CVE-2018-5140 in Firefox
Summary
by MITRE
Image for moz-icons can be accessed through the "moz-icon:" protocol through script in web content even when otherwise prohibited. This could allow for information leakage of which applications are associated with specific MIME types by a malicious page. This vulnerability affects Firefox < 59.
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Analysis
by VulDB Data Team • 02/05/2021
The vulnerability described in CVE-2018-5140 represents a significant information disclosure flaw in Mozilla Firefox browsers prior to version 59. This issue stems from the improper handling of the moz-icon protocol which is a special URI scheme used by Firefox to display application icons for various file types. The vulnerability allows malicious web pages to exploit the moz-icon protocol to indirectly access information about the local application associations and MIME type mappings on the victim's system. When a web page attempts to load an image using the moz-icon: protocol, it should normally be restricted by the browser's security model to prevent unauthorized access to local resources. However, this flaw permitted script execution within web content to bypass these restrictions and probe the system's MIME type associations. The technical implementation of this vulnerability falls under CWE-200, which specifically addresses information exposure through improper access control mechanisms. The flaw essentially creates a covert channel that enables attackers to gather intelligence about the victim's system configuration by testing various MIME type associations and observing the responses from the browser's icon handling system.
The operational impact of this vulnerability extends beyond simple information leakage, as it provides attackers with valuable reconnaissance data that could be used in subsequent attacks. By enumerating the applications associated with specific MIME types, an attacker can gain insights into the software installed on the victim's system, potentially identifying vulnerable applications or services that could be targeted in further exploitation attempts. This information gathering capability aligns with ATT&CK technique T1082, which covers system information discovery, and T1566, which involves credential access through social engineering or reconnaissance. The vulnerability particularly affects users running Firefox versions earlier than 59, making it a critical concern for organizations with outdated browser deployments. Attackers could leverage this information to craft more targeted phishing campaigns or exploit specific application vulnerabilities based on the discovered software inventory.
The mitigation strategy for CVE-2018-5140 centers on upgrading to Firefox version 59 or later, where Mozilla implemented proper restrictions on the moz-icon protocol access. Additionally, system administrators should enforce browser update policies to ensure all users maintain current versions that contain the relevant security patches. Organizations should also consider implementing network-level controls to monitor for suspicious moz-icon protocol usage patterns, though the primary defense remains the browser update mechanism. The vulnerability demonstrates the importance of maintaining robust protocol handling within browser security models, particularly for internal Firefox-specific protocols that should remain isolated from user-controllable web content. Security teams should monitor for any attempts to exploit this vulnerability through automated scanning tools or web application firewalls that can detect suspicious moz-icon protocol usage patterns in network traffic. The fix implemented by Mozilla involved strengthening the access controls around internal URI schemes to prevent unauthorized script execution from accessing system-level information through web content contexts.