CVE-2017-7833 in Firefox
Summary
by MITRE
Some Arabic and Indic vowel marker characters can be combined with Latin characters in a domain name to eclipse the non-Latin character with some font sets on the addressbar. The non-Latin character will not be visible to most viewers. 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 internationalized domain name (IDN) homograph attack that exploits the visual similarity between Latin and non-Latin characters in web browser address bars. The flaw specifically targets Arabic and Indic vowel marker characters that can be concatenated with Latin characters to create deceptive domain names that appear identical or nearly identical to legitimate sites. The vulnerability stems from how Firefox handled Unicode character composition and font rendering in the address bar, where certain font sets would visually obscure the non-Latin components of mixed character domain names. This creates a situation where users cannot distinguish between legitimate domains and maliciously crafted variants that exploit the visual similarities between character sets. The attack vector leverages the fact that these combined domain names do not automatically convert to punycode representation, which would normally alert users to the presence of non-Latin characters. This behavior violates security principles outlined in CWE-1004 which addresses weaknesses in the design of character encoding and handling mechanisms. The vulnerability directly maps to techniques described in the ATT&CK framework under T1071.004 for application layer protocol manipulation and T1566 for credential access through social engineering.
The technical implementation of this flaw involves the Unicode bidirectional algorithm and character composition rules that allow combining characters to be visually merged with base characters. In Arabic and Indic scripts, vowel markers and other diacritical marks can be combined with Latin characters to create domain names that render identically to legitimate sites in certain font contexts. When Firefox displays these domains in the address bar, it fails to properly indicate the presence of non-Latin characters, as the visual rendering masks the actual character composition. This creates a false sense of security for users who rely on visual domain inspection for security validation. The vulnerability specifically affects Firefox versions prior to 57, where the IDN handling and address bar rendering logic did not adequately distinguish between different character sets or properly implement visual warnings for mixed character domains.
The operational impact of this vulnerability extends beyond simple phishing attacks to encompass more sophisticated social engineering campaigns that can exploit user trust in familiar domain names. Attackers can register domains that appear identical to well-known sites such as banks, social media platforms, or government services, leading to successful credential theft and data breaches. The attack succeeds because most users do not check for punycode representation or understand the implications of mixed character domains. This vulnerability particularly affects users who frequently interact with internationalized content or who may not be familiar with the visual characteristics of different character sets. The lack of visual indication in the address bar means that even security-conscious users may not recognize the potential threat. Organizations face significant risk as this vulnerability can bypass traditional security measures that rely on visual domain inspection, making it particularly dangerous in enterprise environments where users may not be adequately trained about IDN security implications.
Mitigation strategies for this vulnerability include implementing automatic punycode conversion for mixed character domains, enhancing browser address bar displays to indicate non-Latin character presence, and deploying user education programs about IDN security risks. Firefox addressed this issue by implementing stricter IDN validation and ensuring that mixed character domains are displayed with appropriate visual warnings or automatic punycode conversion. Organizations should also consider implementing DNS-based security measures, deploying web application firewalls that can detect and block suspicious IDN patterns, and establishing security awareness training that specifically addresses the risks of internationalized domain names. The fix aligns with security recommendations from the Internet Society and other standards bodies that emphasize the importance of proper IDN handling to prevent homograph attacks. Additionally, implementing proper input validation at the application level and ensuring that all domain name displays include appropriate character set indicators can help prevent similar vulnerabilities in other software systems. This vulnerability underscores the importance of considering international character sets in security design and the need for comprehensive testing of Unicode handling in security-sensitive applications.