nosurf is cross-site request forgery (CSRF) protection middleware for Go. A vulnerability in versions prior to 1.2.0 allows an attacker who controls content on the target site, or on a subdomain of the target site (either via XSS, or otherwise) to bypass CSRF checks and issue requests on users behalf. Due to misuse of the Go net/http library, nosurf categorizes all incoming requests as plain-text HTTP requests, in which case the Referer header is not checked to have the same origin as the target webpage. If the attacker has control over HTML contents on either the target website (e.g. example.com), or on a website hosted on a subdomain of the target (e.g. attacker.example.com), they will also be able to manipulate cookies set for the target website. By acquiring the secret CSRF token from the cookie, or overriding the cookie with a new token known to the attacker, attacker.example.com is able to craft cross-site requests to example.com. A patch for the issue was released in nosurf 1.2.0. In lieu of upgrading to a patched version of nosurf, users may additionally use another HTTP middleware to ensure that a non-safe HTTP request is coming from the same origin (e.g. by requiring a Sec-Fetch-Site: same-origin header in the request).
The web application does not, or can not, sufficiently verify whether a well-formed, valid, consistent request was intentionally provided by the user who submitted the request.