CVE-2026-6827

justhtml versions <= 1.11.0 (fixed in 1.12.0) do not sufficiently escape HTML-significant characters (angle brackets) in text nodes when converting to Markdown. Characters like < and > are preserved, allowing raw HTML injection into Markdown output, bypassing sanitization and potentially leading to XSS.

justhtml before 1.16.0 contains multiple HTML sanitization bypass issues that can allow active/dangerous content (e.g., script or style) to survive sanitization, potentially leading to XSS. The issues primarily affect advanced usage rather than the default JustHTML(..., sanitize=True) path.

justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers, HTML serialization, Markdown passthrough (html_passthrough=True), and several custom sanitization-policy edge cases. An attacker can bypass sanitization to inject active HTML and JavaScript, e.g., via encoded javascript: URLs, backslash-based relative URLs, markup-breaking names, raw </textarea> reintroduction, or preserved <style>/<meta http-equiv=refresh>/<base href> tags.

justhtml through 1.9.1 (fixed in 1.10.0) is vulnerable to uncontrolled recursion leading to denial of service. During JustHTML() construction, TreeBuilder.finish() calls _populate_selectedcontent(), which recursively traverses the DOM tree without a depth bound. An attacker can supply deeply nested elements (e.g., ~1000 nested <div> tags) to exceed CPython's recursion limit and trigger an unhandled RecursionError.

justhtml before 1.12.0 (versions <= 1.11.0) contains a mutation XSS (mXSS) vulnerability in the serialization of raw-text elements such as <style> and <script>. When a DOM tree is processed by sanitize_dom() with a custom policy that keeps these elements, text nodes inside them are serialized literally without escaping, allowing attacker-controlled text containing the matching closing tag sequence to break out and inject arbitrary HTML. The default sanitization policy is not affected because it drops the contents of style and script.

justhtml versions 0.9.0 through 1.21.0 contain a cross-site scripting vulnerability in to_markdown() where inline code spans fail to account for blank lines as block boundaries. Attackers can inject blank lines into code or pre element text to break the inline span, causing sanitized HTML to be emitted unescaped and re-parsed as live Markdown by compliant renderers.

justhtml before version 3.11.0 contains a cross-site scripting vulnerability where the default sanitizer bypasses event handler removal in selectedcontent projections. Attackers can inject SVG or MathML elements with event handlers that are cloned and reinserted into output without sanitization, enabling stored or reflected XSS attacks.

justhtml versions 1.13.0 and earlier contain a parser-differential / mutation cross-site scripting (mXSS) vulnerability when using a custom SanitizationPolicy that preserves foreign namespaces (e.g., drop_foreign_namespaces=False with allowlisted SVG/MathML elements or raw-text containers such as <style>). Specially crafted input can sanitize into markup that appears safe but becomes unsafe when re-parsed by a browser or another HTML parser, allowing markup injection. The default safe configuration (sanitize=True) is not affected. Fixed in 1.14.0.

justhtml versions before 1.13.0 contain a cross-site scripting vulnerability in the to_markdown() function when serializing attacker-controlled pre content. Attackers can place backticks inside sanitized pre elements to break out of fixed-length code fences, allowing raw HTML to execute when the generated Markdown is rendered by CommonMark or GFM-style renderers.

justhtml before 1.18.0 contains multiple low-severity denial-of-service issues in CSS selector handling and linkification. Applications that evaluate attacker-controlled selector strings, run selector matching over very large untrusted documents, construct DOM trees from untrusted structure, or enable linkification over attacker-controlled text may consume disproportionate CPU or memory. These are availability-only concerns and do not allow script execution, data disclosure, or sanitizer bypass.