CVE-2026-91122 in Discourse
Summary
by MITRE • 09/24/2026
Discourse is an open-source discussion platform. Prior to 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0, the video placeholder component allowed crafted HTML to cause an attribute breakout and inject an attacker-controlled event handler. An authenticated user with default trust-level posting privileges could store the crafted placeholder in a post. When another user opened the post and clicked the video play overlay, the handler could execute arbitrary JavaScript in the viewer's session. Default Content Security Policy settings block inline event handlers, but instances with CSP disabled or relaxed could allow the script to read page content and make authenticated requests as the viewer. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability identified within Discourse prior to the specified patch versions represents a sophisticated cross-site scripting flaw rooted in improper input validation of HTML attributes within the video placeholder component. This issue allows an attacker with authenticated access and default posting privileges to inject malicious code that breaks out of expected attribute boundaries, effectively enabling the execution of arbitrary JavaScript events. The core technical failure lies in how the application processes user-supplied content for embedded media placeholders; specifically, crafted HTML can manipulate the DOM structure to escape the intended context, leading to an attribute breakout scenario where standard sanitization routines fail to neutralize dangerous event handlers such as onerror or onclick attributes that are subsequently bound to interactive elements like video play overlays.
From a technical perspective, this vulnerability exploits the interaction between user input and dynamic HTML rendering engines used by Discourse for displaying posts containing embedded media. When an authenticated user creates a post with specially crafted markup designed to exploit parsing ambiguities in the placeholder component, the system stores this content without adequately stripping or encoding potentially executable event attributes. The payload remains dormant until another user views the post and interacts with the video element by clicking its play overlay. This interaction triggers the browser's JavaScript engine to execute the injected handler within the context of the viewer’s session, granting the attacker control over client-side operations in real-time.
The operational impact of this vulnerability is significant for any Discourse instance that does not enforce strict Content Security Policy (CSP) directives or has disabled inline event handlers via CSP settings. In environments where default security configurations are relaxed, an attacker can leverage the executed JavaScript to read sensitive page content, steal session cookies, perform authenticated actions on behalf of the victim user, and potentially exfiltrate data from other parts of the application interface that would otherwise be inaccessible due to same-origin policy restrictions. This effectively transforms a stored XSS vector into a powerful tool for account takeover or data theft, compromising the integrity and confidentiality of the community platform.
Industry standards classify this type of flaw under CWE-79, which covers Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting (XSS). More specifically, it aligns with CWE-80 due to the attribute breakout mechanism that allows script injection through HTML attributes rather than direct body insertion. In terms of tactical mapping within the MITRE ATT&CK framework for Enterprise, this vulnerability facilitates techniques associated with Collection via Browser Hijacking and Credential Access through Stealing Web Session Cookies, as the attacker gains the ability to manipulate the victim's browser session state directly after successful exploitation.
Mitigation strategies primarily involve upgrading Discourse to version 2026.1.8 or later for stable releases, or versions 2026.6.3, 2026.7.2, and 2026.8.0 depending on the specific branch in use. These updates contain patches that enforce stricter sanitization rules on HTML attributes within media placeholders, ensuring that event handlers are properly escaped or removed before rendering. Additionally administrators should ensure that Content Security Policy headers are configured to disallow inline scripts using strict-dynamic policies and nonce-based script sources where applicable. Regular security audits of custom plugins and themes are also recommended to prevent similar injection vectors from being introduced through third-party extensions, maintaining a robust defense-in-depth posture against client-side code execution attacks.