Username Impersonation in Spring Security via X.509 CN Handling

Compliance Impact

The vulnerability allows an attacker to impersonate another user by exploiting malformed X.509 certificate Common Name (CN) values, potentially leading to unauthorized access to user accounts.

Such unauthorized user impersonation can undermine the integrity and confidentiality of user data, which are critical requirements under common standards and regulations like GDPR and HIPAA.

Therefore, while this vulnerability could negatively impact compliance by enabling unauthorized access and potential data breaches, it primarily serves as a defense-in-depth concern rather than a direct standalone attack path.

Upgrading to fixed versions and replacing the deprecated SubjectDnX509PrincipalExtractor with SubjectX500PrincipalExtractor is recommended to mitigate this risk and help maintain compliance.

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Executive Summary

CVE-2026-47838 is a vulnerability in Spring Security related to how the SubjectDnX509PrincipalExtractor handles certain malformed X.509 certificate Common Name (CN) values.

Because of this incorrect handling, the extractor can read the wrong username from a certificate. An attacker who crafts a specially malformed certificate can exploit this to impersonate another user.

This vulnerability affects multiple versions of Spring Security, including 5.7.x, 5.8.x, 6.3.x, 6.4.x, and 6.5.x, and is related to the pre-authentication flow where X.509 client certificates are used.

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Impact Analysis

This vulnerability can allow an attacker to impersonate another user by exploiting the incorrect handling of malformed X.509 certificate CN values.

If exploited, an attacker could gain unauthorized access to systems or data by appearing as a legitimate user, potentially bypassing authentication controls.

However, exploitation requires compromising the upstream trust mechanism since the affected components operate behind Spring Security's pre-authentication flow.

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Mitigation Strategies

To mitigate this vulnerability, it is recommended to upgrade Spring Security to the fixed versions available through Enterprise Support for older releases.

Additionally, replace the deprecated SubjectDnX509PrincipalExtractor with SubjectX500PrincipalExtractor as part of the mitigation strategy.

Note that this vulnerability operates behind Spring Security's pre-authentication flow, so ensuring the upstream trust mechanism is secure is also important.

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Detection Guidance

This vulnerability involves the incorrect handling of malformed X.509 certificate Common Name (CN) values by the SubjectDnX509PrincipalExtractor in Spring Security, which can lead to user impersonation.

Detection would involve inspecting the usage of X.509 client certificates in your Spring Security pre-authentication flow and verifying whether the vulnerable SubjectDnX509PrincipalExtractor is in use.

Since the vulnerability is related to certificate parsing, you can check for malformed or suspicious CN values in client certificates presented to your system.

• Use OpenSSL to inspect client certificates for malformed CN values, for example: openssl x509 -in clientcert.pem -noout -subject
• Review your Spring Security configuration to identify if SubjectDnX509PrincipalExtractor is used instead of the recommended SubjectX500PrincipalExtractor.
• Monitor authentication logs for unexpected username mappings or authentication anomalies that could indicate impersonation attempts.

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