CVE-2017-13692 in tidy
Summary
by MITRE
In Tidy 5.5.31, the IsURLCodePoint function in attrs.c allows attackers to cause a denial of service (Segmentation Fault), as demonstrated by an invalid ISALNUM argument.
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Analysis
by VulDB Data Team • 12/16/2022
The vulnerability identified as CVE-2017-13692 represents a critical denial of service flaw within the Tidy HTML parser version 5.5.31. This issue stems from improper input validation within the IsURLCodePoint function located in the attrs.c source file, creating a scenario where maliciously crafted input can trigger segmentation faults and system crashes. The vulnerability specifically manifests when the function receives an invalid ISALNUM argument, demonstrating how seemingly benign parameter handling can become a vector for system instability. Such flaws are particularly concerning in web applications and content processing systems where Tidy is commonly employed for HTML sanitization and normalization.
The technical implementation of this vulnerability resides in the function's handling of character classification operations without proper bounds checking or input validation. When the IsURLCodePoint function processes an argument that does not conform to expected alphanumeric character patterns, it fails to validate the input before performing operations that assume valid character data. This lack of defensive programming practices leads to memory access violations that manifest as segmentation faults, effectively crashing the application and rendering it unavailable to legitimate users. The flaw operates at the intersection of buffer management and character processing, where improper argument validation creates a pathway for exploitation.
From an operational perspective, this vulnerability presents significant risks to systems relying on Tidy for HTML processing, particularly in web applications, content management systems, and automated parsing services. An attacker could exploit this flaw by submitting malformed HTML content containing specially crafted URL parameters that trigger the vulnerable function path. The resulting segmentation fault would cause the application to terminate unexpectedly, leading to service disruption and potential denial of service for legitimate users. This vulnerability impacts systems where HTML sanitization is performed in real-time, as a single malicious input could cause cascading failures in processing pipelines.
The vulnerability aligns with CWE-129, which describes improper validation of input buffers, and demonstrates characteristics consistent with CWE-787, improper access to memory locations, as the function attempts to process invalid memory references. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, which covers network denial of service, and T1566, which involves phishing with malicious attachments, as the exploitation could occur through web-based attack vectors. The flaw represents a classic example of how inadequate input validation in parsing libraries can create widespread impact across applications that depend on these components for content processing.
Mitigation strategies should prioritize immediate patching of Tidy to version 5.5.32 or later, which contains the necessary fixes for the IsURLCodePoint function. Organizations should implement input validation at multiple layers, including application-level sanitization and web application firewalls that can detect and block malformed HTML content. Additionally, deploying intrusion detection systems that monitor for segmentation fault patterns and implementing robust error handling that prevents crash propagation can help contain the impact. Regular security assessments of third-party libraries and maintaining up-to-date dependency management practices are essential for preventing similar vulnerabilities from compromising system integrity. The vulnerability underscores the importance of defensive programming practices and comprehensive testing of character processing functions in security-critical applications.