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 user's 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 cannot, sufficiently verify whether a request was intentionally provided by the user who sent the request, which could have originated from an unauthorized actor.