Htmldoc Vulnerability: Stack Overflow Explained

We've recently discovered a critical vulnerability within the htmldoc software, specifically a stack overflow issue that can be triggered when processing heavily nested HTML files. This vulnerability, reported by michaelrsweet, has significant implications for users who rely on htmldoc for converting HTML documents into formats like PDF. In this article, we'll dive deep into what a stack overflow is, how it affects htmldoc, and what steps can be taken to mitigate this risk. Understanding these technical details is crucial for maintaining the security and stability of your document processing workflows.

Understanding Stack Overflow Vulnerabilities

A stack overflow is a common type of runtime error that occurs when a program tries to use more memory space on the call stack than is available. The call stack, often referred to simply as "the stack," is a region of memory that a program uses to keep track of active function calls. When a function is called, a new frame is added to the top of the stack, containing local variables, function arguments, and the return address (where the program should go back to after the function finishes). If functions call other functions recursively, or if a function calls itself many times without an appropriate exit condition, the stack can grow excessively large. Eventually, it can exceed its allocated boundary, leading to a stack overflow. This usually results in a program crash, often with a segmentation fault or, as seen with htmldoc, a fatal signal.

Why is this a problem? In essence, a stack overflow indicates that the program's execution flow has gone out of control due to an excessive number of nested function calls. Attackers can sometimes exploit this by crafting specific inputs that intentionally trigger this condition. While in this particular htmldoc case, the exploit requires a very deeply nested HTML structure, the underlying principle is that an unhandled condition can lead to instability. The consequences of a stack overflow can range from a simple denial-of-service (DoS) attack, where the program becomes unusable, to more severe security risks if the overflow can be manipulated to overwrite adjacent memory, potentially allowing for code execution. Therefore, addressing stack overflow vulnerabilities is a top priority for software developers to ensure their applications are robust and secure against malicious inputs. This makes the discovery in htmldoc a significant finding that warrants immediate attention.

The htmldoc Stack Overflow: A Deep Dive

The stack overflow vulnerability in htmldoc is triggered by processing HTML files with an extremely high level of nesting. The provided Proof of Concept (PoC) demonstrates this clearly. The Python script generates an HTML file, 10.html, containing an excessive number of nested tags. Specifically, it creates a file with 5 million opening <g> tags followed by 5 million closing </g> tags. When htmldoc attempts to process this file, it enters a state where the function htmlFixLinks is called recursively an enormous number of times. Each call to htmlFixLinks adds a new frame to the call stack. Given the depth of nesting required (millions of tags in the PoC), the stack quickly runs out of space. The AddressSanitizer output clearly shows the stack-overflow error originating from htmlFixLinks within htmllib.cxx, indicating that the program's execution stack has been exhausted.

This recursive nature of htmlFixLinks and its susceptibility to deeply nested structures is the core of the vulnerability. When htmldoc parses the HTML, it likely traverses the Document Object Model (DOM) tree. If the DOM tree is excessively deep, the functions responsible for processing or manipulating this structure can lead to deep recursion. In this scenario, htmlFixLinks appears to be involved in processing links or elements that are deeply nested, leading to the iterative calls that consume the stack. The fact that this occurs in a library function (htmllib.cxx) suggests that this is a fundamental issue in how htmldoc handles structural complexity in its input. The consequence is a complete crash of the htmldoc process, rendering it incapable of converting the malformed or excessively nested HTML into a usable document format. This effectively acts as a denial-of-service vector, as a specially crafted, deeply nested HTML file can crash the htmldoc service or process.

Why Deep Nesting Causes Problems

To understand why deep nesting is problematic, consider how programs typically process hierarchical data, like HTML. HTML documents are structured as a tree, where elements are nested within each other. When a program parses an HTML file, it often builds an internal representation of this tree. For complex operations, like resolving links, performing transformations, or generating output, the program might traverse this tree. Recursive functions are a natural fit for tree traversal. However, each recursive call consumes space on the program's call stack. If the tree is exceptionally deep, the number of nested function calls can grow linearly with the depth. In the htmldoc case, the htmlFixLinks function seems to be the culprit. It's likely designed to handle linking and element resolution, and when faced with an extremely deep structure, it enters a deep chain of recursive calls. Without proper safeguards, such as a maximum recursion depth limit or an iterative approach for extremely deep structures, the stack will inevitably overflow. The cnt = 5000000 in the PoC highlights that htmldoc fails to handle even moderately large, deeply nested structures, let alone extremely deep ones. This points to a lack of defensive programming against such inputs, where the expected input complexity was perhaps underestimated or not rigorously tested.

Technical Details of the htmldoc Vulnerability

The vulnerability in htmldoc stems from its handling of deeply nested HTML structures, leading to a stack overflow. The provided Proof of Concept (PoC) utilizes a Python script to generate an HTML file (10.html) that contains an extremely high degree of nesting. The script initializes a counter cnt to 5000000, representing the desired level of nesting. It then constructs an SVG string that includes cnt opening <g> tags followed by cnt closing </g> tags. This creates a document structure where elements are nested millions of levels deep. When htmldoc is invoked with the command ../htmldoc -f 1+.pdf 10.html, it attempts to process this malformed input. The output shows an AddressSanitizer: DEADLY SIGNAL, specifically an ERROR: AddressSanitizer: stack-overflow. The traceback reveals that the overflow occurs within the htmlFixLinks function located in htmllib.cxx. The stack trace shows repeated calls to htmlFixLinks, indicating a recursive loop or an extremely deep call chain. The specific line numbers mentioned (3572 and 3681) point to the exact location within the source code where the excessive stack usage is happening. The SUMMARY line confirms that the stack-overflow is the primary issue, occurring in htmlFixLinks at line 3572.

This behavior suggests that the htmlFixLinks function is designed to recursively process elements or links within the HTML document. When confronted with a deeply nested structure, this recursion depth exceeds the limits of the program's call stack. The stack is a finite memory region used to store information about active function calls. Each function call adds a new