CVE-2017-7832 in Firefox
Summary
by MITRE
The combined, single character, version of the letter 'i' with any of the potential accents in unicode, such as acute or grave, can be spoofed in the addressbar by the dotless version of 'i' followed by the same accent as a second character with most font sets. This allows for domain spoofing attacks because these combined domain names do not display as punycode. This vulnerability affects Firefox < 57.
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Analysis
by VulDB Data Team • 03/22/2023
This vulnerability represents a sophisticated Unicode-based domain spoofing attack that exploits the visual similarity between certain Unicode characters and standard Latin letters. The flaw specifically targets the letter 'i' and its various accented variants such as í, ì, or î, which can be visually indistinguishable from the standard dotless 'i' character when rendered in typical font sets. This creates a deceptive environment where malicious actors can craft domain names that appear identical to legitimate sites but contain hidden Unicode characters that bypass traditional security mechanisms. The vulnerability stems from how Firefox handles Unicode normalization and character rendering in the address bar, allowing attackers to exploit font rendering inconsistencies to obscure malicious domain names from users.
The technical implementation of this attack leverages the fact that certain Unicode combining characters can be appended to the dotless 'i' character to create visually identical representations of standard Latin letters. When Firefox displays these domain names in the address bar, the rendering engine fails to properly normalize these Unicode sequences, causing the browser to display the spoofed domain without converting it to punycode representation. This normalization failure creates a security gap where users cannot distinguish between legitimate domains and maliciously crafted variants that share the same visual appearance as trusted websites. The vulnerability specifically affects versions prior to Firefox 57, indicating that this was a known issue that required explicit handling of Unicode character sequences in the browser's rendering and validation processes.
The operational impact of this vulnerability extends beyond simple phishing attacks to encompass broader security implications for user trust and browser security models. Attackers can exploit this weakness to create domains that appear legitimate to users, potentially leading to credential theft, financial fraud, or data exfiltration. The attack vector is particularly dangerous because it operates at the user interface level where trust is established, making it difficult for users to identify malicious sites even when they are actively monitoring their browser's address bar. This vulnerability demonstrates the challenges inherent in Unicode handling within web browsers and highlights the importance of proper character normalization and display consistency across different font rendering systems.
The security implications of this vulnerability align with CWE-1160, which addresses improper encoding of Unicode characters in web applications, and relates to ATT&CK technique T1566.001 for spearphishing with a malicious attachment or link. Organizations and security professionals must understand that this vulnerability represents a fundamental flaw in how browsers handle character encoding and display, requiring comprehensive mitigation strategies that include proper Unicode normalization, enhanced input validation, and user education about potential visual spoofing attacks. The fix implemented in Firefox 57 involved stricter Unicode handling and normalization of domain names before display, ensuring that all potentially deceptive character sequences are properly converted to punycode representation to maintain user security awareness.
This vulnerability underscores the critical importance of considering internationalization and localization aspects in security design, as Unicode character handling can create unexpected security gaps in otherwise well-designed systems. The attack demonstrates how seemingly innocuous character encoding issues can be weaponized to bypass user security expectations and trust mechanisms. Security teams should implement monitoring for similar Unicode-based attacks across their systems, particularly in web applications that handle user-provided domain names or URLs, and ensure that all character encoding and display processes follow established security guidelines to prevent similar vulnerabilities from emerging in other software components.