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Bouncy Castle Java CVE-2026-12816

| EUVDEUVD-2026-52135 HIGH
Improper Validation of Integrity Check Value (CWE-354)
2026-08-03 bcorg GHSA-834p-whmm-rg5r
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 known-plaintext observation prerequisite and static-key mode requirement; I:H for MAC forgery; 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:L/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 - 04:25 vuln.today
Analysis Generated
Aug 03, 2026 - 04:25 vuln.today
CVE Published
Aug 03, 2026 - 02:53 cve.org
HIGH 8.7

DescriptionCVE.org

In Bouncy Castle for Java before 1.85, IESEngine stream-mode MAC forgery via length-dependent KDF split. This issue also affects Bouncy Castle for Java LTS before 2.73.12.

AnalysisAI

IESEngine in Bouncy Castle for Java allows cross-message MAC forgery when stream mode is used with static key pairs, affecting bc-java before 1.85 and bc-lts-java before 2.73.12. The flaw stems from a length-dependent KDF output split: in the static-key path (no ephemeral V component), the MAC key K2 was placed at a byte offset equal to the message length after the keystream K1, so an observer of one known-plaintext encryption can recover K2 and forge valid ciphertext-plus-tag pairs for shorter messages. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

Access
Identify application using static-key IES stream mode
Delivery
Observe one ciphertext for a known plaintext of length L
Exploit
XOR plaintext and ciphertext to recover keystream bytes 0..L
Execution
Extract MAC key K2 from bytes L..L+macKeyLen of recovered keystream
Persist
Compute valid HMAC tag over forged shorter ciphertext using recovered K2
Impact
Submit forged message with valid tag, accepted by recipient IESEngine
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Vulnerability AssessmentAI

Exploitation Exploitation requires all three of the following to be simultaneously true: (1) the application uses Bouncy Castle IESEngine initialized without an ephemeral sender key - specifically the static-static initialization path where V.length == 0, typically when IESParameters is constructed without an encoding vector component; (2) stream-cipher mode is selected rather than block-cipher mode within IES; and (3) the attacker can observe at least one encryption of a known plaintext under the target static key pair where the observed plaintext is at least macKeyBytes longer than the message to be forged. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The CVSS 4.0 score of 8.7 with AV:N/AC:L/AT:N/PR:N/UI:N reflects that no authentication or special attacker network position is required at the library level, but the assigned AT:N understates the practical prerequisites. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario An attacker targeting a system that encrypts messages with Bouncy Castle IESEngine in static-key stream mode observes a single ciphertext C for a known plaintext M of length L (obtainable if the attacker can influence what gets encrypted, or if messages contain predictable content). By XOR-ing M and C, the attacker recovers the keystream block, which in the vulnerable layout embeds the MAC key K2 at bytes [L..L+macKeyLen]; the attacker then constructs a forged ciphertext for an arbitrary message shorter than L bytes, computes a valid HMAC tag using the recovered K2, and submits the forgery - which the recipient's IESEngine accepts as authentic. …
Remediation Upgrade bc-java to version 1.85 or later, or bc-lts-java to version 2.73.12 or later; the fix is in commit 6d9e4bbaee9409713ada167e5f901554cbb084ac (https://github.com/bcgit/bc-java/commit/6d9e4bbaee9409713ada167e5f901554cbb084ac). … Detailed patch versions, workarounds, and compensating controls in full report.
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Recommended ActionAI

Within 24 hours, identify all applications and services using bc-java (versions before 1.85) or bc-lts-java (versions before 2.73.12). …

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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-2026-12816 vulnerability details – vuln.today
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