CVE-2026-74900 in openssl_encrypt
Summary
by MITRE • 08/17/2026
openssl_encrypt versions before 1.4.0 contain a critical vulnerability in pqc.py where KEM decapsulation failures silently fall back to simulation mode, generating a deterministic shared secret from only 16 bytes of the private key and publicly available encapsulated key data. Attackers who obtain 16 bytes of the private key can compute the shared secret and decrypt all ciphertext, as the fallback triggers on any KEM failure without raising an error.
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Analysis
by VulDB Data Team • 08/17/2026
The vulnerability in openssl_encrypt versions prior to 1.4.0 represents a critical flaw within the post-quantum cryptography implementation found in pqc.py. This issue stems from improper handling of Key Encapsulation Mechanism decapsulation failures, specifically involving hybrid cryptographic schemes that combine classical and post-quantum algorithms. When the system attempts to decapsulate a shared secret using a public key, it encounters an error condition during the validation or decryption process. Instead of halting execution and raising a security exception, the software silently falls back into simulation mode. This fallback mechanism is designed for testing purposes but was inadvertently left active in production code paths without adequate safeguards against unauthorized access.
In this compromised state, the system generates a deterministic shared secret derived solely from sixteen bytes of the private key material combined with publicly available encapsulated key data. The cryptographic integrity relies on the secrecy of the entire private key and the randomness of ephemeral values. By reducing the required entropy to just sixteen bytes, which may be obtainable through side-channel attacks, partial key exposure, or other information leakage vectors, an attacker can effectively reconstruct the shared secret used for encryption. This undermines the fundamental security assumptions of the hybrid scheme, as the post-quantum component no longer provides its intended protection against classical computational attacks when the fallback is triggered.
The operational impact of this vulnerability is severe, allowing attackers to decrypt all ciphertext protected by the affected implementation if they can obtain or guess sixteen bytes of the private key. Since the failure triggers on any KEM error without raising an alert, there are no immediate indicators of compromise within the application logs for those specific decryption attempts. This silent degradation means that sensitive data remains exposed until a broader security audit reveals the misconfiguration or partial key exposure. The ability to decrypt all ciphertext implies a complete loss of confidentiality for communications relying on this cryptographic module, potentially exposing user credentials, financial transactions, and other highly sensitive information stored in transit or at rest using these keys.
This flaw aligns with CWE-209 Generation of Error Message Containing Sensitive Information if the fallback logic inadvertently leaks state, but more accurately reflects CWE-345 Insufficient Verification of Data Authenticity because the system fails to verify that the decapsulation was successful before proceeding with key derivation. From an ATT&CK perspective, this vulnerability facilitates data exfiltration and potentially credential access by allowing attackers to bypass encryption controls after obtaining partial private key material. It also relates to improper error handling which can mask malicious activity from security monitoring tools.
Mitigation requires immediate upgrading to openssl_encrypt version 1.4.0 or later where the fallback mechanism has been properly secured or removed from production code paths. Developers must ensure that any simulation modes are strictly disabled in non-development environments and that cryptographic operations fail securely by raising exceptions rather than falling back to insecure defaults. Additionally, implementing strict input validation and ensuring that partial key exposure does not lead to full compromise is essential. Security teams should audit their use of post-quantum cryptography libraries for similar silent failure modes and enforce comprehensive logging of all cryptographic errors to detect potential exploitation attempts early in the attack lifecycle.