OAuth2 Proxy is an open-source reverse proxy and static file server that provides authentication using Providers (Google, GitHub, and others) to validate accounts by email, domain or group. In OAuth2 Proxy before version 7.0.0, for users that use the whitelist domain feature, a domain that ended in a similar way to the intended domain could have been allowed as a redirect. For example, if a whitelist domain was configured for ".example.com", the intention is that subdomains of example.com are allowed. Instead, "example.com" and "badexample.com" could also match. This is fixed in version 7.0.0 onwards. As a workaround, one can disable the whitelist domain feature and run separate OAuth2 Proxy instances for each subdomain.

Affected: Go: github.com/oauth2-proxy/oauth2-proxy/v7, Go: github.com/oauth2-proxy/oauth2-proxy

### Impact _What kind of vulnerability is it? Who is impacted?_ For users that use the whitelist domain feature, a domain that ended in a similar way to the intended domain could have been allowed as a redirect.

For example, if a whitelist domain was configured for `.example.com`, the intention is that subdomains of `example.com` are allowed. Instead, `example.com` and `badexample.com` could also match.

### Patches _Has the problem been patched? What versions should users upgrade to?_ This is fixed in version 7.0.0 onwards.

### Workarounds _Is there a way for users to fix or remediate the vulnerability without upgrading?_ Disable the whitelist domain feature and run separate OAuth2 Proxy instances for each subdomain.

# Original Issue Posted by @semoac:

Whitelist Domain feature is not working as expected because is not matching a dot to ensure the redirect is a subdomain.

## Expected Behavior

If whitelist domain is set to `.example.com` , then `hack.alienexample.com` should be rejected as a valid redirect.

## Current Behavior

The code is removing the `dot` from `.example.com` and only checking if the redirect string end with `example.com`

## Possible Solution Here https://github.com/oauth2-proxy/oauth2-proxy/blob/c377466411f2aee180a732187edb638f2f7e57fb/oauthproxy.go#L661

Include the dot when checking the string: ``` strings.HasSuffix(redirectHostname, "." + domainHostname) ```

## Steps to Reproduce (for bugs)

``` package main

import ( "fmt" "strings" )

f…