CVE-2026-12816Public exploitNetwork attack vector

Authenticated ciphertext forgery via length-dependent KDF split

Published Aug 3, 2026 · Updated Aug 3, 2026

MAC validation flaw in Bouncy Castle Java before 1.85 and Bouncy Castle Java LTS before 2.73.12 allows attackers to forge authenticated ciphertexts. IESEngine's four-argument static-key stream mode derives the XOR keystream before the MAC key, so the MAC-key offset varies with message length and known plaintext reveals prefix-stable KDF bytes. A passive observer needs one known plaintext-ciphertext pair and no private key to forge arbitrary shorter ciphertexts accepted by the recipient.

CVSS severity8.7
High
EPSS probability0.15%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

MAC validation flaw in Bouncy Castle Java before 1.85 and Bouncy Castle Java LTS before 2.73.12 allows attackers to forge authenticated ciphertexts. IESEngine's four-argument static-key stream mode derives the XOR keystream before the MAC key, so the MAC-key offset varies with message length and known plaintext reveals prefix-stable KDF bytes. A passive observer needs one known plaintext-ciphertext pair and no private key to forge arbitrary shorter ciphertexts accepted by the recipient.

The record

CVE
CVE-2026-12816
Published
Aug 3, 2026
Updated
Aug 3, 2026
Vendor
Legion of the Bouncy Castle Inc.
Product
Bouncy Castle Java
Classifications
CWE-354, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 3, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <1.85

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Legion of the Bouncy Castle Inc. · Bouncy Castle Javaversion=0 <1.85
Legion of the Bouncy Castle Inc. · Bouncy Castle Java LTSversion=2.73.0 <2.73.12

Published CVSS scores

8.7Legion of the Bouncy Castle Inc.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

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-354Improper Validation of Integrity Check Value
T1565.002Transmitted Data Manipulation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo