CVE-2015-5523 in iOS
Summary
by MITRE
The ParseValue function in lexer.c in tidy before 4.9.31 allows remote attackers to cause a denial of service (crash) via vectors involving multiple whitespace characters before an empty href, which triggers a large memory allocation.
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Analysis
by VulDB Data Team • 06/16/2022
The vulnerability identified as CVE-2015-5523 represents a critical denial of service flaw within the HTML parsing library tidy, specifically affecting versions prior to 4.9.31. This issue resides in the ParseValue function located within the lexer.c file, which forms a fundamental component of the library's parsing mechanism. The vulnerability manifests when processing malformed HTML content containing multiple whitespace characters preceding an empty href attribute, creating a condition that triggers excessive memory allocation patterns. This particular flaw demonstrates the classic characteristics of a resource exhaustion attack vector, where malicious input causes the application to consume disproportionate computational resources leading to system instability.
The technical exploitation of this vulnerability occurs through carefully crafted HTML input that contains multiple whitespace characters positioned before an empty href attribute within HTML tags. When the ParseValue function processes such input, it fails to properly handle the edge case involving excessive whitespace followed by empty href values, resulting in a memory allocation routine that scales disproportionately with the input size. This behavior stems from inadequate input validation and boundary checking within the lexical analysis phase of the parsing process, where the parser does not properly constrain memory allocation based on input characteristics. The flaw operates at the level of the lexer component, which is responsible for tokenizing input streams, making it particularly dangerous as it can be triggered during the initial parsing phase of any HTML document.
The operational impact of CVE-2015-5523 extends beyond simple service disruption to potentially enable more sophisticated attack scenarios. Remote attackers can leverage this vulnerability to cause application crashes, leading to denial of service conditions that affect web applications, content management systems, and any software utilizing the tidy library for HTML processing. The vulnerability is particularly concerning in web-facing applications where user input is processed through the HTML parser, as it allows attackers to craft malicious payloads that can be executed against vulnerable systems. This type of resource exhaustion attack can be particularly effective in environments where multiple parsing operations occur simultaneously, amplifying the impact of individual malicious requests. The vulnerability aligns with CWE-400, which categorizes improper resource management issues, specifically addressing the lack of proper input validation leading to excessive resource consumption.
From a threat modeling perspective, this vulnerability maps to several ATT&CK techniques including T1499.004 for network denial of service and T1059.008 for command and scripting interpreter usage in parsing contexts. The flaw demonstrates how seemingly benign input processing can become a vector for system compromise, particularly in applications that do not properly sanitize or validate input before processing. The memory allocation behavior suggests that the vulnerability could potentially be extended to other resource exhaustion scenarios, making it a significant concern for applications that rely on robust input handling. Organizations utilizing the tidy library in production environments should consider this vulnerability as a potential attack surface that could be exploited to disrupt services, particularly in high-traffic scenarios where multiple parsing operations occur concurrently.
Mitigation strategies for CVE-2015-5523 primarily focus on immediate version upgrading to tidy 4.9.31 or later, which contains the necessary patches to address the memory allocation issue. Additionally, implementing input validation and sanitization measures at the application level can provide defense-in-depth protection against similar vulnerabilities. Organizations should also consider deploying web application firewalls that can detect and block malformed HTML patterns associated with this vulnerability. The fix implemented in the patched versions addresses the root cause by introducing proper boundary checking and memory allocation constraints within the ParseValue function, preventing the excessive memory consumption that previously occurred. Regular security assessments and code reviews focusing on input validation and resource management practices should be implemented to prevent similar vulnerabilities from emerging in other components of the application stack.