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Bouncy Castle Java CVE-2026-12803

| EUVDEUVD-2026-52134 HIGH
Improper Validation of Integrity Check Value (CWE-354)
2026-08-03 bcorg GHSA-4r33-j73q-cw77
Java Information Disclosure Bc Java Bc Lts Java Suse
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:H/VA:N/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
5.9 MEDIUM

AC:H reflects the chosen-plaintext oracle prerequisite with empty AAD; integrity-only forgery maps to I:H with no confidentiality or availability impact.

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

Primary rating from Vendor (bcorg).

CVSS VectorVendor: bcorg

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/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
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
X

Lifecycle Timeline

3
Source Code Evidence Fetched
Aug 03, 2026 - 04:26 vuln.today
Analysis Generated
Aug 03, 2026 - 04:26 vuln.today
CVE Published
Aug 03, 2026 - 02:52 cve.org
HIGH 8.7

DescriptionCVE.org

In Bouncy Castle for Java before 1.85, KCCMBlockCipher MAC does not bind nonce when AAD is absent (cross-nonce AEAD forgery). This issue also affects Bouncy Castle for Java LTS before 2.73.12.

AnalysisAI

Cross-nonce AEAD forgery in Bouncy Castle for Java's KCCMBlockCipher (DSTU 7624 CCM mode) allows an attacker with chosen-plaintext oracle access to forge valid authenticated ciphertext under an un-queried nonce when no associated authenticated data is present. All bc-java releases before 1.85 and bc-lts-java releases before 2.73.12 are affected. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

Recon
Identify KCCMBlockCipher service operating without AAD
Delivery
Submit enc(0,N1) chosen-plaintext query
Exploit
Submit enc(0,N2) and enc(P,N2) chosen-plaintext queries
Install
XOR-cancel nonce-2 keystream terms across the three outputs
C2
Assemble forged C_forge and T_forge via block-level arithmetic
Execute
Present forged ciphertext to decryption endpoint under nonce N1
Impact
Decryption accepts forged message as authentic
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Vulnerability AssessmentAI

Exploitation Exploitation requires all three of the following conditions simultaneously: (1) the application uses KCCMBlockCipher specifically - this flaw does not affect CCMBlockCipher, GCMBlockCipher, or any other Bouncy Castle AEAD mode; (2) encryption operations are performed with absent or zero-length AAD - any non-empty AAD triggers G1 processing and prevents the attack; (3) the attacker has chosen-plaintext oracle access - the ability to submit at least three arbitrary plaintexts to the encryption function and receive the corresponding ciphertext-plus-tag outputs. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The provided CVSS 4.0 score of 8.7 with AC:L and AT:N likely overstates real-world risk by treating exploitation as straightforward and requiring no attack prerequisites. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario An attacker interacting with a service that exposes a KCCMBlockCipher encryption oracle (such as a signing or token-generation API) without any AAD submits three crafted encryption requests: enc(zeros, N1), enc(zeros, N2), and enc(target_plaintext, N2), all with empty AAD. Because the MAC is nonce-independent under the vulnerable code, the attacker XOR-combines the resulting ciphertext blocks and tags to cancel the nonce-2 keystream, producing a forged (ciphertext, tag) pair that the decryptor accepts as valid under nonce N1 - a nonce the attacker never used as an encryption input. …
Remediation Upgrade bc-java to version 1.85 or later, or bc-lts-java to version 2.73.12 or later; these releases unconditionally process the G1 nonce-binding block regardless of AAD presence. … Detailed patch versions, workarounds, and compensating controls in full report.
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Recommended ActionAI

Within 24 hours, inventory all systems and applications using Bouncy Castle for Java and identify which versions are deployed (particularly bc-java before 1.85 or bc-lts-java before 2.73.12). …

Sign in for detailed remediation steps and compensating controls.

Threat intelligence, references, and detailed analysis are available after sign-in.

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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-2026-12803 vulnerability details – vuln.today
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