CVE-2016-5251 in Firefox
Summary
by MITRE
Mozilla Firefox before 48.0 allows remote attackers to spoof the location bar via crafted characters in the media type of a data: URL.
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Analysis
by VulDB Data Team • 03/16/2019
The vulnerability identified as CVE-2016-5251 represents a significant security flaw in Mozilla Firefox versions prior to 48.0 that enables remote attackers to manipulate the browser's location bar display through carefully crafted data URLs. This issue falls under the category of user interface deception attacks where malicious actors can exploit the browser's handling of data URI schemes to present misleading information to users. The vulnerability specifically targets the media type parameter within data URLs, which are designed to embed small data items directly into web pages without requiring separate network requests.
The technical implementation of this flaw involves the manipulation of data URI syntax where the media type portion can be crafted with specific character sequences that cause Firefox to misinterpret and display incorrect information in the location bar. Data URLs follow the format data:mime-type;encoding,parameters,data where the mime-type parameter is particularly susceptible to manipulation. Attackers can exploit this by creating data URLs with carefully constructed media type strings that bypass Firefox's normal validation mechanisms, leading to the display of deceptive content that appears to be a legitimate website address.
This vulnerability creates a serious operational impact by undermining user trust in the browser's security indicators and potentially enabling phishing attacks. When users see a spoofed location bar, they may unknowingly interact with malicious websites believing them to be legitimate, as the deceptive display can make it appear as though they are visiting a trusted domain. The attack vector is particularly dangerous because it operates entirely within the browser environment and does not require any special privileges or system access beyond the ability to craft malicious web content. This allows attackers to conduct sophisticated social engineering campaigns that exploit the user's natural assumption that the location bar represents a reliable indicator of website authenticity.
The security implications of this vulnerability extend beyond simple deception to potentially enable more complex attack scenarios including credential theft, malware distribution, and financial fraud. Users who are tricked into believing they are visiting legitimate websites may enter sensitive information or download malicious content that would otherwise be rejected by security controls. This flaw directly impacts the browser's security model by compromising the integrity of the user interface elements that are expected to provide authentication and trust signals. Organizations and individuals using affected Firefox versions face increased risk of targeted attacks that exploit the trust relationship between users and their browsers. The vulnerability demonstrates how seemingly minor implementation details in URI handling can create significant security risks that affect the entire user base.
Mitigation strategies for this vulnerability involve upgrading to Firefox version 48.0 or later where the issue has been resolved through improved validation of data URI media types and enhanced location bar display mechanisms. Security administrators should implement comprehensive browser update policies to ensure all users are protected against this and related vulnerabilities. Additional protective measures include user education about the importance of verifying website addresses through multiple means, implementing browser security extensions that provide additional validation layers, and monitoring for suspicious URL patterns in network traffic. The fix implemented by Mozilla involved strengthening the parsing and validation of data URI components to prevent malicious character sequences from affecting the location bar display, aligning with security best practices for input validation and user interface security. This vulnerability serves as a reminder of the critical importance of proper URI handling and the need for robust validation mechanisms in web browsers to maintain user trust and security.