---
id: CVE-2021-21291
aliases:
  - GHSA-4mf2-f3wh-gvf2
  - BIT-oauth2-proxy-2021-21291
  - GO-2022-0790
title: >-
  Subdomain checking of whitelisted domains could allow unintended redirects in
  oauth2-proxy
summary: >-
  Subdomain checking of whitelisted domains could allow unintended redirects in
  oauth2-proxy
severity: medium
cvss: 5.4
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N'
vendor: oauth2-proxy
product: github.com/oauth2-proxy/oauth2-proxy/v7
ecosystem: go
affected:
  - github.com/oauth2-proxy/oauth2-proxy/v7 < 7.0.0
  - github.com/oauth2-proxy/oauth2-proxy <= 3.2.0
patched:
  - github.com/oauth2-proxy/oauth2-proxy/v7 7.0.0
published: '2021-05-25'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:49:04.327848096Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-4mf2-f3wh-gvf2'
references:
  - url: >-
      https://github.com/oauth2-proxy/oauth2-proxy/security/advisories/GHSA-4mf2-f3wh-gvf2
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-21291'
  - url: >-
      https://github.com/oauth2-proxy/oauth2-proxy/commit/780ae4f3c99b579cb2ea9845121caebb6192f725
  - url: 'https://github.com/oauth2-proxy/oauth2-proxy/releases/tag/v7.0.0'
  - url: 'https://pkg.go.dev/github.com/oauth2-proxy/oauth2-proxy/v7'
tags:
  - osv
  - go
epss: 0.01576
epssPercentile: 0.74417
ingestedAt: '2026-09-12T03:13:01.753Z'
---

## Overview

### 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"
)

func validOptionalPort(port string) bool {
	if port == "" || port == ":*" {
		return true
	}
	if port[0] != ':' {
		return false
	}
	for _, b := range port[1:] {
		if b < '0' || b > '9' {
			return false
		}
	}
	return true
}

func splitHostPort(hostport string) (host, port string) {
	host = hostport

	colon := strings.LastIndexByte(host, ':')
	if colon != -1 && validOptionalPort(host[colon:]) {
		host, port = host[:colon], host[colon+1:]
	}

	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") {
		host = host[1 : len(host)-1]
	}

	return
}

func main() {
	domain := ".example.com"
	domainHostname, _ := splitHostPort(strings.TrimLeft(domain, "."))
	redirectHostname := "https://hack.alienexample.com"
	if (strings.HasPrefix(domain, ".") && strings.HasSuffix(redirectHostname, domainHostname)) { fmt.Println("This should not have happen.")}
}

```

Users of `github.com/oauth2-proxy/oauth2-proxy` are advised to update to `github.com/oauth2-proxy/oauth2-proxy/v7`

## Affected packages

- `github.com/oauth2-proxy/oauth2-proxy/v7 < 7.0.0`
- `github.com/oauth2-proxy/oauth2-proxy <= 3.2.0`

## Remediation

Upgrade to a patched release:

- `github.com/oauth2-proxy/oauth2-proxy/v7 7.0.0`