CVE-2017-9781 in Check_MK
Summary
by MITRE
A cross site scripting (XSS) vulnerability exists in Check_MK versions 1.4.0x prior to 1.4.0p6, allowing an unauthenticated remote attacker to inject arbitrary HTML or JavaScript via the _username parameter when attempting authentication to webapi.py, which is returned unencoded with content type text/html.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/19/2019
The vulnerability described in CVE-2017-9781 represents a critical cross site scripting flaw in Check_MK monitoring software versions prior to 1.4.0p6. This vulnerability resides within the webapi.py component and specifically targets the authentication handling mechanism where the _username parameter is processed without proper input sanitization. The flaw allows unauthenticated remote attackers to execute malicious code within the context of a victim's browser by injecting arbitrary HTML or JavaScript payloads through the authentication interface.
The technical implementation of this vulnerability stems from improper output encoding practices within the webapi.py script. When an attacker submits a malicious _username parameter during authentication attempts, the system fails to properly encode the user input before returning it in the HTTP response. This occurs because the response is served with content type text/html, making the unescaped user-supplied data directly executable within the browser context. The vulnerability is classified as a classic reflected XSS attack pattern where malicious input is immediately reflected back to the user without adequate sanitization or encoding measures.
The operational impact of this vulnerability extends beyond simple script execution as it provides attackers with the capability to hijack user sessions, steal sensitive monitoring credentials, and potentially escalate privileges within the Check_MK environment. Attackers could craft malicious URLs that, when clicked by authenticated users, would execute malicious scripts to capture session cookies or redirect users to phishing sites. This poses significant risks to system administrators who rely on Check_MK for network monitoring, as compromised authentication interfaces could lead to complete system compromise and unauthorized access to critical infrastructure monitoring data.
Organizations utilizing affected Check_MK versions should immediately implement mitigations including upgrading to version 1.4.0p6 or later, which contains the necessary patches to properly encode user inputs before returning them in HTTP responses. Additional protective measures include implementing proper input validation at the application level, deploying web application firewalls to detect and block suspicious parameter values, and conducting security awareness training for administrators to recognize potential social engineering attempts that might exploit this vulnerability. The vulnerability aligns with CWE-79 which specifically addresses cross site scripting flaws, and represents a technique commonly categorized under ATT&CK tactic TA0001 (Initial Access) and technique T1190 (Exploit Public-Facing Application) in the MITRE ATT&CK framework.