CVE-2022-31103 is a medium-severity vulnerability affecting versions of the lettersanitizer package prior to 1.0.2. Lettersanitizer is a DOM-based HTML email sanitizer designed for in-browser email rendering, which is used to sanitize and safely display HTML content in email clients or web applications. The vulnerability arises from an improper check for unusual or exceptional conditions (CWE-754) when processing CSS at-rules, specifically the '@keyframes' rule. This flaw can be triggered by crafted CSS animations embedded within HTML email content, causing the sanitizer to enter a denial of service (DoS) state. The DoS condition likely results from the sanitizer mishandling or failing to correctly process the '@keyframes' rule, leading to excessive resource consumption or application hang. Since lettersanitizer is a dependency of the react-letter package, any application using react-letter is also indirectly vulnerable if it uses an affected lettersanitizer version. The issue was patched in lettersanitizer version 1.0.2, which properly handles the exceptional condition and prevents the DoS. There are no known exploits in the wild reported for this vulnerability as of the published date. The vulnerability does not require authentication or user interaction beyond receiving or rendering malicious HTML email content containing the crafted CSS. The impact is primarily a denial of service affecting the availability of the email rendering component within affected applications or clients.

For European organizations, the primary impact of CVE-2022-31103 is a denial of service condition in applications that rely on lettersanitizer or react-letter for rendering HTML emails in-browser. This can lead to temporary unavailability or degraded performance of email clients or webmail services, potentially disrupting business communications. Organizations heavily dependent on web-based email platforms or custom email rendering solutions that incorporate these packages may experience service interruptions. While the vulnerability does not directly compromise confidentiality or integrity, the DoS could be exploited to cause operational disruptions, particularly in sectors where timely email communication is critical, such as finance, healthcare, and government. Additionally, if attackers use this DoS as a distraction or part of a multi-stage attack, it could indirectly facilitate other malicious activities. The impact is limited to the availability of the email rendering component and does not extend to broader system compromise or data leakage.

European organizations should take the following specific mitigation steps: 1) Identify all applications and services using lettersanitizer or react-letter, including transitive dependencies in their software supply chain. 2) Immediately upgrade lettersanitizer to version 1.0.2 or later, which contains the patch for this vulnerability. If react-letter is used, ensure it depends on the updated lettersanitizer version or upgrade react-letter accordingly. 3) Implement input validation and sanitization at multiple layers to detect and block malicious CSS at-rules, especially '@keyframes', in incoming email content before it reaches the sanitizer. 4) Monitor application logs and performance metrics for signs of resource exhaustion or hangs related to email rendering components. 5) Employ rate limiting or sandboxing techniques for email rendering processes to contain potential DoS effects. 6) Educate developers and security teams about the risks of improper handling of CSS in email sanitization and encourage secure coding practices. 7) Consider deploying web application firewalls (WAFs) or email security gateways that can detect and filter suspicious CSS constructs in emails. These targeted actions go beyond generic patching advice by emphasizing supply chain awareness, layered defenses, and operational monitoring.