CVE-2025-49763: Apache Traffic Server DoS Vulnerability

CVE-2025-49763 Overview

CVE-2025-49763 is a Resource Exhaustion vulnerability affecting the Edge Side Includes (ESI) plugin in Apache Traffic Server. The ESI plugin lacks a limit for maximum inclusion depth, which allows attackers to craft malicious ESI instructions that trigger excessive memory consumption. This vulnerability can be exploited remotely without authentication to cause denial of service conditions on affected servers.

Critical Impact

Remote attackers can exhaust server memory resources by inserting deeply nested ESI include directives, potentially causing complete service disruption for applications relying on Apache Traffic Server.

Affected Products

• Apache Traffic Server versions 10.0.0 through 10.0.5
• Apache Traffic Server versions 9.0.0 through 9.2.10

Discovery Timeline

• 2025-06-19 - CVE-2025-49763 published to NVD
• 2025-07-01 - Last updated in NVD database

Technical Details for CVE-2025-49763

Vulnerability Analysis

This vulnerability falls under CWE-400 (Uncontrolled Resource Consumption). The ESI plugin in Apache Traffic Server processes Edge Side Includes, which allow dynamic content assembly at the edge. The plugin recursively processes ESI include directives but fails to enforce a maximum recursion or inclusion depth limit.

When processing nested ESI instructions, each level of inclusion consumes additional memory. Without a configurable limit, an attacker can craft ESI content with deeply nested <esi:include> tags that force the server to allocate memory recursively until system resources are exhausted. This network-accessible attack requires no privileges or user interaction, making it particularly dangerous for internet-facing Apache Traffic Server deployments.

Root Cause

The root cause is the absence of depth validation in the ESI parsing logic. When the ESI plugin encounters an <esi:include> directive, it fetches and processes the included content. If that content also contains ESI directives, the plugin continues recursively without checking how deep the inclusion chain has grown. This design flaw allows unbounded recursion that maps directly to unbounded memory allocation.

Attack Vector

The attack vector is network-based. An attacker can exploit this vulnerability by:

1. Crafting malicious ESI content with deeply nested include directives
2. Hosting the malicious content on a server accessible to the Traffic Server
3. Triggering the Traffic Server to process the malicious ESI content through a legitimate request

The vulnerability can be triggered through any request path that causes the ESI plugin to process attacker-controlled content. Since the attack requires no authentication and can be executed remotely, any publicly accessible Apache Traffic Server with the ESI plugin enabled is at risk.

Detection Methods for CVE-2025-49763

Indicators of Compromise

• Unusual memory growth patterns in Apache Traffic Server processes
• Server processes consuming significantly more memory than typical baseline
• ESI-related log entries showing deep nesting or recursive include patterns
• Service degradation or crashes coinciding with specific request patterns

Detection Strategies

• Monitor Apache Traffic Server memory usage for anomalous spikes that correlate with incoming requests
• Implement application-level logging to track ESI include depth during content processing
• Configure alerts for Traffic Server process memory exceeding defined thresholds
• Review access logs for requests targeting ESI-processed content from suspicious sources

Monitoring Recommendations

• Establish memory usage baselines for Apache Traffic Server instances and alert on significant deviations
• Deploy network-level monitoring to identify unusually large or recursive request patterns
• Enable verbose ESI plugin logging during investigation periods to trace include chains
• Integrate Traffic Server metrics with SIEM solutions for correlation with other security events

How to Mitigate CVE-2025-49763

Immediate Actions Required

• Upgrade Apache Traffic Server to version 9.2.11 or 10.0.6 immediately
• If immediate patching is not possible, apply the --max-inclusion-depth configuration option to limit recursion
• Review ESI plugin usage and disable it on servers where ESI functionality is not required
• Monitor affected servers for signs of exploitation until patches are applied

Patch Information

Apache has released security patches addressing this vulnerability. Users running Apache Traffic Server versions 9.0.0 through 9.2.10 should upgrade to version 9.2.11. Users running versions 10.0.0 through 10.0.5 should upgrade to version 10.0.6. These patched versions implement proper inclusion depth limiting in the ESI plugin.

For detailed patch information and discussion, refer to the Apache Mailing List Discussion.

Workarounds

• Configure the new --max-inclusion-depth setting for the ESI plugin to enforce a reasonable recursion limit
• Disable the ESI plugin entirely if Edge Side Includes functionality is not actively used
• Implement resource limits at the operating system level to cap memory consumption per process
• Deploy a web application firewall (WAF) with rules to detect and block requests containing excessively nested ESI directives

# Configuration example - Set maximum ESI inclusion depth
# Add to Traffic Server plugin configuration
traffic_ctl config set proxy.config.http.esi.max_inclusion_depth 10

# Alternatively, configure via records.config
# CONFIG proxy.config.http.esi.max_inclusion_depth INT 10

# To disable ESI plugin entirely if not needed
# Remove or comment out esi.so from plugin.config