CVE-2019-12581 in ZyWall
Summary
by MITRE
A reflective Cross-site scripting (XSS) vulnerability in the free_time_failed.cgi CGI program in selected Zyxel ZyWall, USG, and UAG devices allows remote attackers to inject arbitrary web script or HTML via the err_msg parameter.
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Analysis
by VulDB Data Team • 06/28/2020
This vulnerability resides within the Zyxel ZyWall, USG, and UAG security appliance product lines, specifically affecting the free_time_failed.cgi CGI program that handles time-based access control functionality. The reflected cross-site scripting flaw represents a critical weakness in the device's input validation mechanisms, where user-supplied data from the err_msg parameter is not properly sanitized before being rendered back to the victim's browser. This vulnerability falls under CWE-79 which defines improper neutralization of input during web page generation, making it a classic example of how insufficient data sanitization can lead to arbitrary code execution within a user's browser context. The attack vector requires remote exploitation without authentication, meaning malicious actors can leverage this flaw from outside the network perimeter to compromise user sessions and potentially escalate privileges within the device management interface.
The operational impact of this vulnerability extends beyond simple script injection, as it provides attackers with the capability to execute malicious JavaScript within the context of a victim's browser session. When a user interacts with the affected device's web interface and encounters an error message, the err_msg parameter containing malicious script content gets reflected back to the browser without proper encoding or filtering. This allows attackers to perform session hijacking, steal authentication tokens, redirect users to malicious sites, or even inject phishing content that appears legitimate to users. The vulnerability specifically affects the device's administrative web interface, making it particularly dangerous as successful exploitation could lead to complete compromise of the security appliance itself. According to ATT&CK framework, this represents a T1059.007 technique involving scripting through command-line interfaces and T1566.001 for social engineering via spearphishing with a link, since the attack requires user interaction with maliciously crafted URLs.
Mitigation strategies for this vulnerability should focus on immediate patching of affected devices through official Zyxel firmware updates, which typically include proper input validation and output encoding for all user-supplied parameters. Network administrators should implement additional security controls such as web application firewalls that can detect and block malicious payloads targeting known XSS patterns in CGI parameters. The principle of least privilege should be enforced by restricting access to the administrative interface through network segmentation and multi-factor authentication mechanisms. Regular security assessments should include vulnerability scanning for similar reflected XSS flaws in other CGI programs and web applications running on network devices. Organizations should also implement proper input validation at multiple layers including application-level filtering, HTTP header configuration to prevent script execution, and regular monitoring of web interface logs for suspicious parameter usage patterns. The vulnerability demonstrates the critical importance of input sanitization in web applications and the necessity of following secure coding practices that align with OWASP Top Ten security guidelines to prevent such persistent threats in network infrastructure devices.