Subdomain checking of whitelisted domains could allow unintended redirects in oauth2-proxy

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.4 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

EPSS

• CVE-2021-21291: 1.59% estimated exploitation activity in the next 30 days; percentile 75.76 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

25 May 2021, 18:42 UTC

Last modified by the source

9 Jan 2023, 05:04 UTC

Source last checked

26 Oct 2026, 14:54 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-4mf2-f3wh-gvf2

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# Patch Next advisory brief: CVE-2021-21291

Advisory: Subdomain checking of whitelisted domains could allow unintended redirects in oauth2-proxy
IDs: CVE-2021-21291, GHSA-4mf2-f3wh-gvf2
Record: https://patchnext.com/advisories/cve-2021-21291 (revision 1, source checked 26 Oct 2026, 14:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: github.com/oauth2-proxy/oauth2-proxy/v7 7.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/oauth2-proxy/oauth2-proxy: affected <= 3.2.0
- Go github.com/oauth2-proxy/oauth2-proxy/v7: affected < 7.0.0; fixed in 7.0.0

## Suggested inspection (read-only)
- Go github.com/oauth2-proxy/oauth2-proxy: check the installed version with your package manager.
- Go github.com/oauth2-proxy/oauth2-proxy/v7: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/oauth2-proxy/oauth2-proxy, github.com/oauth2-proxy/oauth2-proxy/v7.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-4mf2-f3wh-gvf2 (GitHub-reviewed advisory GHSA-4mf2-f3wh-gvf2 lists affected ranges: Go github.com/oauth2-proxy/oauth2-proxy/v7 >=0 =0 <=3.2.0)

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