CVE-2026-47838: Spring Security X.509 Certificate DN Parsing Flaw Allows User Impersonation

Technical summary

The SubjectDnX509PrincipalExtractor class fails to properly validate and parse malformed X.509 certificate DN values, specifically in the CN field extraction logic. When a certificate subject name is crafted to contain ambiguous or non-standard CN formatting, the extractor reads an incorrect value and maps it to the wrong principal identity. This violates the authentication contract by allowing a certificate presented by one user to authenticate as a different user. The vulnerability is classified as CWE-287 (Improper Authentication), reflecting the core weakness in identity verification.

Business impact

Organizations relying on X.509 certificate-based authentication with Spring Security face a risk of unauthorized lateral movement and privilege escalation. An authenticated user with a valid certificate could craft a malformed certificate to gain access to another user's resources or permissions. In multi-tenant or federated identity environments, this could enable cross-tenant impersonation. The impact is heightened in systems where certificates are issued by external CAs or where certificate validation is delegated to infrastructure outside direct organizational control.

Affected systems

Spring Security versions 5.7.0–5.7.24, 5.8.0–5.8.26, 6.3.0–6.3.17, 6.4.0–6.4.17, and 6.5.0–6.5.10 are affected. Any Spring application or Spring Boot application using certificate-based authentication via X.509 principal extraction is at risk. The vulnerability applies regardless of whether X.509 authentication is the primary or secondary authentication method.

Exploitability

Exploitation requires an attacker to already be authenticated with a valid certificate (PR:L in the CVSS vector). The attacker must then craft a malformed certificate that causes the extraction logic to resolve to a different username. This is not trivial—it requires understanding the target system's DN parsing behavior and having access to certificate generation or interception capabilities. However, once a valid certificate is obtained, the barrier to crafting a malicious DN is moderate, making this a concern for environments with weaker certificate lifecycle controls.

Remediation

Apply security updates from VMware Spring Security to patch the DN parsing logic. Verify the specific patch versions for your branch (5.7, 5.8, 6.3, 6.4, or 6.5) against the vendor advisory. In addition to patching, implement defense-in-depth by validating certificate DNs at the application level and restricting which certificate issuers are trusted. Monitor authentication logs for anomalous certificate usage or DN values that do not conform to your organization's naming conventions.

Patch guidance

Upgrade to the first patched version available for your currently deployed Spring Security branch. VMware will release patches for all affected versions; consult the official Spring Security security advisory for exact version numbers and timelines. If you are on an older branch (5.7, 5.8), prioritize upgrading to a newer supported release (6.5 or later) rather than waiting for backported patches. Test patches in a staging environment with X.509 authentication enabled before production deployment to ensure certificate validation still works as expected.

Detection guidance

Enable detailed logging in Spring Security's authentication filters and examine DN extraction logs for unexpected or malformed subject names. Alert on authentication success when the extracted DN does not match the expected format for your organization. In environments with SIEM integration, create a correlation rule for certificate authentications where the CN field contains non-standard characters, escape sequences, or length anomalies. Review certificate revocation lists (CRLs) and audit logs for issued certificates with unusual DN values. Network monitoring tools can flag X.509 handshakes followed by privilege escalation or lateral movement.

Why prioritize this

Although the CVSS score of 6.8 (MEDIUM) reflects the requirement for prior authentication, the attack is high-impact and strikes at the root of identity trust. In any system where X.509 authentication is used, this is a critical business logic flaw. Prioritize patching for any organization using certificate-based authentication, especially those with federated or multi-tenant architectures. This vulnerability should not be deprioritized based on CVSS alone—organizational risk depends on reliance on X.509 for authentication.

Risk score, explained

The CVSS 3.1 score of 6.8 reflects a network-exploitable (AV:N) medium-severity impact with high complexity (AC:H) and a requirement for low-level authentication (PR:L). The vector shows high confidentiality and integrity impact (C:H/I:H) but no availability impact. The complexity modifier accounts for the difficulty of crafting a malformed certificate that exploits the specific parsing flaw. Organizations should adjust their risk posture upward if X.509 is a primary authentication mechanism or if certificate issuance is not tightly controlled.

Frequently asked questions