What’s Vulnerable › Vulnerabilities
CVE-2021-21291: Subdomain checking of whitelisted domains could allow unintended redirects
- Severity: medium (CVSS 6.1 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 1.6% probability of exploitation in the next 30 days
- Public exploit or detection template: yes
- Published: 2021-02-02 · updated 2026-06-17
- Weakness: CWE-601
Description
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 products
- oauth2-proxy: before 7.0.0 — fixed in 7.0.0
How to fix
Upgrade to a fixed version: oauth2-proxy 7.0.0. Check the vendor advisory for mitigations.
References
- https://github.com/oauth2-proxy/oauth2-proxy/commit/780ae4f3c99b579cb2ea9845121caebb6192f725
- https://github.com/oauth2-proxy/oauth2-proxy/releases/tag/v7.0.0
- https://github.com/oauth2-proxy/oauth2-proxy/security/advisories/GHSA-4mf2-f3wh-gvf2
- https://pkg.go.dev/github.com/oauth2-proxy/oauth2-proxy/v7
- https://github.com/oauth2-proxy/oauth2-proxy/commit/780ae4f3c99b579cb2ea9845121caebb6192f725
- https://github.com/oauth2-proxy/oauth2-proxy/releases/tag/v7.0.0
- https://github.com/oauth2-proxy/oauth2-proxy/security/advisories/GHSA-4mf2-f3wh-gvf2
- https://pkg.go.dev/github.com/oauth2-proxy/oauth2-proxy/v7
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