CVE-2026-40574: OAuth2 Proxy has an Authorization Bypass in Email Domain Validation via Malformed Multi-@ Email Claims

Executive brief

### Impact

An authorization bypass exists in OAuth2 Proxy as part of the `email_domain` enforcement option. An attacker may be able to authenticate with an email claim such as `attacker@evil.com@company.com` and satisfy an allowed domain check for `company.com`, even though the claim is not a valid email address.

The issue **ONLY** affects deployments that rely on `email_domain` restrictions and accept email claim values from identity providers or claim mappings that do not strictly enforce normal email syntax. The practical risk ONLY exists in self-hosted or custom OIDC environments and federated setups where unexpected claim values can reach oauth2-proxy. Standard hosted providers that enforce valid email formatting ARE NOT effected.

### Patches Users should upgrade to `v7.15.2` or later once available.

### Workarounds The most effective workaround is to ensure the configured identity provider cannot emit malformed or attacker-controlled email claim values.

Affected products

• Go github.com/oauth2-proxy/oauth2-proxy
• Go github.com/oauth2-proxy/oauth2-proxy/v7

References

• https://api.github.com/users/kodareef5
• https://github.com/kodareef5
• https://api.github.com/users/kodareef5/gists%7B/gist_id%7D
• https://api.github.com/users/kodareef5/repos
• https://avatars.githubusercontent.com/u/261977334?v=4
• https://api.github.com/users/kodareef5/events%7B/privacy%7D