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.
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.
- 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.
package main import ( "fmt" "github.com/ory/nosurf" "html/template" "net/http" ) var templateString string = ` <!doctype html> <html> <body> {{ if .name }} <p>Your name: {{ .name }}</p> {{ end }} <form action="/" method="POST"> <input type="text" name="name"> <!-- Try removing this or changing its value and see what happens --> <input type="hidden" name="csrf_token" value="{{ .token }}"> <input type="submit" value="Send"> </form> </body> </html> ` var templ = template.Must(template.New("t1").Parse(templateString)) func myFunc(w http.ResponseWriter, r *http.Request) { context := make(map[string]string) context["token"] = nosurf.Token(r) if r.Method == "POST" { context["name"] = r.FormValue("name") } templ.Execute(w, context) } func main() { myHandler := http.HandlerFunc(myFunc) fmt.Println("Listening on http://127.0.0.1:8000/") http.ListenAndServe(":8000", nosurf.New(myHandler)) }
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 }
- Find an issue that bugs you / open a new one.
- Discuss.
- Branch off, commit, test.
- Make a pull request / attach the commits to the issue.