Authlib Mistune: Nine Vulnerabilities Including DoS and XSS Disclosed Together
Authlib's Mistune parser patched for nine vulnerabilities, including DoS, XSS, and file access flaws, fixed in v3.3.0.
Key findings
- Nine vulnerabilities in Authlib's Mistune patched in version 3.3.0.
- Flaws include denial-of-service via quadratic complexity in parsing.
- URL filtering bypass and arbitrary file access vulnerabilities identified.
- Cross-site scripting (XSS) possible through improper attribute handling.
- Issues with TOC generation and recursive includes also disclosed.
On July 9, 2026, a batch of nine vulnerabilities was disclosed in Authlib's Mistune, a Python Markdown parser. These vulnerabilities, all patched in version 3.3.0, primarily affect how Mistune handles various Markdown constructs, leading to potential denial-of-service conditions, cross-site scripting (XSS), and arbitrary file access. The disclosures highlight issues in URL filtering, directive processing, and parsing logic.
Several vulnerabilities stem from improper handling of Markdown syntax that can lead to quadratic complexity, resulting in denial-of-service (DoS) attacks. CVE-2026-59922 and CVE-2026-59925 describe how specific marker pairs (tilde, equals-sign, caret, double-asterisk, triple-asterisk) can cause the parser to perform excessive work, exhausting CPU resources. Similarly, CVE-2026-59928 details how a large number of reference-link definitions can lead to quadratic work in block parsing, also causing DoS.
Security issues related to URL handling and file access are also present. CVE-2026-59929 and CVE-2026-59923 reveal flaws in the safe_url filter, which fails to block certain schemes like feed:, view-source:, and percent-encoded javascript: URIs, potentially leading to script execution. Furthermore, CVE-2026-59924 describes how the Include directive can be manipulated to access files outside the intended directory due to improper path normalization.
Other vulnerabilities include an XSS flaw and issues with the Table of Contents (TOC) plugin. CVE-2026-59926 details how the render_admonition() function concatenates the :class: option without escaping, enabling attribute injection and XSS. CVE-2026-59930 points out that the TOC plugin generates predictable heading IDs, allowing for same-page navigation hijacking. CVE-2026-59927 highlights an unbounded recursion vulnerability in the Include directive when handling indirect self-includes, which can crash the application.
All nine vulnerabilities were fixed in Mistune version 3.3.0. Users are strongly advised to update to this version to mitigate the risks associated with these security flaws. The consistent patching across multiple components indicates a comprehensive effort by the developers to address the reported issues.
The disclosure of these nine vulnerabilities together underscores the importance of keeping the Mistune library updated. The range of issues, from DoS to XSS and arbitrary file access, presents a significant risk to applications relying on Mistune for Markdown parsing. Promptly updating to version 3.3.0 is crucial for maintaining the security and stability of affected systems.
The vulnerabilities disclosed are: CVE-2026-59922, CVE-2026-59923, CVE-2026-59924, CVE-2026-59925, CVE-2026-59926, CVE-2026-59927, CVE-2026-59928, CVE-2026-59929, and CVE-2026-59930.