CVE-2026-59710 in showdown
Summary
by MITRE • 07/07/2026
showdown contains a stored cross-site scripting vulnerability in the parseHeaders function of src/subParsers/makehtml/tables.js that fails to properly escape table header ID attributes. Attackers can inject arbitrary HTML and script-executing SVG elements through double-quote characters in markdown table headers, achieving stored XSS when untrusted markdown is rendered with the default github flavor configuration.
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Analysis
by VulDB Data Team • 07/07/2026
The vulnerability exists within the showdown markdown parser library where a stored cross-site scripting flaw is present in the parseHeaders function located in src/subParsers/makehtml/tables.js. This weakness specifically affects how table header ID attributes are processed during markdown rendering operations. The function fails to properly sanitize and escape special characters, particularly double-quote characters, which allows malicious input to bypass security controls. When markdown containing table headers with unescaped quotes is processed through the default github flavor configuration, the parser does not adequately neutralize potentially harmful content that could be embedded within the header attributes.
The technical implementation of this vulnerability stems from insufficient input validation and output encoding practices within the markdown parsing pipeline. The parseHeaders function does not perform proper HTML attribute escaping before incorporating user-provided data into the generated HTML structure. This creates an opportunity for attackers to inject malicious payloads that execute in the context of other users who view the affected content. The vulnerability is particularly concerning because it operates at the parsing level, meaning that once malicious input is stored in a system using showdown, any subsequent rendering of that content will execute the injected scripts.
When exploited, this stored XSS vulnerability enables attackers to execute arbitrary JavaScript code within the browser context of victims who view the rendered markdown content. The attack vector specifically utilizes double-quote characters to break out of HTML attribute contexts and inject SVG elements or other malicious payloads. This allows for persistent exploitation where the malicious code remains active until the affected content is modified or deleted, making it particularly dangerous in collaborative environments where multiple users may interact with the same stored content.
The operational impact of this vulnerability extends across various security domains as it represents a fundamental failure in input sanitization within a widely-used markdown processing library. Systems relying on showdown for rendering user-generated content become vulnerable to persistent XSS attacks that can be used to steal session cookies, perform actions on behalf of users, or redirect them to malicious sites. This vulnerability aligns with CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') and follows attack patterns consistent with those documented in the ATT&CK framework under T1059.001 - Command and Scripting Interpreter: PowerShell and similar execution techniques that leverage XSS to establish persistent access.
Organizations should implement immediate mitigations including updating to patched versions of showdown where available, implementing additional input validation layers before markdown processing, and deploying Content Security Policy headers to limit script execution capabilities in rendered content. The vulnerability demonstrates the critical importance of proper escaping and encoding practices in web applications, particularly when handling user-provided data that will be rendered as HTML or JavaScript. Additional defensive measures should include regular security scanning of dependencies, implementation of automated input sanitization, and comprehensive testing of markdown parsing functionality to prevent similar issues from occurring in other components of the application stack.