nosurf

nosurf

nosurf is an HTTP package for Go that helps you prevent Cross-Site Request Forgery attacks. It acts like a middleware and therefore is compatible with basically any Go HTTP application.

Why?

Even though CSRF is a prominent vulnerability, Go's web-related package infrastructure mostly consists of micro-frameworks that neither do implement CSRF checks, nor should they.

nosurf solves this problem by providing a CSRFHandler that wraps your http.Handler and checks for CSRF attacks on every non-safe (non-GET/HEAD/OPTIONS/TRACE) method.

nosurf requires Go 1.1 or later.

Features

• Supports any http.Handler (frameworks, your own handlers, etc.) and acts like one itself.
• Allows exempting specific endpoints from CSRF checks by an exact URL, a glob, or a regular expression.
• Allows specifying your own failure handler. Want to present the hacker with an ASCII middle finger instead of the plain old HTTP 400? No problem.
• Uses masked tokens to mitigate the BREACH attack.
• Has no dependencies outside the Go standard library.

Example

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Manual token verification

In some cases the CSRF token may be send through a non standard way, e.g. a body or request is a JSON encoded message with one of the fields being a token.

In such case the handler(path) should be excluded from an automatic verification by using one of the exemption methods:

	func (h *CSRFHandler) ExemptFunc(fn func(r *http.Request) bool)
	func (h *CSRFHandler) ExemptGlob(pattern string)
	func (h *CSRFHandler) ExemptGlobs(patterns ...string)
	func (h *CSRFHandler) ExemptPath(path string)
	func (h *CSRFHandler) ExemptPaths(paths ...string)
	func (h *CSRFHandler) ExemptRegexp(re interface{})
	func (h *CSRFHandler) ExemptRegexps(res ...interface{})

Later on, the token must be verified by manually getting the token from the cookie and providing the token sent in body through: VerifyToken(tkn, tkn2 string) bool.

Example:

func HandleJson(w http.ResponseWriter, r *http.Request) {
	d := struct{
		X,Y int
		Tkn string
	}{}
	json.Unmarshal(ioutil.ReadAll(r.Body), &d)
	if !nosurf.VerifyToken(nosurf.Token(r), d.Tkn) {
		http.Errorf(w, "CSRF token incorrect", http.StatusBadRequest)
		return
	}
	// do smth cool
}

Contributing

1. Find an issue that bugs you / open a new one.
2. Discuss.
3. Branch off, commit, test.
4. Make a pull request / attach the commits to the issue.