CVE-2021-20137 in Tower Router
Summary
by MITRE • 12/09/2021
A reflected cross-site scripting vulnerability exists in the url parameter of the /cgi-bin/luci/site_access/ page on the Gryphon Tower router's web interface. An attacker could exploit this issue by tricking a user into following a specially crafted link, granting the attacker javascript execution in the context of the victim's browser.
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Analysis
by VulDB Data Team • 12/15/2021
The vulnerability identified as CVE-2021-20137 represents a critical reflected cross-site scripting flaw within the web interface of Gryphon Tower routers, specifically affecting the /cgi-bin/luci/site_access/ endpoint. This issue resides in the handling of the url parameter, where insufficient input validation and output encoding mechanisms fail to properly sanitize user-supplied data before incorporating it into the web response. The vulnerability manifests when the router's web interface processes a maliciously crafted url parameter without adequate protection against script injection attacks, creating an avenue for unauthorized code execution within the victim's browser context.
The technical exploitation of this vulnerability follows the standard reflected XSS attack pattern where an attacker crafts a malicious URL containing JavaScript payload within the url parameter of the vulnerable endpoint. When a victim user navigates to this crafted link while authenticated to the router's web interface, the malicious script executes in the victim's browser with the privileges and context of the authenticated session. This allows the attacker to potentially steal session cookies, modify interface elements, redirect users to malicious sites, or perform actions on behalf of the authenticated user. The vulnerability operates at the application layer and specifically targets the router's web-based management interface, making it particularly dangerous as it can be exploited against authenticated users with administrative privileges.
The operational impact of CVE-2021-20137 extends beyond simple script execution, as it provides attackers with a potential foothold for more sophisticated attacks within the network infrastructure. Successful exploitation could enable attackers to access sensitive network configuration data, modify access controls, or even gain persistent access to the router's management interface. The vulnerability's reflected nature means that the malicious payload does not need to be stored on the server, making it harder to detect through traditional security scanning methods. This type of vulnerability directly violates the principle of least privilege and can lead to complete compromise of the affected router, potentially providing attackers with network access that extends beyond the immediate device. The attack vector relies heavily on social engineering, as victims must be tricked into clicking malicious links, but once exploited, the impact can be significant for network security and integrity.
Mitigation strategies for CVE-2021-20137 should focus on implementing proper input validation and output encoding mechanisms within the router's web interface. The most effective immediate solution involves sanitizing all user-supplied input parameters, particularly those used in dynamic content generation, through proper encoding techniques such as HTML entity encoding for output contexts. Organizations should also implement Content Security Policy headers to limit script execution capabilities and consider implementing proper parameter validation that rejects or filters out potentially malicious input patterns. The vulnerability aligns with CWE-79, which specifically addresses cross-site scripting flaws, and represents a classic example of how inadequate input validation can create security risks. From an ATT&CK perspective, this vulnerability maps to T1566 for social engineering attacks and T1059 for command and scripting interpreter usage, demonstrating how a single vulnerability can enable multiple attack vectors and techniques within the broader adversary lifecycle. Regular firmware updates and security patches from the vendor should be prioritized, while network segmentation and monitoring for suspicious web traffic can provide additional defense layers against exploitation attempts.