CVE-2018-5121 in Firefox
Summary
by MITRE
Low descenders on some Tibetan characters in several fonts on OS X are clipped when rendered in the addressbar. When used as part of an Internationalized Domain Name (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 < 58.
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Analysis
by VulDB Data Team • 07/21/2024
This vulnerability represents a sophisticated text rendering issue that exploits font rendering inconsistencies in macOS operating systems to enable domain name spoofing attacks. The flaw specifically affects the display of Tibetan characters in certain fonts where the low descenders of these characters are being clipped during rendering in the browser address bar. This clipping behavior creates a visual deception opportunity that can be leveraged by attackers to craft malicious Internationalized Domain Names that appear legitimate to users. The vulnerability stems from how macOS handles font rendering for specific character sets, particularly those with complex glyph structures like Tibetan script, where the lower portions of characters are being truncated during display operations. This issue is particularly concerning because it operates at the visual presentation layer rather than the underlying network protocols, making it difficult to detect through traditional security monitoring approaches. The technical implementation involves font rendering engines that fail to properly account for the full glyph dimensions when displaying characters with extensive descenders, creating a visual discrepancy that can be exploited for phishing attacks.
The operational impact of this vulnerability extends beyond simple visual deception to create serious security risks in the context of Internationalized Domain Name attacks. When attackers craft malicious domains using Tibetan characters that appear identical or visually similar to legitimate domains, users may be misled into believing they are visiting trusted websites. This type of attack specifically targets the user's visual confirmation process rather than network-level security mechanisms, making it particularly effective against less security-conscious users. The vulnerability's scope is limited to macOS systems due to the specific font rendering behavior of the operating system's graphics subsystem, while other platforms like Windows or Linux handle these characters differently and remain unaffected. This platform-specific nature means that security teams must consider the operating environment when assessing risk, as the vulnerability does not manifest on all systems. The attack vector relies on the user's visual interpretation of the address bar content, which represents a fundamental weakness in user interface security design where visual presentation can be manipulated to deceive users.
The mitigation strategies for this vulnerability involve multiple layers of defense that address both the immediate rendering issue and broader security implications. Browser vendors have implemented fixes in versions 58 and later by modifying how domain names are displayed and validated, including enhanced IDN validation routines that prevent the use of visually similar characters from different scripts in the same domain. System-level updates to font rendering engines on macOS have been deployed to ensure proper glyph display for all character sets, particularly those with complex visual characteristics. Security awareness training programs should emphasize the importance of verifying full domain names rather than relying solely on visual appearance, particularly when dealing with internationalized content. Organizations should implement network monitoring solutions that can detect and alert on suspicious IDN patterns, even though the vulnerability itself is limited to macOS environments. This issue relates to CWE-20, which addresses improper input validation, and aligns with ATT&CK technique T1566.001 for credential theft through phishing, as the vulnerability enables attackers to create deceptive domains that can trick users into revealing sensitive information. The fix demonstrates the importance of comprehensive character set handling in security-critical applications, as proper font rendering becomes a security consideration rather than merely a user experience issue.