CVE-2026-40574: OAuth2 Proxy has an Authorization Bypass in Email Domain Validation via Malformed Multi-@ Email Claims
Impact
An authorization bypass exists in OAuth2 Proxy as part of the emaildomain 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 emaildomain 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.
Other sources
OAuth2 Proxy is a reverse proxy that provides authentication using OAuth2 providers. Prior to 7.15.2, an authorization bypass exists in OAuth2 Proxy as part of the emaildomain 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 emaildomain restrictions and accept email claim values from identity providers or claim mappings that do not strictly enforce normal email syntax. This vulnerability is fixed in 7.15.2.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-40574?
CVE-2026-40574 has a high severity due to the potential for unauthorized access through an email domain bypass.
How do I fix CVE-2026-40574?
To fix CVE-2026-40574, upgrade to OAuth2 Proxy version 7.15.2 or later.
What is the attack vector for CVE-2026-40574?
The attack vector for CVE-2026-40574 involves an attacker impersonating a user by exploiting the email domain enforcement option.
What impact does CVE-2026-40574 have on applications?
CVE-2026-40574 can allow unauthorized users to authenticate and gain access to sensitive resources.
What is the affected software for CVE-2026-40574?
The affected software for CVE-2026-40574 is the OAuth2 Proxy, specifically versions prior to 7.15.2.