CVE-2021-39161 in Discourse
Summary
by MITRE • 08/27/2021
Discourse is an open source platform for community discussion. In affected versions category names can be used for Cross-site scripting(XSS) attacks. This is mitigated by Discourse's default Content Security Policy and this vulnerability only affects sites which have modified or disabled or changed Discourse's default Content Security Policy have allowed for moderators to modify categories. This issue is patched in the latest stable, beta and tests-passed versions of Discourse. Users are advised to ensure that the Content Security Policy is enabled, and has not been modified in a way which would make it more vulnerable to XSS attacks.
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Analysis
by VulDB Data Team • 09/01/2021
CVE-2021-39161 represents a cross-site scripting vulnerability within the Discourse open source community discussion platform that specifically targets category name handling mechanisms. This vulnerability falls under the CWE-79 category for Cross-Site Scripting and demonstrates how seemingly innocuous user input fields can become attack vectors when proper sanitization controls are bypassed. The flaw exists in the way Discourse processes and renders category names, allowing malicious actors to inject script code that executes in the context of other users' browsers when they view affected category names.
The technical exploitation of this vulnerability requires that organizations have modified or disabled Discourse's default Content Security Policy (CSP) configuration, which serves as a critical defense-in-depth mechanism. This requirement significantly reduces the attack surface since Discourse's default CSP implementation provides robust protection against such attacks. The vulnerability specifically affects instances where moderators have been granted the ability to modify categories, creating an attack path through which malicious input can be introduced into the system. The issue is particularly concerning because it leverages legitimate administrative functionality to create persistent XSS vectors.
The operational impact of this vulnerability extends beyond simple script execution, as it can enable attackers to steal session cookies, perform unauthorized actions on behalf of users, or redirect victims to malicious sites. When combined with the fact that this vulnerability only affects modified CSP configurations, it highlights the critical importance of maintaining default security settings and properly vetting any modifications to security controls. Organizations that have customized their CSP policies may inadvertently create backdoors for attackers to exploit, as demonstrated by this vulnerability. The patch addresses the root cause by implementing proper input sanitization and ensuring that even modified CSP configurations cannot bypass the core security protections.
Mitigation strategies for CVE-2021-39161 should focus on maintaining the integrity of Discourse's default Content Security Policy while implementing proper input validation for all user-modifiable content. Organizations should conduct comprehensive audits of their CSP configurations to ensure no modifications create vulnerabilities, particularly those that might allow script execution from untrusted sources. The vulnerability serves as a reminder of the ATT&CK technique T1213 - Data from Information Repositories, where attackers can exploit platform weaknesses to gain access to user data through compromised community platforms. Regular security assessments of custom configurations and adherence to security best practices for content management systems are essential to prevent similar issues from arising in the future. The vulnerability also underscores the importance of Principle of Least Privilege, where administrative capabilities should be carefully restricted to prevent unauthorized modifications that could compromise security controls.