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
package main
import (
"fmt"
"github.com/justinas/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))
}
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
- 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.
