CVE-2026-59644 in BC-JAVA
Summary
by MITRE • 08/03/2026
In Bouncy Castle for Java before 1.85, MLS hash-ratchet honours arbitrary 32-bit generation counter from sender.
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Analysis
by VulDB Data Team • 08/03/2026
The vulnerability in Bouncy Castle for Java versions prior to 1.85 involves a critical flaw in the Multi-Party Hybrid Encryption (MLS) implementation related to the hash-ratchet mechanism. This issue specifically affects how the system processes generation counters during key ratcheting operations, which are fundamental to maintaining forward secrecy and security in multi-party communication scenarios. The flaw allows an attacker to manipulate the 32-bit generation counter value sent by a legitimate party, potentially compromising the integrity of the cryptographic session.
The technical implementation error stems from insufficient validation of the generation counter parameter within the MLS hash-ratchet protocol. When processing messages containing key ratcheting information, the system fails to properly validate that the generation counter falls within acceptable ranges or follows expected patterns. This lack of input sanitization creates an attack vector where malicious actors can inject arbitrary 32-bit values that may cause unpredictable behavior in the cryptographic state management. The vulnerability primarily affects systems implementing MLS with Bouncy Castle's Java implementation, particularly those utilizing the hash-ratchet mechanism for key derivation and session management.
The operational impact of this vulnerability extends beyond simple data corruption, potentially enabling sophisticated attacks such as replay attacks, session hijacking, or forward secrecy violations in multi-party encrypted communications. An attacker who successfully manipulates the generation counter could force the system into an inconsistent cryptographic state, potentially allowing them to derive keys that should remain secret or disrupt the normal flow of key ratcheting operations. This could result in complete breakdown of the secure communication channel between parties, making it impossible to establish or maintain confidential exchanges.
The vulnerability aligns with CWE-129, Input Validation and Canonicalization Issues, as it represents a failure to properly validate input parameters within cryptographic protocols. From an ATT&CK framework perspective, this weakness maps to T1583.001 - Adversary Operate on Air Gapped Networks and T1584.002 - Establish Accounts, as attackers could potentially manipulate cryptographic sessions to gain unauthorized access or disrupt communications. Organizations using Bouncy Castle for Java in MLS implementations should immediately update to version 1.85 or later, which includes proper validation of generation counter values. Additionally, network monitoring should be enhanced to detect unusual patterns in MLS message exchanges that might indicate exploitation attempts. Implementing proper cryptographic protocol testing and static analysis of the MLS implementation can help identify similar vulnerabilities in other cryptographic libraries used within the organization's security infrastructure.