CVE-2021-38751 in ExponentCMS
Summary
by MITRE • 08/16/2021
A HTTP Host header attack exists in ExponentCMS 2.6 and below in /exponent_constants.php. A modified HTTP header can change links on the webpage to an arbitrary value, leading to a possible attack vector for MITM.
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Analysis
by VulDB Data Team • 08/18/2021
The vulnerability identified as CVE-2021-38751 represents a critical security flaw in ExponentCMS versions 2.6 and earlier, specifically within the /exponent_constants.php file. This issue manifests as an HTTP Host header attack that exploits the application's improper handling of HTTP headers during web request processing. The vulnerability stems from the application's reliance on the Host header value without proper validation or sanitization, creating an environment where malicious actors can manipulate the header to inject arbitrary values into generated URLs and links throughout the web interface.
The technical implementation of this vulnerability allows attackers to modify the Host header in HTTP requests to include malicious domain names or IP addresses. When ExponentCMS processes these requests, it incorporates the modified Host header value directly into the generated links and URLs displayed on web pages, effectively creating a man-in-the-middle attack vector. This flaw directly violates security principles outlined in CWE-20, which addresses improper input handling, and specifically relates to CWE-310, concerning cryptographic issues. The vulnerability enables attackers to redirect users to malicious domains, potentially facilitating phishing attacks, credential theft, or further exploitation of the compromised system.
The operational impact of this vulnerability extends beyond simple link manipulation, as it creates a persistent attack surface that can be exploited across multiple user sessions. When users interact with affected web pages, they may unknowingly navigate to attacker-controlled domains, particularly if the malicious links are embedded in navigation menus, form actions, or redirect mechanisms. This vulnerability is particularly dangerous in environments where users may not carefully inspect URLs or where the application is accessed over untrusted networks, making it susceptible to various attack patterns documented in the MITRE ATT&CK framework under T1190 for exploit public-facing application and T1566 for credential harvesting through social engineering. The attack can be executed without requiring authentication, making it accessible to any user who can send HTTP requests to the vulnerable system.
Mitigation strategies for CVE-2021-38751 should prioritize immediate patching of ExponentCMS to versions 2.7 or later, where the Host header validation has been properly implemented. Organizations should also implement strict input validation for all HTTP headers, particularly the Host header, by implementing whitelisting mechanisms that only accept known, legitimate domain names. Network-level protections such as web application firewalls can provide additional defense-in-depth by monitoring and blocking suspicious Host header values. Security teams should also conduct comprehensive audits of all web applications to identify similar vulnerabilities in other systems and ensure proper header sanitization practices are implemented across the entire infrastructure. The vulnerability demonstrates the critical importance of proper HTTP header handling in web applications and aligns with security best practices outlined in OWASP Top Ten Project, specifically addressing the risks associated with improper input validation and insecure communication protocols.