CVE-2019-10010

Vulnerability

Cross-site scripting (XSS) vulnerability in the PHP League CommonMark library before version 0.18.3 allows remote attackers to insert unsafe links into HTML output. The issue occurs when double-encoded HTML entities are not properly escaped during rendering, leading to injection of arbitrary HTML/JavaScript [1]. All versions prior to 0.18.3 are affected.

Exploitation

An attacker can craft Markdown content containing double-encoded HTML entities (e.g., <script>). When a vulnerable application renders this Markdown to HTML, the library fails to decode and escape these entities, resulting in the injection of unsafe links or script content. No authentication is required; the attacker only needs to supply the malicious input via any channel the application processes Markdown (e.g., user comments, chat messages) [1][2].

Impact

Successful exploitation allows an attacker to inject arbitrary HTML and JavaScript into the rendered page, leading to cross-site scripting (XSS). This can result in session theft, credential harvesting, defacement, or other malicious actions within the security context of the affected application [1][4].

Mitigation

Upgrade to version 0.18.3 or later, which properly decodes and escapes double-encoded HTML entities [3]. For additional defense in depth, the library documentation recommends setting 'allow_unsafe_links' => false and 'html_input' => 'strip' when processing untrusted input [2][4]. No other workarounds are available.