CVE-2025-57821 is a medium severity vulnerability classified as CWE-601 (Open Redirect) found in the Basecamp google_sign_in library, which integrates Google Sign-In functionality into Ruby on Rails applications. Versions prior to 1.3.0 are affected. The vulnerability arises because it is possible to craft a malformed URL that bypasses the "same origin" check implemented by the library, allowing an attacker to redirect users to an arbitrary external site. This redirection can be exploited in phishing attacks or to facilitate other social engineering techniques. The risk is elevated in Rails applications that store flash messages in session cookies if an attacker can also inject arbitrary data into the session cookie, potentially chaining attacks to manipulate user sessions or steal sensitive information. The vulnerability does not require authentication but does require user interaction (clicking a crafted URL). The CVSS v3.1 base score is 4.2, reflecting a network attack vector with high complexity, no privileges required, user interaction required, and limited impact on confidentiality and integrity but no impact on availability. The issue was patched in version 1.3.0 of the google_sign_in library. If upgrading is not immediately feasible, mitigating the risk involves setting the SameSite attribute on session cookies to Lax or Strict to reduce the risk of session cookie injection and chaining attacks. No known exploits are reported in the wild as of the publication date.

For European organizations using Ruby on Rails applications with the Basecamp google_sign_in library versions older than 1.3.0, this vulnerability could facilitate phishing attacks by redirecting users to malicious external sites that appear to be legitimate. This could lead to credential theft, session hijacking, or delivery of malware. Organizations that rely on flash messages stored in session cookies are at higher risk if attackers can inject data into these cookies, potentially compromising user sessions and data integrity. The impact on confidentiality and integrity is limited but non-negligible, especially for applications handling sensitive user data or financial transactions. The vulnerability does not affect availability directly but could undermine user trust and lead to reputational damage. Since the vulnerability requires user interaction, the risk is somewhat mitigated by user awareness and training but remains significant in environments with high user turnover or less security-conscious users.

1. Upgrade the Basecamp google_sign_in library to version 1.3.0 or later immediately to apply the official patch that fixes the open redirect vulnerability. 2. For applications where upgrading is not immediately possible, explicitly set the SameSite attribute on session cookies to Lax or Strict to prevent session cookie injection and reduce the risk of chained attacks. 3. Implement strict input validation and URL whitelisting on redirect parameters to ensure only trusted URLs are used in redirects. 4. Educate users about the risks of clicking on suspicious links and implement anti-phishing measures such as email filtering and link scanning. 5. Monitor application logs for unusual redirect patterns or suspicious URL parameters that could indicate exploitation attempts. 6. Conduct security code reviews and penetration testing focused on authentication and session management components to identify and remediate related weaknesses.