CVE-2008-2168 in HTTP Server
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in Apache 2.2.6 and earlier allows remote attackers to inject arbitrary web script or HTML via UTF-7 encoded URLs that are not properly handled when displaying the 403 Forbidden error page.
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Analysis
by VulDB Data Team • 07/31/2021
The vulnerability described in CVE-2008-2168 represents a critical cross-site scripting flaw within the Apache HTTP Server version 2.2.6 and earlier releases. This security weakness specifically manifests when the server encounters UTF-7 encoded URLs that trigger a 403 Forbidden error response, creating an avenue for remote attackers to execute malicious scripts within the context of a victim's browser session. The flaw exploits the server's inadequate handling of character encoding during error page generation, particularly when processing malformed or maliciously crafted URLs that utilize UTF-7 encoding techniques.
The technical implementation of this vulnerability stems from Apache's insufficient sanitization of user-supplied input during error response generation. When a client requests a resource that triggers a 403 Forbidden error, the server constructs an error page that displays information about the requested URL. However, the server fails to properly decode or sanitize UTF-7 encoded characters in the URL before incorporating them into the HTML output of the error page. This oversight allows attackers to craft URLs containing UTF-7 encoded payloads that, when processed by the server, get interpreted as regular ASCII characters but are actually executed as HTML or JavaScript code within the victim's browser context. The vulnerability specifically impacts the server's error handling mechanism rather than core functionality, making it particularly insidious as it can be triggered through normal web browsing activities without requiring special privileges or authentication.
The operational impact of this vulnerability extends beyond simple script execution, as it provides attackers with the capability to perform session hijacking, defacement of web pages, and data theft from authenticated users. When a victim's browser encounters a maliciously crafted URL that triggers the vulnerable 403 error page, the embedded scripts can access cookies, session tokens, and other sensitive information that the browser stores for the affected website. This vulnerability aligns with CWE-79, which describes cross-site scripting flaws in web applications, and demonstrates how improper input validation can create persistent security risks. The attack vector requires minimal sophistication, as attackers can simply craft URLs with UTF-7 encoding that exploit the server's error handling, making this vulnerability particularly dangerous in environments where users may inadvertently click on malicious links or where automated attacks can be deployed at scale.
Organizations affected by this vulnerability should prioritize immediate patching of their Apache installations to version 2.2.7 or later, which includes the necessary fixes for proper UTF-7 handling during error page generation. Additionally, administrators should implement comprehensive input validation mechanisms at the application level, including the use of Content Security Policy headers to limit script execution capabilities, and deploy web application firewalls to detect and block suspicious URL patterns. The remediation approach should also include regular security assessments of web applications to identify similar encoding-related vulnerabilities, as this flaw demonstrates how seemingly benign character encoding issues can create significant security risks. This vulnerability serves as a critical reminder of the importance of proper input sanitization in web server implementations and aligns with ATT&CK technique T1212, which covers exploitation of software vulnerabilities through encoding techniques. The fix implemented in Apache 2.2.7 specifically addresses the root cause by ensuring that UTF-7 encoded characters are properly decoded or escaped before inclusion in error page content, thereby preventing the injection of malicious scripts through this particular attack vector.