CVE-2018-6829 in Communications Interactive Session Recorder
Summary
by MITRE
cipher/elgamal.c in Libgcrypt through 1.8.2, when used to encrypt messages directly, improperly encodes plaintexts, which allows attackers to obtain sensitive information by reading ciphertext data (i.e., it does not have semantic security in face of a ciphertext-only attack). The Decisional Diffie-Hellman (DDH) assumption does not hold for Libgcrypt's ElGamal implementation.
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Analysis
by VulDB Data Team • 03/21/2024
The vulnerability identified as CVE-2018-6829 represents a critical weakness in the Libgcrypt cryptographic library version 1.8.2 and earlier, specifically affecting the ElGamal encryption implementation within the cipher/elgamal.c module. This flaw fundamentally undermines the security guarantees that should be provided by modern cryptographic systems, creating a situation where attackers can exploit the encryption scheme to extract sensitive information from ciphertext data. The issue manifests when ElGamal encryption is used to directly encrypt messages without proper padding or encoding mechanisms, leaving the system vulnerable to ciphertext-only attacks that would normally be prevented by semantic security properties.
The technical root cause of this vulnerability lies in the improper encoding of plaintexts during the ElGamal encryption process, which violates the fundamental assumptions underlying the Decisional Diffie-Hellman (DDH) problem. In a properly implemented ElGamal system, the DDH assumption should hold, meaning that an attacker cannot distinguish between certain types of ciphertext pairs. However, Libgcrypt's implementation fails to maintain this mathematical property, creating a scenario where patterns in the plaintext become discernible within the ciphertext structure. This weakness stems from the lack of proper randomization and encoding mechanisms that should be applied to ensure that identical plaintexts produce different ciphertexts when encrypted multiple times, a requirement that falls under the semantic security standard defined in cryptographic literature.
The operational impact of this vulnerability extends far beyond simple information disclosure, as it fundamentally compromises the confidentiality assurances that users rely upon when employing Libgcrypt for secure communications. Attackers can exploit this weakness to perform ciphertext-only attacks that reveal information about the encrypted data, potentially exposing sensitive communications, private keys, or other confidential information. The vulnerability affects any system using Libgcrypt versions 1.8.2 or earlier for ElGamal encryption, including but not limited to email encryption systems, secure messaging applications, and any cryptographic implementations that depend on this library for their security properties. This weakness particularly impacts systems where the same plaintext might be encrypted multiple times, as the patterns that should be randomized are instead preserved in the ciphertext structure.
Security practitioners should address this vulnerability by upgrading to Libgcrypt version 1.8.3 or later, which contains the necessary patches to correct the ElGamal implementation. Additionally, organizations should review their cryptographic implementations to ensure that they are not directly using ElGamal encryption without proper padding mechanisms, as this vulnerability specifically affects direct encryption without additional security layers. The mitigation strategy should include implementing proper padding schemes such as OAEP or similar randomized encodings that ensure semantic security properties. From an ATT&CK framework perspective, this vulnerability maps to techniques involving credential access and information discovery, as attackers can extract sensitive information from encrypted communications through ciphertext analysis. The vulnerability also aligns with CWE-327, which addresses the use of weak or broken encryption algorithms, and CWE-326, which covers inadequate encryption strength, demonstrating how improper implementation of well-established cryptographic primitives can create severe security weaknesses that compromise system integrity. Organizations should also consider implementing cryptographic protocol monitoring to detect potential exploitation attempts and establish incident response procedures for dealing with potential information disclosure events that could result from this vulnerability.