Description

Joplin is an open source note-taking and to-do application that organises notes and lists into notebooks. From 1.4.0 until 3.7.2, packages/renderer/MdToHtml/rules/katex.ts enables KaTeX's trust option for note content, allowing a note author to place a \href URL into rendered output without passing Joplin's normal URL allowlist. On Windows, clicking a link whose target is an attacker-controlled UNC path causes pathExists() to initiate SMB authentication and disclose the current user's NTLMv2 challenge-response without a warning. The unfiltered URL can also invoke other registered URL handlers, but the credential disclosure through KaTeX \href is the distinguishing demonstrated impact. This issue is fixed in version 3.7.2.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "2.5",
  "pubDate": "2026-09-21T21:17:03.593Z",
  "pubdate": "2026-09-21T21:17:03.593Z",
  "executiveSummary": "Joplin versions 1.4.0 through 3.7.2 are vulnerable to a security flaw within the KaTeX rendering logic that permits the circumvention of URL allowlist protections. This vulnerability involves an improper trust configuration in the renderer that allows note authors to inject arbitrary URIs, including UNC paths, into rendered output.\nThe primary risk stems from the application's interaction with the underlying Windows operating system. When a victim interacts with a malicious link, the application triggers an SMB authentication process to a remote server, resulting in the involuntary disclosure of the user's NTLMv2 challenge-response hash.\nThis vulnerability is critical for Windows-based installations where the application relies on underlying OS primitives for URI handling. An attacker can craft a malicious note containing an unauthorized link; upon the user clicking this link, the system facilitates the authentication request without user notification or confirmation. This exploitation vector is particularly effective for credential harvesting and potential relay attacks within local network environments.\nSuccessful exploitation requires the victim to render and interact with the malicious note content. The issue is resolved in version 3.7.2, which restricts the trust settings for KaTeX, ensuring that all rendered URLs are subject to the application's centralized security policies.",
  "technicalDetails": "The vulnerability originates in the file packages/renderer/MdToHtml/rules/katex.ts, where the Joplin application facilitates the rendering of mathematical notation via the KaTeX library. Within this module, the KaTeX 'trust' option is incorrectly configured, allowing the rendering engine to bypass global URL validation mechanisms that would normally constrain user-supplied input.\nIn a secure configuration, Joplin enforces a strict allowlist for URLs embedded within note content. However, the specific implementation of KaTeX integration enables the use of the \\href command without subjecting the target URI to these sanitization routines. This allows for the injection of arbitrary protocols and URI schemes that are otherwise prohibited by the application's security policy.\nThe attack flow proceeds as follows: First, an attacker creates a malicious note containing a KaTeX block with an embedded \\href pointing to an attacker-controlled UNC path (e.g., \\\\attacker-ip\\share). Second, when a victim opens the note, the Joplin Markdown-to-HTML renderer processes the KaTeX rule and generates an HTML anchor element that references the malicious UNC path. Third, upon the victim clicking this link, the Windows operating system's internal URL handling routines trigger. Because the link targets a network path, the OS automatically initiates the SMB authentication process as per standard Windows networking behavior.\nDuring this process, the OS transparently attempts to authenticate against the remote host by sending the local user's NTLMv2 challenge-response hash. This interaction occurs without triggering any warnings or prompting the user for intervention, effectively exfiltrating the credential material to the attacker's server.\nBeyond NTLMv2 disclosure, the flaw allows the invocation of arbitrary registered URL handlers on the host system. This provides a mechanism for attackers to interact with local applications or system services via their respective URI schemes. The vulnerability exists because the rendered output is processed with an implicit trust level that contradicts the intended security posture of the application's note-rendering pipeline. Version 3.7.2 addresses this by enforcing the application's URL allowlist requirements within the KaTeX rendering context, ensuring that all links are properly validated before they are processed by the underlying OS."
}