OAuth2 Proxy before 5.0 has an open redirect vulnerability. Authentication tokens could be silently harvested by an attacker. This has been patched in version 5.0.

Affected: Go: github.com/oauth2-proxy/oauth2-proxy

### Impact An open redirect vulnerability has been found in `oauth2_proxy`. Anyone who uses `oauth2_proxy` may potentially be impacted.

For a context [detectify] have an in depth blog post about the potential impact of an open redirect. Particularly see the OAuth section.

**tl;dr**: People's authentication tokens could be silently harvested by an attacker. e.g: `facebook.com/oauth.php?clientid=123&state=abc&redirect_url=https://yourdomain.com/red.php?url%3dhttps://attacker.com/`

### Patches

@sauyon found the issue, and has submitted a patch.

``` diff --git a/oauthproxy.go b/oauthproxy.go index 72ab580..f420df6 100644 --- a/oauthproxy.go +++ b/oauthproxy.go @@ -517,7 +517,7 @@ func (p *OAuthProxy) GetRedirect(req *http.Request) (redirect string, err error) // 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) ```

This patch will be applied to the next release, which is scheduled for when this is publicly disclosed.

### Workarounds

At this stage there is no work around.