CVE-2016-8734 in Subversion
Summary
by MITRE
Apache Subversion's mod_dontdothat module and HTTP clients 1.4.0 through 1.8.16, and 1.9.0 through 1.9.4 are vulnerable to a denial-of-service attack caused by exponential XML entity expansion. The attack can cause the targeted process to consume an excessive amount of CPU resources or memory.
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Analysis
by VulDB Data Team • 10/04/2022
The vulnerability identified as CVE-2016-8734 represents a critical denial-of-service weakness in Apache Subversion's mod_dontdothat module and affected HTTP clients spanning versions 1.4.0 through 1.8.16 and 1.9.0 through 1.9.4. This flaw stems from the improper handling of XML entity expansion within the software's processing pipeline, creating a condition where maliciously crafted XML input can trigger exponential resource consumption. The vulnerability falls under the category of XML External Entity (XXE) processing issues and aligns with CWE-400, which specifically addresses unchecked resource consumption in software systems. The mod_dontdothat module, designed to prevent certain operations within Subversion's HTTP interface, becomes a vector for resource exhaustion attacks when processing malformed XML content.
The technical mechanism behind this vulnerability exploits the way XML parsers handle entity references and expansion processes. When the affected Subversion components process XML data containing recursively defined entities or entities with excessive expansion ratios, the parser enters a state where each entity reference can exponentially increase the processing requirements. This behavior creates a scenario where a small XML payload can trigger massive computational overhead, causing the targeted process to consume disproportionate amounts of CPU cycles and memory resources. The exponential nature of the resource consumption means that even modest input sizes can quickly overwhelm system capabilities, leading to service unavailability for legitimate users. This vulnerability directly relates to ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion.
The operational impact of CVE-2016-8734 extends beyond simple service disruption, as it can affect the entire Subversion infrastructure and potentially compromise system availability for development teams relying on version control services. Attackers can exploit this vulnerability by crafting XML payloads that trigger the exponential expansion behavior, causing servers to become unresponsive or crash entirely. The vulnerability affects both the server-side mod_dontdothat module and client-side HTTP implementations, meaning that an attacker could potentially compromise either endpoint in the Subversion communication chain. Organizations using affected Subversion versions face significant risk of operational disruption, particularly in environments where Subversion serves as a critical component of development workflows and code management processes. The vulnerability also poses risks to continuous integration systems and automated deployment pipelines that depend on Subversion for source code management.
Mitigation strategies for CVE-2016-8734 require immediate action to upgrade to patched versions of Apache Subversion, specifically versions 1.8.17, 1.9.5, or later releases that contain fixes for the XML entity expansion handling. Organizations should implement XML parser configuration changes that disable external entity resolution and limit entity expansion ratios to prevent exploitation. Network-level protections such as rate limiting and input validation should be deployed to reduce the impact of potential attacks. System administrators must also consider implementing monitoring solutions that can detect unusual resource consumption patterns indicative of this attack vector. Additionally, security teams should conduct vulnerability assessments to identify all systems running affected Subversion versions and ensure proper patch management procedures are in place. The remediation process should include thorough testing of patched environments to ensure that the fixes do not introduce compatibility issues with existing Subversion workflows and integrations. Organizations should also review their incident response procedures to prepare for potential exploitation attempts targeting this vulnerability.