CVE-2019-11730 in Firefox
Summary
by MITRE
A vulnerability exists where if a user opens a locally saved HTML file, this file can use file: URIs to access other files in the same directory or sub-directories if the names are known or guessed. The Fetch API can then be used to read the contents of any files stored in these directories and they may uploaded to a server. It was demonstrated that in combination with a popular Android messaging app, if a malicious HTML attachment is sent to a user and they opened that attachment in Firefox, due to that app's predictable pattern for locally-saved file names, it is possible to read attachments the victim received from other correspondents. This vulnerability affects Firefox ESR < 60.8, Firefox < 68, and Thunderbird < 60.8.
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Analysis
by VulDB Data Team • 11/06/2023
This vulnerability represents a critical cross-origin access flaw that exploits the Fetch API's ability to read local files through file: URIs when HTML content is opened locally. The security issue stems from Firefox's insufficient restriction of file access when processing local HTML files, allowing malicious content to traverse directory structures and access sensitive files stored in the same directory or subdirectories. The vulnerability specifically targets the Fetch API implementation which permits file: URI access without proper sandboxing mechanisms, creating an attack vector where local file contents can be read and exfiltrated. This flaw exists because the browser fails to properly enforce the same-origin policy when dealing with local file access patterns, particularly when HTML files are opened directly from the filesystem rather than served over HTTP protocols.
The operational impact of this vulnerability extends beyond simple information disclosure to include potential data exfiltration and privacy breaches. When combined with predictable filename patterns used by popular Android messaging applications, attackers can exploit the vulnerability to read sensitive attachments from victims' devices. The attack chain begins with a malicious HTML attachment delivered via email or messaging, which when opened in Firefox triggers the Fetch API to access local files using file: URIs. The vulnerability affects multiple Mozilla products including Firefox ESR versions prior to 60.8, Firefox versions prior to 68, and Thunderbird versions prior to 60.8, indicating a widespread impact across the browser ecosystem. The exploitability is particularly concerning because it requires no network connectivity once the malicious file is opened locally, making it a client-side attack that can persistently access victim data.
The technical implementation of this vulnerability aligns with CWE-20: Improper Input Validation and CWE-284: Improper Access Control, as it demonstrates inadequate validation of file access requests and insufficient access controls for local file operations. The attack pattern follows the MITRE ATT&CK technique T1059.007 for Command and Scripting Interpreter: JavaScript, where malicious JavaScript code leverages the Fetch API to execute file access operations. The vulnerability essentially bypasses the browser's security model by allowing local file access through the Fetch API without proper origin validation, creating a path for arbitrary file reading that can be exploited to access sensitive user data. This flaw represents a fundamental weakness in how Firefox handles local file access and demonstrates the importance of proper sandboxing mechanisms for local content execution.
Mitigation strategies should focus on updating affected browser versions to the patched releases, which implement proper restrictions on file: URI access through the Fetch API. Users should avoid opening untrusted HTML files locally and should be educated about the risks of opening attachments from unknown sources. The recommended remediation includes applying security patches promptly, implementing content security policies that restrict file access, and using browser security features such as sandboxing for local file handling. Organizations should also consider network-level protections such as email filtering to prevent delivery of malicious HTML attachments and implement security awareness training to reduce the likelihood of users opening potentially malicious files. The vulnerability highlights the need for comprehensive security testing of browser APIs and the importance of maintaining strict access controls even for local file operations.