Description

commonmark versions from 2.0.0 before 2.8.4 contain a denial of service vulnerability in XmlRenderer that emits depth-proportional indentation for every XML tag. Attackers can provide deeply nested Markdown or AST structures to cause quadratic memory consumption and output amplification, exhausting server resources.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-09-07T13:20:42.600Z",
  "pubdate": "2026-09-07T13:20:42.600Z",
  "executiveSummary": "The CommonMark library, specifically within versions 2.0.0 through 2.8.3, is susceptible to a denial of service (DoS) vulnerability located in the XmlRenderer component.\nThis vulnerability is rooted in an algorithmic complexity issue where the rendering process generates depth-proportional indentation for XML tags, leading to quadratic memory consumption and output amplification.\nBy submitting maliciously crafted Markdown or Abstract Syntax Tree (AST) structures featuring deep nesting, an attacker can exhaust system memory and CPU resources, effectively crashing the service or rendering it unresponsive.\nThis vulnerability does not require authentication and can be exploited over any network interface that accepts Markdown input, posing a significant risk to availability.\nThe issue highlights the dangers of processing untrusted input that influences resource allocation in linear or super-linear ways relative to nesting depth.",
  "technicalDetails": "The vulnerability resides within the XmlRenderer class of the commonmark package. The root cause is the implementation of an indentation mechanism that scales linearly with the depth of the parsed structure for every individual XML tag generated during the rendering phase.\nWhen commonmark parses input that is deeply nested—such as nested blockquotes, lists, or other hierarchical elements—the resulting AST reflects this depth. During the rendering process, the XmlRenderer attempts to format the output with whitespace to reflect this hierarchy. Because the indentation logic is applied to every tag output, the total volume of generated output grows quadratically relative to the nesting level of the input.\nThe attack flow begins when an attacker provides a crafted Markdown payload designed with an extremely high degree of nesting. Upon processing this payload, the XmlRenderer initiates the generation of the XML representation. As it traverses the deep AST nodes, the renderer calculates the necessary indentation padding for each node. This process results in an exponential increase in the number of whitespace characters and total bytes stored in memory or written to the output stream for each level of nesting.\nThis behavior forces the allocation of memory buffers that grow significantly faster than the input size, eventually leading to heap exhaustion or excessive CPU cycles consumed by the renderer attempting to manage these large strings. This effectively constitutes a resource-exhaustion denial of service.\nThe vulnerability affects versions 2.0.0 through 2.8.3. Authentication is not a prerequisite for exploitation, as the renderer is typically exposed at the boundary of web applications processing user-submitted Markdown. There is no specific privilege requirement, as the impact is realized purely through the manipulation of input data delivered via standard network channels.\nPost-exploitation, the server may experience an out-of-memory (OOM) error, triggering a process crash or causing severe latency that impacts all users of the application."
}