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Bouncy Castle Java CVE-2026-12852

| EUVDEUVD-2026-52137 HIGH
Memory Allocation with Excessive Size Value (CWE-789)
2026-08-03 bcorg GHSA-p2mp-577q-8vh2
Java Information Disclosure Bc Java Legion Of The Bouncy Castle Inc
8.7
CVSS 4.0 · Vendor: bcorg
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Severity by source

Vendor (bcorg) PRIMARY
8.7 HIGH
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber
vuln.today AI
7.5 HIGH

Network-reachable with no auth on unauthenticated MLS structures; availability-only impact via heap exhaustion; no scope change or confidentiality impact evidenced.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat
7.5 HIGH
qualitative

Primary rating from Vendor (bcorg).

CVSS VectorVendor: bcorg

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
X

Lifecycle Timeline

3
Source Code Evidence Fetched
Aug 03, 2026 - 04:23 vuln.today
Analysis Generated
Aug 03, 2026 - 04:23 vuln.today
CVE Published
Aug 03, 2026 - 02:55 cve.org
HIGH 8.7

DescriptionCVE.org

In Bouncy Castle for Java before 1.85, MLS wire decoder allocates attacker-declared opaque length before bounds check.

AnalysisAI

Heap exhaustion via uncontrolled memory allocation in Bouncy Castle for Java's MLS wire decoder allows remote unauthenticated attackers to crash any application using the bc-java MLS module. The MLSInputStream.readOpaque() method decodes an attacker-supplied Varint length field (up to 0x3FFFFFFF, approximately 1 GiB) and allocates that buffer before verifying the input stream actually contains that many bytes; a handful of concurrent few-byte crafted messages can exhaust the JVM heap. No public exploit has been identified at time of analysis and CISA KEV listing is absent, though the attack is trivially constructable from the published patch and regression test.

Technical ContextAI

The vulnerability is in the MLS (RFC 9420, Messaging Layer Security) wire format decoder within the mls submodule of bc-java (CPE: cpe:2.3:a:legion_of_the_bouncy_castle_inc.:bc-java:*). MLS wire messages encode variable-length opaque fields using a Varint prefix. Before the fix, MLSInputStream.readOpaque() decoded the Varint length and immediately invoked new byte[size] without first checking that the stream contained at least that many remaining bytes. CWE-789 (Uncontrolled Memory Allocation) precisely describes this flaw: attacker-controlled data flows directly into an allocation call without an upper-bound constraint derived from actual input size. Critically, the affected message types - PublicMessage, KeyPackage, Welcome, and PrivateMessage - carry opaque fields in their opening bytes and are processed entirely before any signature or MAC verification, meaning the allocation path is reachable by any unauthenticated network peer.

RemediationAI

Upgrade Bouncy Castle for Java to version 1.85 or later, which incorporates the bounds check that validates the declared opaque length against the bytes actually remaining in the input stream before any buffer allocation (patch commit: https://github.com/bcgit/bc-java/commit/a747038bb5bbd5e29fb2b7607ab38af1fd8d1790). The vendor advisory is at https://github.com/bcgit/bc-java/wiki/CVE-2026-12852. If an immediate upgrade is not feasible, apply network-layer rate limiting and enforce a strict maximum inbound message size before bytes reach the MLS decoder, which prevents the large Varint value from being presented to the vulnerable code path; note this requires a proxy or application-level size gate and cannot be implemented via JVM flags alone. Setting a JVM heap ceiling (-Xmx) will not prevent the DoS but will reduce time-to-OOM and may improve process restart behaviour. Applications not using the bc-java MLS module require no remediation.

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Vendor StatusVendor

SUSE

Severity: Important
Product Status
openSUSE Leap 16.0 Fixed
openSUSE Tumbleweed Fixed
SUSE Linux Enterprise High Performance Computing 15 SP4-ESPOS Affected
SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS Affected
SUSE Linux Enterprise High Performance Computing 15 SP5-ESPOS Affected

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CVE-2026-12852 vulnerability details – vuln.today
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