CVE-2021-35489 in Thruk
Summary
by MITRE • 11/10/2021
Thruk 2.40-2 allows /thruk/#cgi-bin/extinfo.cgi?type=2&host={HOSTNAME]&service={SERVICENAME]&backend={BACKEND] Reflected XSS via the host or service parameter. An attacker could inject arbitrary JavaScript into extinfo.cgi. The malicious payload would be triggered every time an authenticated user browses the page containing it.
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Analysis
by VulDB Data Team • 11/12/2021
The vulnerability identified as CVE-2021-35489 affects Thruk version 2.40-2, a web-based monitoring interface for Nagios and other monitoring systems. This security flaw resides within the extinfo.cgi component of the application's web interface, specifically in how it handles user-supplied parameters during the display of host and service information. The vulnerability manifests as a reflected cross-site scripting issue that occurs when the application fails to properly sanitize or encode user input before rendering it in the web response. Attackers can exploit this weakness by crafting malicious URLs that include JavaScript code within the host or service parameters, which are then reflected back to authenticated users who visit the affected page.
The technical implementation of this vulnerability follows the classic reflected XSS pattern where user-controllable data flows from the HTTP request directly to the HTTP response without proper sanitization. When an authenticated user accesses a maliciously crafted URL containing JavaScript code within the host or service parameters, the web application includes this unvalidated input in the HTML response. The malicious payload executes in the context of the victim's browser session, potentially allowing attackers to perform actions on behalf of the authenticated user. This type of vulnerability is categorized under CWE-79 as "Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')", which represents one of the most common and dangerous web application security flaws. The vulnerability is particularly concerning because it requires no special privileges beyond access to the Thruk web interface, and the attack can be executed through social engineering techniques such as phishing emails or compromised web links.
The operational impact of CVE-2021-35489 extends beyond simple script execution, as it can enable attackers to hijack user sessions, steal sensitive monitoring data, or manipulate the display of critical system information. Since Thruk serves as a monitoring interface for network infrastructure, successful exploitation could allow attackers to access detailed system information, potentially including credentials, configuration details, or status information about monitored services. The reflected nature of the vulnerability means that attackers must convince victims to click on malicious links, but once executed, the payload can perform actions such as stealing session cookies, redirecting users to malicious sites, or modifying the displayed content to mislead administrators about system status. This vulnerability directly aligns with ATT&CK technique T1566.001 "Phishing: Spearphishing Attachment" and T1071.004 "Application Layer Protocol: DNS" as attackers can leverage the XSS to establish persistent access or redirect users to malicious infrastructure. The authentication requirement for exploitation means that attackers need to target users with valid monitoring credentials, but the impact on system integrity and information confidentiality remains significant.
Organizations should implement immediate mitigations including input validation and output encoding for all user-supplied parameters in the extinfo.cgi component. The recommended approach involves implementing proper HTML escaping for all dynamic content before rendering it in web responses, ensuring that any special characters in user input are properly encoded to prevent execution as HTML or JavaScript. Additionally, organizations should deploy Content Security Policy (CSP) headers to limit the sources from which scripts can be executed, providing an additional layer of protection against XSS attacks. The most effective long-term solution requires updating to Thruk version 2.41 or later, where the vulnerability has been patched through proper input sanitization and parameter validation. Security teams should also implement monitoring for suspicious URL patterns in web server logs, particularly focusing on requests containing unusual characters or JavaScript payloads in the host and service parameters. Regular security assessments of web applications should include thorough testing for XSS vulnerabilities, with particular attention to reflected parameters in CGI scripts and web interfaces that handle user input for display purposes.