CVE-2026-77088 in justhtml
Summary
by MITRE • 08/23/2026
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.
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Analysis
by VulDB Data Team • 08/23/2026
The vulnerability identified in justhtml versions 0.9.0 through 1.21.0 represents a significant cross-site scripting risk rooted in improper input validation during the conversion of HTML content to markdown format. This flaw specifically affects the to_markdown() function, which is responsible for sanitizing and transforming structured web data into plain text representations suitable for various downstream applications. The core technical deficiency lies in how the library handles inline code spans when encountering blank lines within their textual content. In standard Markdown syntax, a blank line typically serves as a delimiter that separates block-level elements from one another. However, justhtml fails to recognize these blank lines as valid boundaries for breaking out of an inline code span context. This oversight allows an attacker who controls the input data to inject specific sequences containing newlines or empty paragraphs into code or preformatted text blocks.
When such maliciously crafted content is processed by the vulnerable function, the parser does not correctly terminate the inline code scope at the point where a blank line occurs. Instead, it continues treating subsequent characters as part of the same inline span until another specific delimiter is encountered. This misinterpretation leads to a failure in proper escaping mechanisms that are normally applied to ensure special HTML and Markdown characters are rendered safely rather than executed. Consequently, any content appearing after the injected blank line within the affected code block may be emitted without adequate sanitization or escaping. If this output is then consumed by another system or renderer that interprets it as live Markdown, those unescaped sequences can include executable script tags or other malicious payloads.
The operational impact of this vulnerability extends beyond simple data corruption to active security breaches in environments where user-generated content flows through justhtml before being displayed via a compliant Markdown renderer. An attacker could craft a payload consisting of an HTML script tag embedded within what appears to be innocuous code text, separated by blank lines from the rest of the block. Because the library fails to break the inline span at these boundaries, the script tag remains unescaped in the final output. When this output is rendered on a web page, the browser interprets it as executable JavaScript rather than static text. This enables persistent cross-site scripting attacks that can steal session cookies, hijack user accounts, deface websites, or redirect users to malicious sites without their knowledge. The severity is compounded by the fact that many developers rely on such libraries for content management systems where input validation might be assumed but not strictly enforced at every stage of processing.
From a classification perspective, this issue aligns with CWE-79 Improper Neutralization of Input During Web Page Generation commonly known as Cross-Site Scripting (XSS). Specifically, it falls under the subcategory of stored or reflected XSS depending on whether the malicious content is saved to a database and served later or processed immediately. Furthermore, in terms of tactical mapping within the MITRE ATT&CK framework, this vulnerability facilitates initial access via phishing if used in conjunction with social engineering, but more directly supports execution techniques where attacker-controlled code runs in the victim's browser context. It also relates to CWE-601 URL Redirection to Untrusted Site which can occur as a secondary effect of script injection leading to unauthorized redirects.
Mitigation strategies must address both immediate remediation and long-term defensive coding practices. The most effective solution is for organizations using justhtml versions 0.9.0 through 1.21.0 to upgrade immediately to the latest patched version where this boundary detection logic has been corrected. If upgrading is not feasible due to legacy system constraints, developers should implement additional input validation layers that strip or escape newline characters within code blocks before they reach the markdown conversion function. Alternatively, applying strict Content Security Policy headers can mitigate the impact by preventing inline script execution even if it successfully bypasses the library's flawed escaping logic. Regular security audits of third-party dependencies and continuous integration testing with fuzzing tools designed to detect edge cases in text parsing libraries are essential for maintaining robust application security posture against such subtle but dangerous flaws.