Full Report
A security flaw in Apache Traffic Server (ATS) is targeting cloud service providers worldwide. The vulnerability, identified as CVE-2025-49763, exposes affected systems to denial-of-service (DoS) attacks that exploit a critical ACL issue in the server’s Edge Side Includes (ESI) plugin, enabling attackers to exhaust server memory and disrupt operations. Apache Traffic Server is widely used as a high-performance, scalable caching proxy and traffic management system. The newly reported Apache Traffic Server vulnerability centers on the ESI plugin, a component designed to assemble web content at the edge dynamically. This feature, while valuable, contains a flaw in its processing of inclusion depth, a mechanism that controls how many nested ESI requests the server will follow. Decoding CVE-2025-49763 Vulnerability Attackers can craft malicious requests that recursively force the ESI plugin to process deeper inclusion layers than intended. This triggers excessive memory consumption, ultimately overwhelming the server’s resources and leading to a DoS condition that can take critical infrastructure offline. In an official advisory, the Apache Software Foundation highlighted not only this flaw but also a related ACL issue affecting the PROXY protocol client IP address handling. These combined vulnerabilities pose a multifaceted threat to systems running vulnerable ATS versions. Details of CVE-2025-49763 and Related Issues CVE-2025-49763: A remote DoS vulnerability via memory exhaustion in the ESI plugin. Affected Versions: ATS versions 9.0.0 through 9.2.10 and 10.0.0 through 10.0.5. Reporter: The DoS flaw was reported by security researcher Yohann Sillam. Related ACL Issue: CVE-2025-31698, involving incorrect client IP address handling for access control, was reported by Masakazu Kitajo. Mitigation Strategies and Recommendations In response to these vulnerabilities, the Apache Software Foundation promptly released patched versions—ATS 9.2.11 and 10.0.6—that introduce new configurable settings aimed at mitigating the risks rather than applying an automatic fix. Users are strongly encouraged to upgrade to these versions or later releases. Key mitigation steps include: Upgrading ATS: Organizations should update their servers to version 9.2.11 or 10.0.6 or above. Configuring ESI Plugin Limits: The new --max-inclusion-depth setting, defaulting to 3, limits the depth of nested ESI includes, effectively preventing infinite recursive processing that leads to memory exhaustion. Addressing the ACL Issue: For deployments using the PROXY protocol, administrators should configure the proxy.config.acl.subject setting to correctly determine which IP addresses are subject to access control lists (ACLs), as outlined in ip_allow.config and remap.config. If left unaddressed, CVE-2025-49763 could allow remote attackers to incapacitate ATS servers by exhausting memory resources, causing service interruptions that impact user experience and potentially incur financial and reputational damage. Conclusion By promptly upgrading affected ATS versions and applying the recommended configuration changes, especially around the ESI plugin inclusion depth and ACL rules, organizations can reduce their exposure to disruptive DoS attacks. Administrators running ATS versions 9.0.0 to 9.2.10 or 10.0.0 to 10.0.5 should prioritize these actions to protect their web infrastructure from the damaging effects of memory exhaustion-based attacks.
Analysis Summary
# Vulnerability: Apache Traffic Server ESI Plugin Memory Exhaustion (CVE-2025-49763)
## CVE Details
- CVE ID: CVE-2025-49763
- CVSS Score: Information on the specific CVSS score is not provided in the text, but the impact is high (Denial of Service). (Severity: High assumed due to DoS potential)
- CWE: Related to improper resource management/inclusion depth handling in ESI plugin.
## Affected Systems
- Products: Apache Traffic Server (ATS)
- Versions: Vulnerable versions are not explicitly listed, but the flaw affects systems running the ESI plugin.
- Configurations: Systems using the Edge Side Includes (ESI) plugin.
## Vulnerability Description
CVE-2025-49763 is a denial-of-service (DoS) vulnerability residing within Apache Traffic Server's Edge Side Includes (ESI) plugin. The flaw is specifically related to the processing of inclusion depth within the ESI mechanism. An attacker can send specially crafted requests that recursively trigger deeper ESI inclusion layers than the system is designed to handle, causing excessive and uncontrolled memory consumption until server resources are overwhelmed, resulting in a DoS condition. The advisory also mentions a related ACL issue concerning PROXY protocol client IP address handling.
## Exploitation
- Status: Not explicitly stated as exploited in the wild, but the vulnerability enables DoS attacks.
- Complexity: Likely Medium, as crafting recursive requests requires understanding the ESI implementation.
- Attack Vector: Network (Remote exploitation possible via HTTP interaction).
## Impact
- Confidentiality: Unknown/Minor (Focus is on availability/integrity).
- Integrity: Medium (System crash/disruption).
- Availability: High (Leads to Denial of Service, taking critical infrastructure offline).
## Remediation
### Patches
Specific patch versions are not individually listed in the provided text. Users should consult the latest official Apache Traffic Server advisories for patched versions that correctly handle ESI inclusion depth.
### Workarounds
The vulnerability is tied to the ESI plugin's processing. Potential workarounds might involve:
1. Disabling or limiting the functionality of the ESI plugin if possible.
2. Implementing strict rate limiting or request inspection to prevent recursive ESI calls from originating from a single source.
## Detection
- Indicators of compromise: Unexplained, high consumption of server memory leading to process crashes or unresponsive service operations.
- Detection methods and tools: Monitoring resource utilization (memory usage) on ATS instances, especially during periods of heavy traffic or unusual request patterns targeting ESI endpoints.
## References
- Vendor advisories: Official advisory from the Apache Software Foundation (mentioned as "official advisory").
- Relevant links - defanged:
* https://lists.apache.org/[email protected]
* https://thecyberexpress.com/apache-traffic-server-cve-2025-49763/
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*Note: This summary is based solely on the provided article text. Specific severity scores, precise version numbers, and Proof-of-Concept (PoC) status were not detailed.*