CVE-2026-115355 in Mistune
Summary
by MITRE • 10/23/2026
Mistune before 3.3.0 contains a path traversal vulnerability in the Include directive that allows attackers to read arbitrary files by supplying ../ sequences in include paths. Attackers can also inject HTML through unescaped error messages and included file contents to perform cross-site scripting even when escape=True is set.
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Analysis
by VulDB Data Team • 10/23/2026
The Mistune library, prior to version 3.3.0, contains a critical path traversal vulnerability within its Include directive functionality. This security flaw arises from insufficient validation of user-supplied input used in the include paths for markdown content processing. Specifically, attackers can manipulate these paths by injecting directory traversal sequences such as ../ into the configuration or input data. When the parser processes this malformed input, it fails to properly canonicalize or restrict the file system access scope, allowing the application to resolve references outside of the intended base directory. This lack of strict boundary enforcement enables an attacker to read arbitrary files from the underlying server's file system that would otherwise be inaccessible due to standard security controls like chroot jails or restricted web roots.
The operational impact of this vulnerability is severe, particularly in applications where Mistune is used to render user-generated content for display on a website. By exploiting the path traversal mechanism, an attacker can extract sensitive configuration files, source code containing hardcoded credentials, private keys, or other confidential data stored on the server. This leads directly to unauthorized information disclosure and potential compromise of system integrity. Furthermore, the vulnerability extends beyond simple file reading due to how Mistune handles error states and included content. The library fails to properly escape HTML entities in both unescaped error messages generated during parsing failures and in the contents of files that are successfully included via the directive.
This deficiency creates a secondary vector for Cross-Site Scripting attacks, even when the global escape=True setting is enabled within Mistune's configuration. Typically, this flag is intended to sanitize output by escaping special HTML characters such as <, >, &, and quotes, thereby preventing script injection. However, because the error messages and included file contents are not subjected to adequate sanitization before being rendered in the browser context, an attacker can craft malicious include paths or inject content that results in executable JavaScript code running within a victim's session. This allows for account hijacking, session theft, defacement of web pages, or redirection to phishing sites, significantly expanding the attack surface beyond mere data exfiltration.
From a classification perspective, this vulnerability aligns with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, which describes flaws where software does not properly neutralize special elements within file paths that could cause files to be located in unintended directories. Additionally, the cross-site scripting component falls under CWE-79: Improper Neutralization of Input During Web Page Generation, commonly known as Cross-Site Scripting (XSS). In terms of tactical mapping, this behavior corresponds to ATT&CK technique T1083: File and Directory Discovery for information gathering and T1189: Drive-by Client Side Code Injection or T1059.007: JavaScript Execution via XSS depending on the specific exploitation method used against end users.
To mitigate these risks, organizations must immediately upgrade Mistune to version 3.3.0 or later where these validation checks and escaping mechanisms have been corrected. For applications unable to update instantly, it is crucial to implement strict input validation at the application layer before passing any include paths to the markdown parser. This includes rejecting inputs containing relative path traversal sequences like ../ or ..\ and ensuring that only whitelisted file names are permitted. Furthermore, developers should avoid relying solely on library-level escaping flags for security; instead, they must ensure that all dynamic content rendered in HTML contexts is properly sanitized using robust output encoding libraries consistent with OWASP guidelines. Regular code reviews focusing on markdown processing components can help identify similar logic flaws before deployment.