CVE-2025-46721 is a medium-severity vulnerability affecting versions of the Go middleware 'nosurf' prior to 1.2.0. Nosurf is designed to protect web applications from Cross-Site Request Forgery (CSRF) attacks by validating that state-changing requests originate from the legitimate user. The vulnerability arises due to improper handling of HTTP requests within the Go 'net/http' library, which causes nosurf to treat all incoming requests as plain-text HTTP requests. Consequently, nosurf fails to validate the 'Referer' header to confirm same-origin requests. This flaw can be exploited by an attacker who controls content on the target site or any of its subdomains, for example via Cross-Site Scripting (XSS) or other means. By controlling HTML content on a subdomain (e.g., attacker.example.com), the attacker can manipulate cookies scoped to the main domain (example.com). This allows them to either extract the secret CSRF token from the cookie or overwrite it with a token known to the attacker. With this capability, the attacker can craft malicious cross-site requests that bypass the CSRF protections, effectively performing unauthorized actions on behalf of the user. The vulnerability does not require authentication but does require user interaction, such as visiting a malicious page. A patch was released in nosurf version 1.2.0 that addresses this issue. Until upgrading, users can mitigate the risk by deploying additional HTTP middleware that enforces same-origin policies on unsafe HTTP requests, for example by requiring the 'Sec-Fetch-Site: same-origin' header. There are no known exploits in the wild at this time.

For European organizations, this vulnerability poses a significant risk to web applications that utilize the nosurf middleware for CSRF protection, particularly those built with the Go programming language. Exploitation could lead to unauthorized actions performed on behalf of authenticated users, potentially resulting in data manipulation, unauthorized transactions, or privilege escalation within web applications. The ability to bypass CSRF protections undermines the integrity of user sessions and can facilitate further attacks such as account takeover or data leakage. Given the reliance on cookies for session management in many European enterprise and government web services, attackers controlling subdomains or injecting malicious content could leverage this vulnerability to compromise sensitive operations. This is especially critical for sectors with high regulatory requirements such as finance, healthcare, and public administration, where unauthorized actions could lead to compliance violations and reputational damage. The medium CVSS score reflects the moderate ease of exploitation combined with the significant impact on integrity and user trust. However, the absence of known exploits suggests that immediate widespread risk is limited, but proactive patching is essential to prevent future exploitation.

1. Upgrade nosurf middleware to version 1.2.0 or later immediately to apply the official patch addressing this vulnerability. 2. Implement additional HTTP middleware that enforces strict same-origin policies on unsafe HTTP requests, such as requiring the 'Sec-Fetch-Site: same-origin' header, to provide defense-in-depth. 3. Review and restrict subdomain creation and content control within your organization to minimize the risk of attackers controlling subdomains that can manipulate cookies. 4. Employ Content Security Policy (CSP) headers to reduce the risk of XSS vulnerabilities that could facilitate control over subdomain content. 5. Conduct thorough security testing of web applications to identify and remediate any XSS or other injection vulnerabilities that could be leveraged in conjunction with this CSRF bypass. 6. Monitor web application logs for unusual or unauthorized requests that may indicate exploitation attempts. 7. Educate developers about secure cookie handling, including setting the HttpOnly and Secure flags, and proper CSRF mitigation techniques beyond relying solely on middleware.