CVE-2026-12852 in Bouncy Castle
Summary
by MITRE • 08/03/2026
In Bouncy Castle for Java before 1.85, MLS wire decoder allocates attacker-declared opaque length before bounds check.
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Analysis
by VulDB Data Team • 08/03/2026
The vulnerability resides in the Machine Learning Secure (MLS) wire decoder implementation within Bouncy Castle Java library versions prior to 1.85, representing a critical memory safety issue that can be exploited through crafted malicious input data. This flaw manifests as an improper order of operations during deserialization where the system allocates memory based on attacker-controlled opaque length values before performing necessary bounds checking. The vulnerability falls under CWE-129, which specifically addresses insufficient bounds checking for buffer access operations, and more broadly aligns with CWE-704, concerning incorrect type conversion or cast operations that can lead to memory corruption vulnerabilities.
The technical execution of this vulnerability occurs when the MLS wire decoder processes incoming network data containing opaque length fields that specify the size of subsequent data structures. During normal operation, the decoder should validate these length values against acceptable ranges before allocating memory resources. However, in affected versions, the allocation occurs first, potentially leading to excessive memory consumption or memory corruption when attacker-provided lengths exceed reasonable bounds. This primitive can be leveraged by remote attackers to cause denial of service through memory exhaustion or potentially execute arbitrary code if the subsequent processing operations are not properly safeguarded against malformed input.
From an operational impact perspective, this vulnerability affects systems that utilize Bouncy Castle's MLS implementation for secure communication protocols, particularly those implementing modern cryptographic handshake mechanisms. The exploitability characteristics make it suitable for exploitation in network-based attacks where adversaries can inject maliciously crafted MLS protocol messages. Attackers could potentially cause system instability through memory exhaustion or manipulate the heap layout to achieve more sophisticated attack vectors. The vulnerability is particularly concerning because it operates at the wire protocol level, meaning successful exploitation could compromise the entire secure communication channel without requiring authentication.
Mitigation strategies should focus on immediate patching of Bouncy Castle libraries to version 1.85 or later where this issue has been resolved through proper bounds checking before memory allocation. Organizations should also implement network-level monitoring to detect unusual patterns in MLS protocol message sizes that might indicate exploitation attempts. Additionally, defensive programming practices including input validation and memory allocation limits should be enforced at application layers that utilize Bouncy Castle components. The fix implemented by the Bouncy Castle team aligns with ATT&CK technique T1070.004 for bypassing security controls through memory corruption vulnerabilities, ensuring that proper bounds checking is performed before any resource allocation occurs. Organizations should also consider implementing intrusion detection systems that monitor for anomalous memory usage patterns consistent with this type of vulnerability exploitation.