A critical security update addresses multiple vulnerabilities in the Mistune Markdown parsing library that could allow remote attackers to achieve arbitrary file read, cross-site scripting, and denial of service. These flaws stem from insufficient input sanitization, improper path handling, unhandled recursion, and algorithmic complexity issues within core parsing engines and directive plugins. Depending on the flaw exploited, severity ranges from medium impact to severe security compromises, reaching CVSS scores up to 8.6. Successful exploitation enables unauthenticated attackers to view sensitive local files, execute arbitrary client-side code, or disrupt service availability. Organizations utilizing vulnerable library versions in web applications or document processing pipelines are strongly urged to apply patched releases immediately.
CVE-2026-59924: This vulnerability is an arbitrary file read flaw within the Include directive module, caused by improper validation of file path traversal sequences. Remote unauthenticated attackers can supply crafted directory paths to read arbitrary local files accessible to the application process. High impact on confidentiality allows exposure of system configurations, source code, and credentials. Risk of exploitation is severe for applications accepting untrusted Markdown inputs.[/subscribe_to_unlock_form]
A critical security update addresses multiple vulnerabilities in the Mistune Markdown parsing library that could allow remote attackers to achieve arbitrary file read, cross-site scripting, and denial of service. These flaws stem from insufficient input sanitization, improper path handling, unhandled recursion, and algorithmic complexity issues within core parsing engines and directive plugins. Depending on the flaw exploited, severity ranges from medium impact to severe security compromises, reaching CVSS scores up to 8.6. Successful exploitation enables unauthenticated attackers to view sensitive local files, execute arbitrary client-side code, or disrupt service availability. Organizations utilizing vulnerable library versions in web applications or document processing pipelines are strongly urged to apply patched releases immediately.
CVE-2026-59924: This vulnerability is an arbitrary file read flaw within the Include directive module, caused by improper validation of file path traversal sequences. Remote unauthenticated attackers can supply crafted directory paths to read arbitrary local files accessible to the application process. High impact on confidentiality allows exposure of system configurations, source code, and credentials. Risk of exploitation is severe for applications accepting untrusted Markdown inputs.[emaillocker id="1283"]
CVE-2026-59923: A cross-site scripting vulnerability exists in the URL sanitization component due to improper handling of percent-encoded JavaScript protocol URIs. Attackers can bypass URL safety checks by supplying encoded payloads that execute arbitrary JavaScript within a victim's browser session. Successful exploitation risks session hijacking, unauthorized data modification, and credential theft. The vulnerability heavily impacts web applications rendering untrusted content.
CVE-2026-59926: This issue is a cross-site scripting flaw in the Admonition directive plugin caused by failing to sanitize class attributes. Unauthenticated attackers can inject arbitrary HTML and malicious client-side code through unescaped class names within directives. The impact includes arbitrary script execution in user browser contexts and potential session takeover. Exploitation risk is elevated in environments rendering dynamic, user-generated Markdown documentation.
CVE-2026-59929: A URL sanitization bypass exists in the safe rendering engine due to incomplete filtering of legacy and chained protocol schemes. Attackers can craft specialized link targets that bypass harmful protocol checks and trigger client-side script execution. The flaw leads to cross-site scripting risks, compromising the confidentiality and integrity of web sessions. Applications depending on default URL safety filters remain highly vulnerable.
CVE-2026-59927: This denial of service flaw in the include directive plugin arises from unhandled mutual file recursion leading to a renderer crash. Remote attackers can trigger application-level denial of service by providing two recursively referencing Markdown files. The flaw compromises service availability with minimal effort required from the adversary. Exploitation risk is highest in automated documentation generation systems.
CVE-2026-59928: An algorithmic complexity vulnerability in the block parser results in quadratic-time parsing when processing long repeated reference-link definitions. Remote attackers can transmit specially crafted Markdown payloads that consume excessive CPU resources and exhaust server power. The impact is limited to denial of service through performance degradation and responsiveness loss.
CVE-2026-59925: The inline parsing engine suffers from a quadratic-time complexity flaw when handling extended runs of nested emphasis syntax markers. Processing malicious inputs containing repeated emphasis pairs causes elevated CPU usage and rendering delays. The primary risk involves denial of service against applications parsing user-supplied text.
CVE-2026-59922: A quadratic-time parsing issue within formatting plugins affects the handling of long sequences of strikethrough, mark, and insert tags. Attackers can supply complex formatting markers to consume application resources and disrupt server responsiveness. The impact focuses on denial of service without compromising system confidentiality or integrity.
CVE-2026-59930: A structural collision flaw in the table of contents directive generates predictable heading identifiers without proper slugification. Attackers can craft custom HTML content containing matching identifier attributes to hijack or overwrite table of contents references. The impact includes potential content spoofing and UI confusion, carrying low to moderate overall risk.
Prompt adoption of updated library releases and robust input validation strategies are critical to mitigating these risks and ensuring application resilience against unauthorized file access and script execution. Organizations must prioritize updating affected dependencies across all production environments to eliminate potential attack vectors.
We recommend you to update mistune to version 3.3.0.
The following reports contain further technical details:
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