CVE-2023-4421 identifies a timing side-channel vulnerability in the Network Security Services (NSS) library's implementation of PKCS#1 v1.5 RSA decryption depadding. The NSS code prior to version 3.61 leaks timing information related to the correctness of the padding and the length of the encrypted message during the depadding process. This leakage enables an attacker to perform Bleichenbacher-like adaptive chosen ciphertext attacks by sending a large number of attacker-selected ciphertexts. Such attacks can decrypt previously intercepted ciphertexts, for example, TLS sessions that use RSA key exchange, or allow forging of digital signatures using the victim's private key. The root cause is the non-constant-time handling of padding errors, which reveals subtle timing differences. The vulnerability was addressed by adopting the implicit rejection algorithm, as described in the Marvin Attack paper, where invalid padding results in a deterministic random message instead of early rejection, thus eliminating timing differences. NSS versions prior to 3.61 are vulnerable, and the fix requires upgrading to 3.61 or later. No public exploits have been reported yet, but the attack vector is well-understood and theoretically feasible. This vulnerability impacts any system relying on NSS for RSA-based cryptographic operations, including TLS clients and servers, VPNs, and other security-sensitive applications.

For European organizations, the impact of CVE-2023-4421 can be significant, especially for those using NSS in their cryptographic stacks for TLS or digital signature verification. Successful exploitation can lead to the decryption of confidential TLS sessions, exposing sensitive data such as credentials, personal information, or proprietary business communications. Additionally, the ability to forge signatures undermines trust in digital identities and can facilitate further attacks like unauthorized code signing or fraudulent document approvals. This compromises confidentiality, integrity, and potentially availability if attackers leverage forged credentials to escalate privileges or disrupt services. Sectors such as finance, healthcare, government, and critical infrastructure that rely heavily on secure communications and cryptographic assurances are particularly at risk. The attack requires active network access and the ability to submit numerous ciphertexts, which may limit exposure but does not eliminate risk. The absence of known exploits in the wild reduces immediate threat but does not preclude future exploitation, especially as the vulnerability is publicly disclosed.

European organizations should prioritize upgrading NSS to version 3.61 or later to eliminate the timing side-channel vulnerability. Where upgrading is not immediately feasible, organizations should consider mitigating exposure by disabling RSA key exchange in TLS configurations in favor of more secure key exchange methods like ECDHE. Network monitoring should be enhanced to detect unusual volumes of TLS handshake attempts or anomalous ciphertext submissions that could indicate an ongoing Bleichenbacher-like attack. Implementing strict rate limiting and anomaly detection on cryptographic operations can reduce attack feasibility. Additionally, organizations should audit their cryptographic libraries and dependencies to ensure no legacy NSS versions remain in use. For signature verification, consider transitioning to algorithms not vulnerable to this attack vector, such as RSA-PSS or elliptic curve-based schemes. Finally, maintain up-to-date threat intelligence feeds and apply security patches promptly to reduce the window of exposure.