CVE-2017-7764 in Firefox
Summary
by MITRE
Characters from the "Canadian Syllabics" unicode block can be mixed with characters from other unicode blocks in the addressbar instead of being rendered as their raw "punycode" form, allowing for domain name spoofing attacks through character confusion. The current Unicode standard allows characters from "Aspirational Use Scripts" such as Canadian Syllabics to be mixed with Latin characters in the "moderately restrictive" IDN profile. We have changed Firefox behavior to match the upcoming Unicode version 10.0 which removes this category and treats them as "Limited Use Scripts.". This vulnerability affects Firefox < 54, Firefox ESR < 52.2, and Thunderbird < 52.2.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2017-7764 represents a sophisticated domain name spoofing attack vector that exploits Unicode character handling in web browsers. This security flaw specifically targets the rendering behavior of the Canadian Syllabics Unicode block, which contains characters that can be visually confused with Latin characters. The issue arises from the browser's handling of internationalized domain names where certain Unicode scripts are classified as "Aspirational Use Scripts" under the prevailing Unicode standard. These characters were previously permitted to be mixed with Latin characters in the moderately restrictive internationalized domain name (IDN) profile, creating opportunities for attackers to craft deceptive domain names that appear legitimate but actually redirect to malicious sites.
The technical implementation of this vulnerability stems from the browser's address bar rendering logic, which fails to properly apply punycode encoding for certain Unicode characters. When a domain name contains characters from the Canadian Syllabics block mixed with Latin characters, the browser displays these characters in their visual form rather than converting them to punycode representation. This visual representation allows attackers to create domain names that look identical or nearly identical to legitimate domains, exploiting the human tendency to overlook subtle character differences when navigating web addresses. The vulnerability specifically affects browsers that implement the older Unicode standard where Canadian Syllabics characters were categorized as "Aspirational Use Scripts" rather than "Limited Use Scripts," enabling their inclusion in mixed-character domain names without proper encoding.
The operational impact of this vulnerability extends beyond simple phishing attacks, as it fundamentally undermines the trust model of web navigation and DNS resolution. Attackers can exploit this weakness to create domain names that visually mimic well-known organizations, financial institutions, or government websites, potentially leading to credential theft, financial fraud, or data breaches. The attack surface is particularly concerning because it targets the most critical part of web browser security - the address bar where users make trust decisions about website authenticity. This vulnerability aligns with the ATT&CK framework's technique T1566, which covers social engineering through phishing and spoofing, and specifically relates to CWE-1013, which addresses the improper handling of Unicode characters in user interfaces. The impact affects not only individual users but also enterprise environments where users may be less familiar with the technical aspects of domain name validation and character encoding.
The mitigation strategy implemented by Firefox developers involved aligning the browser's behavior with the upcoming Unicode 10.0 standard, which reclassified Canadian Syllabics characters as "Limited Use Scripts" rather than "Aspirational Use Scripts." This change effectively prevents the mixing of Canadian Syllabics characters with Latin characters in the address bar, forcing all such characters to be rendered in their proper punycode form. The fix addresses the root cause by modifying the IDN processing algorithm to enforce stricter character classification and rendering rules. Organizations should ensure their browser versions are updated to Firefox 54, Firefox ESR 52.2, or Thunderbird 52.2 to receive this protection, as older versions remain vulnerable to character confusion attacks. The solution demonstrates the importance of proactive Unicode standard adoption in security implementations and highlights the ongoing challenge of balancing internationalization support with security requirements in web applications.