CVE-2021-40906 in CheckMK Raw Edition
Summary
by MITRE • 03/26/2022
CheckMK Raw Edition software (versions 1.5.0 to 1.6.0) does not sanitise the input of a web service parameter that is in an unauthenticated zone. This Reflected XSS allows an attacker to open a backdoor on the device with HTML content and interpreted by the browser (such as JavaScript or other client-side scripts) or to steal the session cookies of a user who has previously authenticated via a man in the middle. Successful exploitation requires access to the web service resource without authentication.
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Analysis
by VulDB Data Team • 05/01/2025
The vulnerability identified as CVE-2021-40906 affects CheckMK Raw Edition software versions 1.5.0 through 1.6.0, representing a critical security flaw that stems from inadequate input sanitisation within an unauthenticated web service parameter. This issue creates a reflected cross-site scripting vulnerability that can be exploited by attackers who gain access to the web service resource without authentication requirements. The vulnerability resides in the software's handling of user-supplied data within a web service endpoint that should not require authentication, making it particularly dangerous as it can be targeted by attackers without prior credentials or access rights. The flaw operates by allowing malicious input to be reflected back to the user's browser without proper sanitisation, creating opportunities for various malicious activities including session hijacking and backdoor establishment.
The technical implementation of this vulnerability follows the CWE-79 pattern for cross-site scripting, specifically classified as reflected XSS where attacker-supplied data is immediately reflected back in the web response without proper sanitisation or encoding. This allows attackers to inject malicious scripts that execute in the victim's browser context, leveraging the browser's interpretation of HTML content to execute JavaScript or other client-side code. The vulnerability's impact extends beyond simple script execution as it enables attackers to steal session cookies from authenticated users, effectively allowing session hijacking attacks. The reflected nature of the vulnerability means that the malicious payload must be crafted to match the specific request parameters, making it particularly challenging to detect and prevent through traditional security measures.
The operational impact of CVE-2021-40906 is significant as it provides attackers with multiple attack vectors that can compromise system integrity and user privacy. Successful exploitation can lead to complete session takeover, allowing unauthorized access to administrative functions within the CheckMK environment, and potentially enabling persistent backdoor access through the injection of malicious JavaScript code. The vulnerability's accessibility through unauthenticated access points means that any attacker with network access to the affected service can attempt exploitation, making it particularly dangerous in environments where such services are exposed to untrusted networks. The potential for man-in-the-middle attacks to capture authenticated session cookies further amplifies the risk, as these attacks can occur even when users are properly authenticated, creating a window of opportunity for attackers to hijack legitimate sessions.
Mitigation strategies for CVE-2021-40906 should focus on immediate patching of affected CheckMK Raw Edition versions to address the input sanitisation deficiencies. Organizations should implement proper input validation and output encoding mechanisms to prevent malicious content from being reflected back to users, following established security practices such as those outlined in the OWASP Top Ten and the NIST Cybersecurity Framework. Network segmentation and access controls should be strengthened to limit exposure of unauthenticated web service endpoints, while monitoring systems should be deployed to detect anomalous traffic patterns that may indicate exploitation attempts. Additionally, security teams should conduct thorough penetration testing and vulnerability assessments to identify other potentially vulnerable endpoints within the CheckMK environment, ensuring comprehensive protection against similar vulnerabilities that may exist in related components or integrations. The ATT&CK framework categorizes this vulnerability under T1059.007 for command and scripting interpreter with reflected XSS, and T1566 for phishing with social engineering elements, highlighting the multi-faceted nature of the threat landscape this vulnerability creates.