentication or user interaction, making it relatively straightforward for attackers to exploit against vulnerable endpoints.

The vulnerability impacts availability by causing CPU or memory exhaustion during the parsing phase of ETag headers. While the integrity and confidentiality of data remain unaffected, the availability impact can effectively take down services that rely on ETag-based caching or conditional request handling.

Root Cause

The root cause is inadequate input validation and resource limits when processing ETag values from HTTP request headers. The parsing logic does not enforce appropriate size constraints on incoming If-Match and If-None-Match header values, allowing attackers to submit excessively large or complex ETag strings that consume disproportionate processing resources.

Attack Vector

An attacker can exploit this vulnerability remotely over the network by sending HTTP requests containing maliciously crafted If-Match or If-None-Match headers. The attack does not require authentication or any special privileges, and no user interaction is needed.

The exploitation involves sending HTTP requests with oversized or algorithmically complex ETag header values to endpoints that process conditional requests. When the application attempts to parse these headers, it expends excessive CPU cycles or memory, leading to service degradation or complete unavailability.

For technical details on the vulnerability mechanism, refer to the Spring Security Advisory CVE-2024-38809.

Detection Methods for CVE-2024-38809

Indicators of Compromise

• Abnormally large If-Match or If-None-Match HTTP headers in request logs
• Unusual spikes in CPU utilization during HTTP request processing
• Increased memory consumption correlated with ETag header parsing operations
• Multiple requests from single sources with oversized conditional headers

Detection Strategies

• Implement request header size monitoring at the web application firewall (WAF) level
• Configure alerting for requests with If-Match or If-None-Match headers exceeding normal thresholds
• Monitor application performance metrics for anomalous resource consumption patterns
• Deploy intrusion detection rules to flag requests with unusually large ETag headers

Monitoring Recommendations

• Enable verbose logging for HTTP header processing to capture header sizes
• Set up real-time dashboards tracking CPU and memory utilization for web application components
• Configure automated alerts when request processing latency exceeds baseline thresholds
• Review web server access logs for patterns of requests targeting endpoints with ETag parsing

How to Mitigate CVE-2024-38809

Immediate Actions Required

• Upgrade to the fixed version of the affected framework as specified in the security advisory
• Implement a servlet filter or WAF rule to enforce size limits on If-Match and If-None-Match headers
• Review application endpoints that utilize ETag-based conditional request handling
• Consider temporarily disabling ETag parsing on critical endpoints if immediate patching is not possible

Patch Information

Users of affected versions should upgrade to the corresponding fixed version as specified in the Spring Security Advisory CVE-2024-38809. For additional guidance, refer to the NetApp Security Advisory NTAP-20240920-0003.

Workarounds

• Implement a servlet Filter to enforce size limits on If-Match and If-None-Match request headers before they reach application logic
• Configure web application firewall rules to block or reject requests with oversized conditional headers
• Use reverse proxy configurations to validate and limit header sizes at the edge
• For unsupported versions, deploy request filtering middleware to sanitize incoming ETag headers

# Example: Configure header size limits in Apache HTTPD
# Add to httpd.conf or virtual host configuration
LimitRequestFieldSize 8190
RequestHeader unset If-Match early "expr=%{req:If-Match} =~ /^.{8000,}$/"
RequestHeader unset If-None-Match early "expr=%{req:If-None-Match} =~ /^.{8000,}$/"