CVE-2021-32735 in Kirby
Summary
by MITRE • 07/02/2021
Kirby is a content management system. In Kirby CMS versions 3.5.5 and 3.5.6, the Panel's `ListItem` component (used in the pages and files section for example) displayed HTML in page titles as it is. This could be used for cross-site scripting (XSS) attacks. Malicious authenticated Panel users can escalate their privileges if they get access to the Panel session of an admin user. Visitors without Panel access can use the attack vector if the site allows changing site data from a frontend form. Kirby 3.5.7 patches the vulnerability. As a partial workaround, site administrators can protect against attacks from visitors without Panel access by validating or sanitizing provided data from the frontend form.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 07/09/2021
The vulnerability identified as CVE-2021-32735 affects Kirby CMS versions 3.5.5 and 3.5.6, specifically targeting the Panel's `ListItem` component that handles page and file titles. This flaw represents a classic cross-site scripting vulnerability where HTML content is rendered directly without proper sanitization, creating an environment where malicious actors can inject harmful scripts into page titles. The vulnerability exists within the Panel's user interface component that displays content, making it particularly dangerous as it operates within the administrative context of the CMS. The issue stems from insufficient input validation and output escaping mechanisms that should have prevented HTML content from being interpreted as executable code rather than plain text.
The technical exploitation of this vulnerability requires an authenticated Panel user to leverage the XSS vector, though the impact extends beyond simple script execution. When malicious HTML is injected into page titles, it can be executed in the context of other administrators' sessions, potentially enabling privilege escalation attacks. This occurs because the vulnerability allows attackers to inject JavaScript code that can manipulate the Panel interface or steal session cookies, thereby compromising administrative access. The attack vector becomes particularly concerning when considering that authenticated users with limited privileges might use this vulnerability to escalate their access level within the CMS environment.
The operational impact of CVE-2021-32735 extends beyond traditional XSS consequences, as it creates a potential pathway for persistent attacks against CMS administrators. The vulnerability affects both authenticated Panel users and unauthenticated visitors who can exploit frontend forms that allow data modification. This dual attack surface means that even users without direct Panel access can leverage the vulnerability through frontend submission points, making the attack vector more accessible. The vulnerability essentially undermines the security boundary between frontend and backend operations, allowing attackers to bridge these contexts through manipulated page titles.
Security professionals should recognize this vulnerability as a manifestation of CWE-79 (Cross-site Scripting) and potentially CWE-20 (Improper Input Validation) within the context of content management systems. The ATT&CK framework categorizes this vulnerability under T1059.007 (Scripting) and potentially T1548.001 (Abuse Elevation of Privilege) when considering privilege escalation scenarios. The vulnerability's remediation through the release of Kirby 3.5.7 demonstrates the importance of proper input sanitization and output encoding in web applications, particularly in CMS environments where user-generated content is processed. Organizations should implement comprehensive security measures including regular updates, input validation, and proper sanitization of all user-supplied content that appears in administrative interfaces.
The recommended mitigation strategies include immediate deployment of the patched Kirby 3.5.7 version, which addresses the core sanitization issue in the `ListItem` component. Additionally, administrators should implement proper data validation and sanitization for all frontend forms that can modify site content, particularly those that feed into Panel displays. This partial workaround addresses the specific attack vector involving unauthenticated visitors while the primary patch resolves the underlying issue in the Panel's rendering logic. The vulnerability serves as a reminder of the critical importance of secure coding practices in CMS platforms where user inputs directly influence the presentation layer and administrative interfaces, emphasizing the need for defense-in-depth strategies that protect both frontend and backend components from cross-site scripting attacks.