Apache Traffic Server Vulnerabilities Allows Attackers to Cause DoS via Memory Exhaustion

A critical vulnerabilities in Apache Traffic Server (ATS), a high-performance caching proxy server widely deployed in content delivery networks.

Security researcher Masakazu Kitajo announced two distinct vulnerabilities, including CVE-2025-49763, which enables denial-of-service (DoS) attacks through uncontrolled memory consumption during Edge Side Includes (ESI) processing.

This vulnerability allows malicious actors to craft specialized requests that trigger exponential memory growth, eventually exhausting server resources and crashing critical infrastructure components.

The discovery coincides with the release of patched versions 10.0.6 and 9.2.11, which address these security gaps amid heightened concerns over network infrastructure resilience.

The ESI processing vulnerability stems from improper bounds checking during dynamic content assembly. When processing nested ESI directives, Traffic Server fails to limit recursion depth or resource allocation for fragmented content assembly operations.

Attackers exploit this by crafting requests containing deeply recursive ESI tags or maliciously fragmented content segments, each forcing ATS to allocate new memory buffers without releasing prior allocations.

This creates unbounded memory growth during request handling, where a single malicious connection can consume gigabytes of RAM within seconds.

The protocol-level nature of this attack bypasses conventional rate-limiting defenses, as the damage occurs within legitimate session boundaries.

Compounding the risk, the vulnerability operates at the core content assembly layer, making detection challenging.

Security analysts note that normal operation patterns—such as complex webpage assembly for e-commerce platforms—can mask malicious payloads until memory utilization spikes occur.

Unlike network-level DDoS attacks, this vulnerability enables application-layer DoS through semantically valid requests, requiring minimal bandwidth from attackers while maximizing server-side damage.

The memory exhaustion persists beyond request completion due to ATS’s failure in garbage collection of orphaned ESI processing contexts.

Apache Traffic Server Vulnerability

The Apache Traffic Server project responded rapidly with coordinated releases of versions 9.2.11 and 10.0.6, both containing fixes for the memory exhaustion vulnerability.

The patches implement three critical safeguards: recursive depth limitation for ESI tag processing, memory allocation quotas per request context, and immediate buffer deallocation upon error conditions.

Administrators should prioritize upgrading immediately, particularly for public-facing caching nodes handling user-generated content. As Chris McFarlen emphasized in the release announcement, “These updates address critical stability issues affecting production environments”.

For organizations unable to immediately upgrade, interim mitigations include disabling ESI processing via proxy.config.esi.enabled 0 in records.config.

However, this workaround breaks functionality for websites relying on dynamic content assembly. Alternative protection involves deploying memory usage monitors with aggressive thresholds that terminate suspicious worker processes.

Network operators should also implement strict ACL rules limiting ESI requests to trusted origins, though this conflicts with the concurrently disclosed ACL bypass vulnerability (CVE-2025-31698). Security teams must balance these tradeoffs while planning upgrades.

Ecosystem Impact

This vulnerability exposes fundamental challenges in secure proxy design, highlighting how performance optimization often conflicts with security isolation.

As Bryan Call noted during the recent ATS Spring Summit, “Modern CDN architectures increasingly require reassessing resource management models”.

The incident has spurred development of new TS APIs for safer configuration overrides, including Masaori Koshiba’s proposal for TSHttpTxnConfigParse/Set functions that enforce validation boundaries during runtime adjustments.

These enhancements aim to prevent misconfigurations that could exacerbate memory management issues.

Long-term security improvements include the proposed “Snowflake ID” system for uniquely identifying connections across distributed infrastructures.

By replacing sequential connection identifiers with collision-resistant UUIDs, administrators gain improved attack tracing capabilities during forensic analysis.

Additionally, Brian Neradt’s proposal for enhanced TLS logging fields (cqssg) improves cryptographic visibility, helping detect malicious sessions attempting to disguise exploitation traffic.

These measures reflect a broader shift toward defense-in-depth within the ATS ecosystem following these critical vulnerabilities.

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