CVE-2011-1156 in Universal Feed Parser
Summary
by MITRE
feedparser.py in Universal Feed Parser (aka feedparser or python-feedparser) before 5.0.1 allows remote attackers to cause a denial of service (application crash) via a malformed DOCTYPE declaration.
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Analysis
by VulDB Data Team • 02/02/2025
The vulnerability identified as CVE-2011-1156 resides within the feedparser.py module of Universal Feed Parser, a widely used Python library for parsing RSS and Atom feeds. This library serves as a critical component in numerous applications and services that process syndicated content, making its stability and security paramount to system integrity. The flaw manifests in the library's handling of malformed DOCTYPE declarations within XML feed structures, creating a potential avenue for remote attackers to disrupt service availability through carefully crafted malicious feed content.
The technical implementation of this vulnerability stems from inadequate input validation within the feedparser library's XML parsing routines. When processing feeds containing malformed DOCTYPE declarations, the parser fails to properly handle certain edge cases in the XML structure, leading to unhandled exceptions that cause the application to crash. This represents a classic denial of service vulnerability where the attacker can trigger application instability by submitting specifically crafted feed data that exploits the parser's insufficient error handling mechanisms. The vulnerability specifically affects versions prior to 5.0.1, indicating that the issue was recognized and patched in subsequent releases.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect entire content delivery systems that rely on Universal Feed Parser. Applications consuming feeds from untrusted sources, including news aggregators, social media platforms, and content management systems, could experience complete service outages when processing maliciously crafted feeds. This vulnerability is particularly dangerous in automated environments where feed processing occurs continuously, as it can lead to cascading failures across dependent systems. The remote nature of the attack means that malicious actors can exploit this vulnerability from anywhere on the internet without requiring local access or authentication credentials.
Mitigation strategies for CVE-2011-1156 primarily focus on upgrading to version 5.0.1 or later of the Universal Feed Parser library, which includes proper handling of malformed DOCTYPE declarations. Organizations should also implement feed validation mechanisms that sanitize input before processing, employ network segmentation to limit exposure, and establish monitoring systems to detect unusual feed processing patterns. From a cybersecurity perspective, this vulnerability aligns with CWE-400, which categorizes it as an uncontrolled resource consumption issue, and maps to ATT&CK technique T1499.004 for denial of service attacks. Regular security assessments and dependency updates form essential components of defense in depth strategies to prevent exploitation of similar parsing vulnerabilities in other XML processing libraries.
The broader implications of this vulnerability highlight the critical importance of robust input validation in parsing libraries that handle external data sources. Given the widespread adoption of Universal Feed Parser across numerous applications, the potential for cascading effects makes this vulnerability particularly concerning. Security practitioners should consider implementing additional layers of protection such as feed sanitization proxies, rate limiting for feed processing, and comprehensive logging of feed parsing activities to detect and respond to exploitation attempts effectively.