Using a Custom Cipher with NID_undef may lead to NULL encryption

Root

Cause

OpenSSL supports custom ciphers via the deprecated EVP_CIPHER_meth_new() function. In versions 3.0.0 through 3.0.5, when a custom cipher is initialized using EVP_EncryptInit_ex2(), EVP_DecryptInit_ex2(), or EVP_CipherInit_ex2(), the library incorrectly attempts to fetch an equivalent cipher from registered providers instead of using the custom cipher directly. This lookup is keyed by the NID passed to EVP_CIPHER_meth_new(). If an application passes NID_undef (a sentinel value meaning “no defined cipher”), the provider lookup matches the NULL cipher, which performs no encryption and simply emits plaintext as ciphertext [1][3].

Exploitation

Exploitation requires that the application calls EVP_CIPHER_meth_new() with NID_undef and then uses that custom cipher in an encryption or decryption initialization routine. The attack surface is limited to applications that programmatically create custom ciphers using the legacy API. Standard SSL/TLS users are not affected [1][2]. The default OpenSSL provider, if loaded, supplies the NULL cipher, so the misbehavior occurs automatically; no special attacker configuration is needed beyond the vulnerable application logic [3].

Impact

An attacker who can supply plaintext to be processed by the vulnerable initialization path will receive that plaintext unchanged as “ciphertext.” This defeats the confidentiality guarantee of encryption, effectively turning the cipher operation into a no-op. The issue does not affect data integrity or availability directly, but the loss of encryption can lead to exposure of sensitive information in transit or at rest [2][3].

Mitigation

The vulnerability is fixed in OpenSSL 3.0.6. Users of OpenSSL 3.0.0–3.0.5 should upgrade immediately. Workarounds include avoiding the use of NID_undef in custom cipher creation or not using the legacy EVP_CIPHER_meth_new() API at all; OpenSSL 1.1.1 and 1.0.2 are not affected [1][3]. The Rust crate openssl-src has been patched in version >=300.0.10 [2].