Merge pull request from GHSA-qqxw-m5fj-f7gv

check for /\ redirects

CHANGELOG.md

@@ -17,7 +17,7 @@
 - DigitalOcean provider support added
 
 ## Important Notes
-N/A
+- (Security) Fix for open redirect vulnerability..  a bad actor using `/\` in redirect URIs can redirect a session to another domain
 
 ## Breaking Changes
 

oauthproxy.go

@@ -558,7 +558,7 @@ func validOptionalPort(port string) bool {
 // IsValidRedirect checks whether the redirect URL is whitelisted
 func (p *OAuthProxy) IsValidRedirect(redirect string) bool {
 	switch {
-	case strings.HasPrefix(redirect, "/") && !strings.HasPrefix(redirect, "//"):
+	case strings.HasPrefix(redirect, "/") && !strings.HasPrefix(redirect, "//") && !strings.HasPrefix(redirect, "/\\"):
 		return true
 	case strings.HasPrefix(redirect, "http://") || strings.HasPrefix(redirect, "https://"):
 		redirectURL, err := url.Parse(redirect)