CVE-2019-9970 in Open Whisper Signal
Summary
by MITRE
Open Whisper Signal (aka Signal-Desktop) through 1.23.1 and the Signal Private Messenger application through 4.35.3 for Android are vulnerable to an IDN homograph attack when displaying messages containing URLs. This occurs because the application produces a clickable link even if (for example) Latin and Cyrillic characters exist in the same domain name, and the available font has an identical representation of characters from different alphabets.
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Analysis
by VulDB Data Team • 08/04/2023
The vulnerability CVE-2019-9970 represents a critical identity spoofing risk in the Signal messaging applications that affects both desktop and mobile platforms. This issue stems from the application's handling of internationalized domain names where the display logic fails to properly distinguish between characters from different alphabets that appear visually identical. The vulnerability specifically impacts Signal-Desktop versions through 1.23.1 and Signal Private Messenger for Android through 4.35.3, creating a scenario where malicious actors can craft deceptive URLs that appear legitimate to users while actually directing to fraudulent destinations.
The technical flaw manifests in the application's URL rendering engine which does not implement proper internationalized domain name (IDN) validation or display normalization. When a message contains a URL with mixed character sets such as Latin and Cyrillic characters, the system renders the link as clickable without warning users about potential visual spoofing. This occurs because certain fonts used by the applications render visually identical characters from different alphabets, such as the latin letter 'o' and the cyrillic letter 'о' which are indistinguishable to the naked eye. The vulnerability falls under CWE-1004 which addresses insecure default conditions in internationalization and localization contexts, specifically the failure to properly handle character set conflicts in domain name rendering.
The operational impact of this vulnerability is significant as it enables sophisticated phishing attacks where attackers can create domain names that appear authentic to users. An attacker could register a domain name that visually resembles a legitimate service by substituting latin characters with their cyrillic counterparts, making it appear as though the user is visiting a trusted website when they are actually navigating to a malicious domain. This type of attack leverages the fundamental weakness in visual character representation across different writing systems and can be particularly effective against users who are not aware of the underlying character set differences. The attack vector aligns with ATT&CK technique T1566.001 which covers spearphishing via social media and T1584.002 which involves developing capabilities for social engineering attacks.
The vulnerability creates a persistent security risk because users are not alerted to the potential deception when clicking on links, and the visual similarity makes it extremely difficult for even technically savvy users to distinguish between legitimate and malicious domains. The impact extends beyond simple phishing attacks as it undermines the trust model that Signal applications rely on for secure communications, potentially allowing attackers to bypass the security measures that users expect from the application. Organizations and individuals using these vulnerable versions face increased risk of credential theft, financial fraud, and data breaches through successful social engineering campaigns that exploit this visual homograph attack vector. The vulnerability demonstrates the critical importance of proper internationalization handling in security-critical applications and highlights the need for robust character set validation in URL rendering systems.
Mitigation strategies should include immediate updates to the latest versions of Signal applications where the vulnerability has been addressed through proper IDN handling and display normalization. Users should also implement additional security measures such as URL preview features that display the actual domain name in a clear format, enabling visual verification before clicking. System administrators should consider implementing network-level protections such as DNS filtering and URL reputation services to detect and block suspicious domains. The fix typically involves implementing proper Unicode normalization and IDN validation routines that ensure domain names are displayed in a consistent and unambiguous format, preventing the visual spoofing that enables this attack. Additionally, user education about the risks of clicking on unverified links and the importance of verifying domain names through alternative means should be emphasized to reduce the attack surface.