CVE-2017-7763 in Firefox
Summary
by MITRE
Default fonts on OS X display some Tibetan characters as whitespace. When used in the addressbar as part of an IDN this can be used for domain name spoofing attacks. Note: This attack only affects OS X operating systems. Other operating systems are unaffected. 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-2017-7763 represents a sophisticated internationalized domain name (IDN) spoofing attack that exploits font rendering inconsistencies in Apple's operating systems. This flaw specifically impacts macOS versions where certain Tibetan characters are displayed as whitespace when rendered using default system fonts, creating a critical security gap that can be exploited by attackers to deceive users into visiting malicious websites. The vulnerability stems from the fundamental mismatch between how Unicode characters are processed and displayed in different font contexts, particularly when dealing with complex scripts that have multiple valid character representations.
The technical implementation of this vulnerability relies on the fact that macOS default fonts handle certain Tibetan Unicode code points in a manner that renders them invisible or as empty spaces, while the same characters would normally appear as distinct glyphs in other font contexts. This rendering inconsistency becomes particularly dangerous when combined with IDN homograph attacks, where attackers register domain names that contain visually similar but technically different characters. The attack specifically targets the address bar display mechanism, where the invisible or whitespace characters can be strategically placed within domain names to create deceptive URLs that appear legitimate to users. This flaw operates at the intersection of typography, internationalization, and web security, demonstrating how seemingly benign font rendering decisions can create significant attack vectors.
The operational impact of this vulnerability extends beyond simple visual deception to potentially enable sophisticated phishing attacks and credential theft operations. When combined with other IDN-related vulnerabilities and the specific font rendering behavior on macOS, attackers can create domain names that visually appear identical to legitimate websites while actually resolving to malicious endpoints. The fact that this affects Firefox, Firefox ESR, and Thunderbird versions below the specified thresholds indicates that the vulnerability exists at the application layer rather than being purely a system-level issue, suggesting that the browser's handling of internationalized domain names and font rendering processes contains the exploitable flaw. This vulnerability directly relates to CWE-1004 which addresses insecure default configurations in software applications, and can be mapped to ATT&CK technique T1071.004 for Application Layer Protocol: DNS to demonstrate the exploitation pathway through domain name resolution and display mechanisms.
Mitigation strategies for this vulnerability require a multi-layered approach that addresses both the immediate rendering issue and broader security practices. System administrators and users should immediately update affected browsers to versions that properly handle IDN character validation and display, particularly focusing on Firefox 54, Firefox ESR 52.2, and Thunderbird 52.2 releases which contain the necessary patches. The recommended approach includes implementing strict IDN validation policies that prevent the use of visually similar characters in domain name registration, combined with user education about recognizing suspicious URL patterns. Organizations should also consider implementing network-level filtering solutions that can detect and block potentially spoofed domain names, while browser vendors should enhance their IDN handling to ensure consistent character rendering across all font contexts. Additionally, the vulnerability highlights the importance of comprehensive internationalization testing in security reviews, particularly when dealing with complex scripts that have multiple valid character representations, and should be considered within the broader context of secure software development practices that address Unicode handling and internationalization concerns.