CVE-2026-115348 in Mistune
Summary
by MITRE • 10/23/2026
Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity vulnerability in the inline link parser's parse_link_text function that allows unauthenticated attackers to exhaust CPU with unbalanced brackets. Attackers can submit Markdown containing many unbalanced '[' or '![' characters, forcing repeated unbounded forward scans and tying up worker processes for minutes.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified within Mistune versions prior to 3.3.0 represents a significant algorithmic complexity flaw located specifically in the inline link parser's parse_link_text function. This issue is categorized under CWE-400, which addresses Uncontrolled Resource Consumption resulting from an algorithm with quadratic or higher time complexity relative to input size. The core technical deficiency lies in how the parser handles malformed Markdown syntax containing unbalanced brackets. When a user submits content featuring numerous unmatched opening square bracket characters such as [ or image markers like ![, the parsing logic is forced into repeated and unbounded forward scans of the text buffer. Instead of failing fast upon detecting invalid syntax, the algorithm attempts to resolve potential link structures by scanning ahead for closing delimiters that do not exist, leading to exponential growth in computational effort relative to the length of the input string.
From an operational perspective, this vulnerability enables a denial-of-service attack vector against any service relying on Mistune to process user-supplied Markdown content without prior sanitization or rate limiting. Because many web applications utilize worker processes that are blocked while parsing incoming requests, an attacker can exploit this flaw by submitting specially crafted payloads designed to trigger the worst-case scenario of the parser's logic. The result is a sustained exhaustion of CPU resources on the host system. These malicious inputs tie up server-side worker threads for extended periods, potentially minutes per request, which effectively starves legitimate users of computational resources and service availability. This behavior aligns with MITRE ATT&CK technique T1496, Resource Hijacking, where an adversary consumes significant amounts of computing capacity to disrupt normal operations or cause a denial of service condition without necessarily compromising the confidentiality or integrity of the data itself.
The impact extends beyond simple CPU spikes; it can lead to cascading failures in production environments if not properly mitigated. In high-throughput applications, even a small number of such requests can saturate available processing cores, causing latency increases for all concurrent users and potentially triggering automatic scaling events or application crashes depending on the infrastructure's resilience configurations. Since Mistune is often used as part of larger content management systems or documentation platforms that accept user input, this vulnerability poses a direct risk to public-facing services unless specific countermeasures are implemented at the network or application layer.
To mitigate this vulnerability, organizations should immediately upgrade their dependencies to version 3.3.0 or later where the parsing logic has been optimized to handle malformed inputs more efficiently and with bounded complexity. For environments that cannot be patched instantly, implementing input validation rules is critical. Security teams should enforce strict length limits on Markdown fields and reject payloads containing excessive numbers of unbalanced brackets before they reach the parser. Additionally, deploying Web Application Firewalls configured to detect patterns indicative of algorithmic complexity attacks can provide an additional layer of defense by blocking requests that exhibit characteristics consistent with this exploitation vector. Regular auditing of third-party library versions is essential to ensure that such known computational flaws are addressed promptly and do not remain exposed in production systems.