CVE-2022-24880 is a medium-severity vulnerability affecting versions of the Tethik flask-session-captcha package prior to 1.2.1. This package is used to extend Flask web applications by adding an image-based CAPTCHA mechanism that stores the CAPTCHA solution in a server-side session. The vulnerability arises from an incorrect check of the return value of the captcha.validate() function. Specifically, when no value is passed to captcha.validate()—for example, when a form is submitted with an empty CAPTCHA field—the function returns None instead of False. If application developers implement their CAPTCHA validation logic by explicitly checking for a False return value, they may inadvertently allow a bypass of the CAPTCHA verification because None is not equal to False. This logical flaw means that an attacker could submit forms without solving the CAPTCHA and still pass the verification check, effectively bypassing the CAPTCHA protection. The issue is fixed in version 1.2.1 of flask-session-captcha, where the return value handling is corrected. As a workaround, developers can avoid explicitly checking for False and instead use a less strict validation check that treats None as a failure. This vulnerability falls under several CWE categories: CWE-253 (Incorrect Check of Function Return Value), CWE-394 (Unexpected Status Code or Return Value), and CWE-754 (Improper Check for Unusual or Exceptional Conditions). There are no known exploits in the wild at this time, and no CVSS score has been assigned. However, the vulnerability can lead to CAPTCHA bypass, which may facilitate automated attacks such as spam, brute force, or credential stuffing on affected web applications that rely on this package for CAPTCHA protection.

For European organizations, the impact of this vulnerability depends largely on the extent to which flask-session-captcha is used in their web applications. Organizations using vulnerable versions (<1.2.1) of this package may face increased risk of automated abuse, including spam submissions, brute force login attempts, and other forms of automated attacks that CAPTCHA mechanisms are designed to mitigate. This can lead to degraded service quality, increased operational costs due to handling malicious traffic, and potential compromise of user accounts if CAPTCHA bypass is combined with other vulnerabilities. Sectors such as e-commerce, government portals, and financial services that rely on Flask-based web applications with CAPTCHA protection are particularly at risk. While the vulnerability does not directly lead to remote code execution or data leakage, the bypass of CAPTCHA can be a stepping stone for further attacks, undermining the integrity and availability of web services. Given that the flaw requires no authentication and no user interaction beyond submitting a form, exploitation is relatively easy for attackers with basic knowledge. However, the scope is limited to applications using the vulnerable package and implementing the flawed validation logic. The absence of known exploits suggests limited active targeting, but the vulnerability should be addressed promptly to prevent future abuse.

European organizations should take the following specific mitigation steps: 1) Identify all Flask-based web applications using the flask-session-captcha package and determine the version in use. 2) Upgrade all instances of flask-session-captcha to version 1.2.1 or later, where the issue is fixed. 3) Review and update CAPTCHA validation logic in the application code to avoid explicit checks for False and instead use a more robust validation approach that treats None or any non-True value as a failure. 4) Implement additional layers of defense against automated abuse, such as rate limiting, IP reputation checks, and behavioral analysis, to reduce reliance on CAPTCHA alone. 5) Conduct security testing and code reviews focused on input validation and authentication bypass scenarios. 6) Monitor web application logs for unusual patterns of form submissions that might indicate CAPTCHA bypass attempts. 7) Educate development teams about the importance of correctly handling function return values and edge cases in security-critical code. These steps go beyond generic advice by focusing on code-level fixes, layered defenses, and proactive monitoring tailored to this vulnerability's nature.