CVE-2026-75803 in OpenSSL
Summary
by MITRE • 08/25/2026
Issue summary: ChaCha20-Poly1305 and AES-OCB decryption with an empty ciphertext can report success without verifying the supplied authentication tag when the operation is finalized by calling the EVP_Cipher() function.
Impact summary: Applications calling EVP_Cipher() on an empty ciphertext and expecting the call to check the AEAD tag may accept forged messages.
CWE: CWE-354 (Improper Validation of Integrity Check Value)
Description: The EVP_Cipher() API call for AEAD ciphers behaves like a one shot encryption and decryption call. It also verifies the AEAD tag after the decryption operation. However for AES-OCB and ChaCha20-Poly1305 ciphers it skipped the AEAD tag verification when an empty ciphertext was passed to the function. The callers of this function might believe that a successful return indicates a valid AEAD tag for these ciphers, even when that has not truly been validated in this case.
FIPS impact: no The FIPS modules in 4.0, 3.6, 3.5, 3.4, and 3.0 are not affected by this CVE as the affected algorithms are not FIPS approved and thus not implemented in the FIPS module.
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Analysis
by VulDB Data Team • 08/25/2026
The cryptographic library implementation contains a critical logic flaw within the EVP_Cipher API when processing Authenticated Encryption with Associated Data (AEAD) ciphers, specifically affecting AES-OCB and ChaCha20-Poly1305 modes of operation. This vulnerability arises from an incorrect handling of edge cases where the input ciphertext is empty or has zero length. Under normal operational circumstances, the EVP_Cipher function serves as a unified interface for both encryption and decryption operations, performing necessary cryptographic transformations and subsequently verifying the authentication tag to ensure data integrity and authenticity. However, in this specific scenario involving AEAD algorithms with an empty payload, the internal logic fails to invoke the finalization step that validates the supplied authentication tag against the computed value derived from the associated data and nonce. This deviation from standard protocol execution means that the function returns a success status code without having performed the mandatory integrity check required by modern cryptographic standards for authenticated encryption schemes.
The technical root cause lies in the premature exit condition within the decryption routine for these specific cipher suites. When the length of the ciphertext buffer is determined to be zero, the control flow bypasses the block that computes and compares the expected tag with the provided one. This oversight creates a situation where an application developer might rely on the return value of EVP_Cipher as a definitive indicator of message validity. Since the function returns success despite the lack of verification, any subsequent processing of the decrypted data proceeds under the false assumption that the message has been authenticated by the sender and has not been tampered with during transmission or storage. This behavior contradicts the fundamental security guarantees provided by AEAD constructions, which are designed to provide both confidentiality and integrity simultaneously, ensuring that even if an attacker cannot decrypt the message, they also cannot modify it without detection.
The operational impact of this vulnerability is severe for applications that utilize these specific cipher suites in protocols where empty messages or zero-length payloads are valid or possible within the protocol state machine. An adversary capable of intercepting network traffic could craft a forged packet containing an empty ciphertext and any arbitrary authentication tag, such as all zeros or random bytes. Upon receiving this malformed but syntactically correct message, the vulnerable application would decrypt it successfully and accept it as legitimate due to the false positive return value from EVP_Cipher. This effectively nullifies the integrity protection offered by AES-OCB and ChaCha20-Poly1305 for these specific cases, allowing attackers to inject malicious commands, alter session states, or bypass access controls that rely on message authentication. The risk is particularly acute in protocols like TLS 1.3 or QUIC where empty application data might be transmitted during certain handshake phases or keep-alive mechanisms, although the exploitability depends heavily on how the higher-level protocol interprets successful decryption of an empty payload.
This vulnerability maps directly to CWE-354, which describes Improper Validation of Integrity Check Value, as the system fails to properly validate the integrity check value (the authentication tag) before accepting the data as authentic. From a threat modeling perspective aligned with MITRE ATT&CK techniques, this flaw facilitates Data Injection and potentially Man-in-the-Middle attacks by allowing an attacker to bypass cryptographic verification mechanisms. The lack of proper validation means that the security boundary provided by the encryption layer is compromised at the point of message acceptance, enabling unauthorized data injection without triggering standard integrity failure alerts.
To mitigate this risk, developers must ensure they are using a patched version of the underlying cryptographic library where the logic for empty ciphertexts has been corrected to enforce tag verification regardless of payload size. For applications unable to immediately upgrade their dependencies, defensive programming practices should be adopted. This includes explicitly checking if the decrypted output is empty and applying additional application-level integrity checks or sequence number validations before processing the data further. Furthermore, security audits should review code paths that invoke AEAD decryption functions with variable-length inputs to ensure no other edge cases are similarly mishandled. It is also important to note that this issue does not affect FIPS-approved modules because AES-OCB and ChaCha20-Poly1305 are not currently approved algorithms within the Federal Information Processing Standards framework, meaning systems relying strictly on FIPS-compliant modes of operation for regulatory compliance remain unaffected by this specific implementation flaw.