CVE-2017-8808 in MediaWiki
Summary
by MITRE
MediaWiki before 1.27.4, 1.28.x before 1.28.3, and 1.29.x before 1.29.2 has XSS when the $wgShowExceptionDetails setting is false and the browser sends non-standard URL escaping.
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Analysis
by VulDB Data Team • 01/23/2021
MediaWiki versions prior to 1.27.4, 1.28.x before 1.28.3, and 1.29.x before 1.29.2 contain a cross-site scripting vulnerability that arises from improper handling of URL encoding when the $wgShowExceptionDetails configuration setting is disabled. This vulnerability specifically manifests when browsers send non-standard URL escaping sequences that are not properly sanitized or validated by the application's input processing mechanisms. The flaw occurs because MediaWiki fails to adequately normalize or validate URL parameters before processing them, creating an opportunity for malicious actors to inject malicious scripts into the application's response. When $wgShowExceptionDetails is set to false, the application typically suppresses detailed error information from being displayed to users, but this setting does not prevent the underlying input sanitization issues from persisting. The vulnerability stems from the application's reliance on browser-specific URL decoding behavior rather than implementing consistent server-side validation of all incoming URL parameters. This creates a scenario where attackers can craft malicious URLs containing encoded script payloads that bypass normal input filtering mechanisms. The technical implementation flaw aligns with CWE-79 Cross-site Scripting, specifically targeting the improper neutralization of input during web page generation. Attackers can exploit this vulnerability by constructing URLs with non-standard percent-encoding sequences that, when processed by the vulnerable MediaWiki instance, result in script execution within the context of a victim's browser session. This vulnerability represents a classic case of inadequate input validation and sanitization that allows malicious content to be rendered as part of the web application's output. The operational impact is significant as it enables attackers to execute arbitrary JavaScript code in the context of authenticated users, potentially leading to session hijacking, data theft, or privilege escalation. The vulnerability operates at the application layer and can be classified under ATT&CK technique T1213 Data from Information Repositories, as it allows for unauthorized access to user data through script injection. The attack vector requires a user to navigate to a maliciously crafted URL, making it a server-side request forgery variant that leverages browser-specific URL handling behaviors. Organizations running vulnerable MediaWiki instances should immediately upgrade to the patched versions to mitigate this risk. The fix implemented in the patched versions typically involves implementing more robust URL parameter validation and normalization routines that ensure consistent processing regardless of browser-specific URL encoding behaviors. This vulnerability demonstrates the critical importance of server-side input validation and the dangers of relying on client-side behaviors for security purposes, as it can be exploited even when the application is configured to suppress detailed error information. The security implications extend beyond simple script execution, as this vulnerability could potentially be chained with other exploits to gain deeper access to the application or its underlying systems.
The vulnerability represents a sophisticated attack surface that combines improper input validation with browser-specific URL handling behaviors. The root cause lies in MediaWiki's failure to implement consistent URL parameter sanitization across all supported browsers and their varying approaches to URL decoding. When $wgShowExceptionDetails is false, the application assumes that error details are not exposed to users, but this configuration does not address the fundamental input validation weakness that allows malicious payloads to be processed and rendered. The flaw specifically exploits the difference between standard URL decoding and non-standard encoding sequences that browsers may send, creating a gap in the application's security controls. This type of vulnerability is particularly dangerous because it can be exploited without requiring user interaction beyond visiting a malicious URL, making it a server-side scripting vulnerability that bypasses traditional client-side security measures. The attack pattern follows established security patterns described in ATT&CK framework under T1566 Phishing with Malicious Attachments, as the malicious URL serves as the delivery mechanism for the XSS payload. The vulnerability's exploitation requires understanding of both MediaWiki's internal URL processing logic and browser-specific URL encoding behaviors, making it a sophisticated target for advanced persistent threats. Organizations should implement comprehensive input validation policies that normalize all URL parameters before processing, regardless of their encoding format. The remediation strategy involves updating the application's URL handling routines to ensure consistent parameter validation and sanitization, which addresses the underlying CWE-79 weakness while maintaining compatibility across different browser implementations. This vulnerability underscores the importance of defensive programming practices and the necessity of treating all input as potentially malicious, even when application-level protections such as error suppression are in place.