CVE-2025-68121 is a security vulnerability identified in the Go programming language's standard library, specifically within the crypto/tls package responsible for implementing TLS protocols. The issue arises during TLS session resumption, a performance optimization allowing clients and servers to reuse previously negotiated session parameters to avoid a full handshake. The vulnerability is triggered if the TLS Config structure's ClientCAs or RootCAs fields are altered between the initial handshake and the resumed handshake. This situation can occur when developers use Config.Clone to duplicate a Config and then mutate the copy, or when Config.GetConfigForClient is used to dynamically provide configurations per client. Because the resumed handshake relies on the mutated Config, it may incorrectly accept a resumed session that would have been rejected during the initial handshake, effectively bypassing certificate validation checks. This improper validation violates CWE-295 (Improper Certificate Validation), potentially allowing an attacker to resume a session with a client or server that should not have trusted the peer, undermining the TLS trust model. The affected versions include all Go versions prior to 1.26.0-rc.1, including 1.25.0-0 and earlier. No public exploits have been reported to date, and no CVSS score has been assigned. The vulnerability impacts the confidentiality and integrity of TLS sessions by enabling unauthorized session resumptions, which could facilitate man-in-the-middle attacks or unauthorized access if exploited. The flaw requires a specific development pattern (mutating Config between handshakes) but does not require user interaction or authentication to exploit once conditions are met.

For European organizations, this vulnerability poses a significant risk to the security of TLS communications in applications developed with Go. Since TLS is widely used to secure web services, APIs, and internal communications, improper certificate validation during session resumption could allow attackers to bypass trust checks, potentially leading to unauthorized data access, session hijacking, or man-in-the-middle attacks. This risk is particularly critical for sectors handling sensitive data such as finance, healthcare, and government services. The impact is amplified in environments where Go-based microservices or cloud-native applications are prevalent. Additionally, organizations relying on dynamic TLS configurations or cloning Config objects without strict controls are more vulnerable. The absence of known exploits suggests the vulnerability is not yet actively exploited, but the potential for future attacks remains. Confidentiality and integrity of communications are at risk, while availability impact is minimal unless combined with other attack vectors. The vulnerability could undermine compliance with European data protection regulations if exploited to leak or manipulate personal data.

To mitigate CVE-2025-68121, European organizations should: 1) Audit Go applications to identify usage of crypto/tls Config cloning or dynamic configuration via GetConfigForClient, ensuring that ClientCAs and RootCAs fields are not mutated between initial and resumed handshakes. 2) Avoid mutating TLS Config objects after the initial handshake or implement strict immutability controls on these fields. 3) Monitor Go project updates and apply patches or upgrade to Go version 1.26.0 or later once officially released with the fix. 4) Implement additional TLS session resumption controls at the application level, such as disabling session resumption where appropriate or enforcing strict certificate validation policies. 5) Conduct penetration testing and code reviews focused on TLS session management to detect improper usage patterns. 6) Educate developers on secure use of the crypto/tls package and the risks of mutating Config objects post-handshake. 7) Employ network monitoring to detect anomalous TLS session resumptions that deviate from expected behavior. These steps go beyond generic TLS hardening by focusing on the specific mutation patterns that trigger the vulnerability.