CVE-2017-7814 in Firefox
Summary
by MITRE
File downloads encoded with "blob:" and "data:" URL elements bypassed normal file download checks though the Phishing and Malware Protection feature and its block lists of suspicious sites and files. This would allow malicious sites to lure users into downloading executables that would otherwise be detected as suspicious. This vulnerability affects Firefox < 56, Firefox ESR < 52.4, and Thunderbird < 52.4.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2017-7814 represents a critical flaw in the web browser security architecture of Firefox and Thunderbird applications. This weakness specifically targets the phishing and malware protection mechanisms that are designed to safeguard users from downloading malicious content. The vulnerability operates by exploiting the handling of specific URL schemes that are commonly used for embedding content directly within web pages or for creating temporary file references. When browsers encounter these encoded URLs, they fail to properly validate the content against established security policies that would normally prevent downloads from suspicious sources.
The technical implementation of this vulnerability stems from how the browser processes "blob:" and "data:" URL protocols which are standard web features for handling binary data and inline content. These URL schemes allow web applications to create temporary references to data without requiring actual file system access or network requests. However, the flaw in Firefox and Thunderbird versions prior to the patched releases enabled attackers to craft malicious URLs that would bypass the normal file download validation procedures. This bypass mechanism specifically targets the block list functionality that should prevent downloads from known malicious domains or file types.
The operational impact of this vulnerability creates a significant risk for users who rely on the built-in security features of these email clients and browsers. Attackers could exploit this weakness by hosting malicious executables on seemingly legitimate websites or by embedding malicious content within email messages that would normally be blocked by the security systems. The vulnerability essentially allows threat actors to circumvent the protection mechanisms that are specifically designed to prevent phishing attacks and malware distribution through email attachments or web downloads. Users would be tricked into downloading executables that would otherwise be detected and blocked by the security systems, potentially leading to full system compromise.
The security implications extend beyond simple file download bypasses and represent a fundamental flaw in the application's threat detection and prevention architecture. This vulnerability aligns with CWE-20, which describes improper input validation, and demonstrates how improper handling of URL schemes can lead to security bypasses. From an attack perspective, this vulnerability maps to several MITRE ATT&CK techniques including T1193 for Spearphishing Attachments and T1059 for Command and Scripting Interpreter, as attackers could leverage this weakness to deliver malicious payloads more effectively. Organizations using affected versions of Firefox or Thunderbird face increased risk of successful phishing campaigns and malware infections, particularly in environments where these applications are commonly used for email and web browsing activities.
The remediation strategy involves updating to the patched versions of Firefox, Firefox ESR, and Thunderbird as specified in the vulnerability details. System administrators should prioritize deployment of these updates across all affected systems, particularly in enterprise environments where users may be exposed to higher risk web content. Additional defensive measures include implementing network-level content filtering, monitoring for suspicious URL patterns, and maintaining updated threat intelligence feeds to identify potential exploitation attempts. Regular security assessments should verify that the updated security features are functioning correctly and that no other similar bypass mechanisms exist within the browser's security architecture.