fix API handler to respect referer + exit on CORS

this commit makes the API handler short circuit the request if the
CORS headers say its not allowed. (the CORS handler only sets the
headers, but does not short-circuit)

It also makes the handler respect the referer again. See security
discussion at ipfs/kubo#1532

License: MIT
Signed-off-by: Juan Batiz-Benet <juan@benet.ai>

http/handler.go

@@ -45,6 +45,13 @@ const (
 	applicationJson        = "application/json"
 	applicationOctetStream = "application/octet-stream"
 	plainText              = "text/plain"
+	originHeader           = "origin"
+)
+
+const (
+	ACAOrigin      = "Access-Control-Allow-Origin"
+	ACAMethods     = "Access-Control-Allow-Methods"
+	ACACredentials = "Access-Control-Allow-Credentials"
 )
 
 var localhostOrigins = []string{
@@ -115,6 +122,13 @@ func (i Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 func (i internalHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	log.Debug("Incoming API request: ", r.URL)
 
+	if !allowOrigin(r, i.cfg) || !allowReferer(r, i.cfg) {
+		w.WriteHeader(http.StatusForbidden)
+		w.Write([]byte("403 - Forbidden"))
+		log.Warningf("API blocked request to %s. (possible CSRF)", r.URL)
+		return
+	}
+
 	req, err := Parse(r, i.root)
 	if err != nil {
 		if err == ErrNotFound {
@@ -310,3 +324,60 @@ func sanitizedErrStr(err error) string {
 	s = strings.Split(s, "\r")[0]
 	return s
 }
+
+// allowOrigin just stops the request if the origin is not allowed.
+// the CORS middleware apparently does not do this for us...
+func allowOrigin(r *http.Request, cfg *ServerConfig) bool {
+	origin := r.Header.Get("Origin")
+
+	// curl, or ipfs shell, typing it in manually, or clicking link
+	// NOT in a browser. this opens up a hole. we should close it,
+	// but right now it would break things. TODO
+	if origin == "" {
+		return true
+	}
+
+	for _, o := range cfg.CORSOpts.AllowedOrigins {
+		if o == "*" { // ok! you asked for it!
+			return true
+		}
+
+		if o == origin { // allowed explicitly
+			return true
+		}
+	}
+
+	return false
+}
+
+// allowReferer this is here to prevent some CSRF attacks that
+// the API would be vulnerable to. We check that the Referer
+// is allowed by CORS Origin (origins and referrers here will
+// work similarly in the normla uses of the API).
+// See discussion at https://github.com/ipfs/go-ipfs/issues/1532
+func allowReferer(r *http.Request, cfg *ServerConfig) bool {
+	referer := r.Referer()
+
+	// curl, or ipfs shell, typing it in manually, or clicking link
+	// NOT in a browser. this opens up a hole. we should close it,
+	// but right now it would break things. TODO
+	if referer == "" {
+		return true
+	}
+
+	// check CORS ACAOs and pretend Referer works like an origin.
+	// this is valid for many (most?) sane uses of the API in
+	// other applications, and will have the desired effect.
+	for _, o := range cfg.CORSOpts.AllowedOrigins {
+		if o == "*" { // ok! you asked for it!
+			return true
+		}
+
+		// referer is allowed explicitly
+		if o == referer {
+			return true
+		}
+	}
+
+	return false
+}