CVE-2017-7807 in Firefox
Summary
by MITRE
A mechanism that uses AppCache to hijack a URL in a domain using fallback by serving the files from a sub-path on the domain. This has been addressed by requiring fallback files be inside the manifest directory. This vulnerability affects Thunderbird < 52.3, Firefox ESR < 52.3, and Firefox < 55.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2017-7807 represents a sophisticated web application security flaw that exploits the Application Cache (AppCache) mechanism to perform unauthorized URL hijacking attacks. This vulnerability specifically targets browsers that implement the HTML5 Application Cache API, which allows web applications to cache resources locally for offline access and improved performance. The flaw enables malicious actors to manipulate the fallback behavior of AppCache to redirect users to unintended destinations, potentially leading to phishing attacks or content injection exploits. The vulnerability affects major browser implementations including Firefox ESR versions prior to 52.3, Firefox versions prior to 55, and Thunderbird versions prior to 52.3, highlighting the widespread impact across web browsers and email clients that utilize similar caching mechanisms.
The technical implementation of this vulnerability relies on the improper handling of AppCache fallback mechanisms within web applications. When a web application defines a fallback entry in its AppCache manifest file, the system should ensure that fallback resources are properly scoped to the manifest directory to prevent cross-domain resource access. However, the flaw allowed attackers to specify fallback URLs that could reference files located in sub-paths of the originating domain, effectively creating a path traversal scenario within the caching system. This misconfiguration enabled attackers to serve malicious content from subdirectories that should have been restricted, allowing them to intercept or modify requests intended for legitimate resources. The vulnerability operates at the level of resource management and caching policies, where the boundary checking between the manifest file and fallback resource locations was insufficient to prevent unauthorized access patterns. This type of flaw aligns with CWE-22 Path Traversal and CWE-200 Information Exposure, as it enables unauthorized access to resources and potentially sensitive data through improper boundary validation.
The operational impact of CVE-2017-7807 extends beyond simple web application manipulation to encompass potential security breaches and user deception attacks. Attackers could leverage this vulnerability to create convincing phishing pages by hijacking legitimate URLs and serving malicious content through the AppCache fallback mechanism. The attack surface includes scenarios where users might be redirected to malicious sites when attempting to access specific resources, particularly in environments where AppCache is actively used for web applications. Email clients like Thunderbird that implement similar caching mechanisms were also affected, suggesting that the vulnerability could be exploited in email content delivery or web-based email interfaces. The impact is particularly concerning for enterprise environments where users might encounter legitimate-looking URLs that actually redirect to malicious destinations, potentially leading to credential theft, malware distribution, or other malicious activities. This vulnerability aligns with several ATT&CK techniques including T1189 Drive-by Compromise and T1071.004 Application Layer Protocol: Web Protocols, as it exploits web application protocols to establish unauthorized access paths.
The remediation approach for CVE-2017-7807 focuses on implementing stricter validation of fallback resource locations within AppCache manifest files. Browser vendors addressed this vulnerability by requiring that fallback files must be located within the same directory as the manifest file, effectively preventing cross-directory access patterns that enabled the exploitation. This fix aligns with defensive programming principles and access control best practices, ensuring that resource boundaries are properly enforced. Organizations should ensure their browser versions are updated to include the patched implementations, particularly in environments where legacy software may still be in use. Security teams should also review existing web applications that utilize AppCache functionality to verify that fallback mechanisms are properly configured and do not introduce similar vulnerabilities. The patch implementation demonstrates the importance of proper resource scoping and boundary validation in web application security, particularly in caching and offline access mechanisms. This vulnerability serves as a reminder of the critical need for comprehensive security testing of web application features that handle resource caching and access control, as seemingly benign functionality can introduce significant security risks when not properly validated.