CVE-2026-81688 in openssl_encrypt
Summary
by MITRE • 08/27/2026
openssl_encrypt versions before 1.4.9 store an unkeyed SHA-256 hash of the plaintext in the cleartext file header metadata. Attackers can read this hash without the password to confirm guessed plaintexts offline or fingerprint identical plaintexts across separately-encrypted files.
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability identified in openssl_encrypt versions prior to 1.4.9 represents a critical flaw in how encrypted data is structured and secured, specifically concerning the handling of metadata within the cleartext file header. This implementation error results in the storage of an unkeyed SHA-256 hash of the plaintext directly into the header information that accompanies the ciphertext. Because this hash is computed without any cryptographic keying material or salt derived from the user's password, it remains static for identical input data regardless of when or how many times the encryption operation is performed with different passwords. This design choice fundamentally undermines the confidentiality guarantees expected from encrypted storage solutions, as it exposes a deterministic signature of the original message that can be extracted without any authentication credentials.
From a technical perspective, this flaw creates a significant attack surface for offline cryptanalysis and data correlation attacks. An attacker who gains access to an encrypted file does not need to break the encryption algorithm itself or guess the password immediately to gain valuable intelligence about the contents. By extracting the unkeyed SHA-256 hash from the header metadata, the adversary can perform brute-force or dictionary attacks against this specific value rather than the full ciphertext. More critically, because the hash is deterministic and independent of the key, it allows for fingerprinting operations across multiple files. If an attacker possesses a known plaintext sample or suspects the content of another encrypted file, they can compute the SHA-256 hash of that suspected text and compare it against the hashes found in various encrypted archives to confirm matches without ever decrypting them. This capability effectively neutralizes the obfuscation provided by encryption when dealing with structured data or common message formats.
The operational impact of this vulnerability is severe, particularly for applications handling sensitive personal information, financial records, or proprietary business documents where integrity and confidentiality are paramount. The ability to confirm guessed plaintexts offline means that weak passwords offer no protection against targeted attacks if the attacker has access to multiple encrypted files containing similar content. Furthermore, the fingerprinting capability allows adversaries to correlate data across different systems or time periods, potentially revealing patterns in user behavior, communication frequency, or document types even when the actual contents remain hidden from casual inspection. This violates the principle of semantic security, which dictates that ciphertexts should reveal no information about the plaintext other than its length, and instead leaks a unique identifier for specific content blocks.
To mitigate this risk, organizations must immediately upgrade openssl_encrypt to version 1.4.9 or later, where the implementation has been corrected to ensure that metadata does not expose unkeyed hashes of the plaintext. In environments where upgrading is not immediately feasible, administrators should enforce strong, high-entropy passwords to reduce the feasibility of offline brute-force attacks against any exposed hash values. Additionally, applying data classification policies and limiting access to encrypted files can help minimize the risk of correlation attacks by reducing the number of samples available for fingerprinting analysis. Long-term remediation strategies should also involve auditing encryption implementations to ensure compliance with modern cryptographic standards that mandate unique salts or nonces for each encryption operation, thereby preventing deterministic outputs even if similar plaintexts are processed. This aligns with industry best practices outlined in CWE-327 regarding the use of broken or risky cryptographic algorithms and ensures adherence to ATT&CK techniques related to offline credential dumping and data exfiltration through pattern recognition.