CVE-2017-7825 in Firefox
Summary
by MITRE
Several fonts on OS X display some Tibetan and Arabic 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 < 56, Firefox ESR < 52.4, and Thunderbird < 52.4.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2017-7825 represents a sophisticated character rendering issue that exploits font display inconsistencies on macOS systems to enable domain name spoofing attacks. This flaw specifically manifests when certain Tibetan and Arabic characters are displayed as whitespace in the operating system's font rendering engine, creating a potential security risk that can be leveraged by attackers to deceive users through deceptive web addresses. The issue stems from how macOS handles character display for specific Unicode ranges, particularly affecting fonts that contain glyphs for these complex scripts. When these characters are embedded within internationalized domain names, they can appear as empty spaces or invisible characters, making it possible for malicious actors to craft domain names that visually appear identical to legitimate sites while containing hidden or different character sequences.
The technical implementation of this vulnerability involves the interaction between macOS font rendering subsystems and Unicode character handling, specifically when processing internationalized domain names that contain characters from the Tibetan and Arabic script ranges. The flaw operates at the presentation layer of the operating system, where font glyphs for certain Unicode code points are either missing, improperly rendered, or displayed as whitespace rather than their intended visual representations. This rendering inconsistency creates a situation where attackers can register domain names that contain these problematic characters in positions that are visually indistinguishable from legitimate domains. The vulnerability is particularly dangerous because it exploits the inherent complexity of Unicode character sets and font rendering, which are fundamental components of modern operating systems. The attack vector specifically targets the address bar display functionality, where users expect to see consistent and predictable character rendering for domain names, making it difficult for users to detect when they are being directed to malicious sites.
The operational impact of CVE-2017-7825 extends beyond simple visual deception to represent a significant threat to user security and trust in web navigation. This vulnerability creates conditions where users may be tricked into visiting malicious websites by domain name spoofing techniques that exploit the font rendering inconsistencies. The attack is particularly effective because it targets the user's natural assumption that domain names will be displayed consistently and predictably, which is a fundamental expectation in web browsing behavior. When combined with the fact that this vulnerability affects major browser applications including Firefox, Firefox ESR, and Thunderbird, the potential attack surface becomes substantial. The vulnerability affects versions of these applications that were widely deployed, meaning that a large number of users were potentially exposed to this risk. The impact is further amplified because the deception occurs at the user interface level, making it difficult for users to detect the attack without specialized knowledge or tools. This type of vulnerability falls under the broader category of visual spoofing attacks that leverage the complexity of Unicode character sets and rendering systems to bypass traditional security mechanisms.
The mitigation strategies for CVE-2017-7825 primarily focus on updating affected software components and implementing additional validation mechanisms. Users should immediately update their browsers to versions that address this vulnerability, specifically Firefox 56 or later, Firefox ESR 52.4 or later, and Thunderbird 52.4 or later. System administrators should ensure that all macOS systems are updated to the latest security patches that address font rendering inconsistencies and Unicode handling issues. The vulnerability also highlights the importance of implementing proper domain name validation and display techniques that account for Unicode normalization and character rendering variations. Organizations should consider implementing additional security measures such as DNS-based security solutions and browser security extensions that can detect and warn users about potentially suspicious domain name patterns. From a security architecture perspective, this vulnerability demonstrates the need for robust input validation and character set handling in user-facing applications, particularly those that process internationalized domain names. The issue also underscores the importance of comprehensive testing of font rendering and character display systems in security-critical applications, as these components can become attack vectors when they fail to properly handle complex Unicode character sets. This vulnerability aligns with common weakness enumerations such as CWE-174, which deals with insufficient character set handling, and represents a specific manifestation of broader security concerns related to Unicode normalization and display consistency in web browsers and operating systems.