CVE-2021-31409: 
Java vulnerability analysis and mitigation

CVE-2021-31409 is a Regular expression Denial of Service (ReDoS) vulnerability discovered in the EmailValidator class within the V7 compatibility module of Vaadin 8. The vulnerability affects Vaadin versions 8.0.0 through 8.12.4, specifically in the com.vaadin:vaadin-compatibility-server component. The issue was discovered and reported by Stefan Penndorf on March 12, 2021, and was publicly disclosed on April 30, 2021 (Vaadin Security).

The vulnerability stems from an unsafe validation regular expression pattern in the EmailValidator component that is susceptible to exponential backtracking. This vulnerability has been assigned a High severity rating with a CVSS v3.1 base score of 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H). The issue is classified under CWE-400: Uncontrolled Resource Consumption, affecting the server-side email validation functionality (Vaadin Security).

When exploited, this vulnerability can lead to uncontrolled resource consumption and potential Denial of Service conditions. The UI thread of the server can be forced to spend an indefinite amount of time processing malicious input, which can result in thread pool or resource exhaustion. This ultimately makes the application unresponsive to normal users (Vaadin Security).

Users of affected versions are advised to upgrade to Vaadin version 8.13.0 or newer, which contains the fix for this vulnerability. The fix involves updating the regular expression pattern used in the legacy EmailValidator. The issue was addressed through a pull request that modified the validation pattern to prevent the exponential backtracking behavior (Vaadin Security, GitHub PR).