VulnerabilityPublished 3 Aug 2026Updated 2 Sep 2026

CVE-2026-12816: In Bouncy Castle for Java before 1.85, IESEngine stream-mode MAC forgery via length-dependent KDF split

Source GCVE Database

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Authoritative record:CVE-2026-12816· High 8.7

Not known to be exploited · CVSS 8.7 High : upgrade to 1.85 soon.

Data as of 1 Nov 2026, 11:02 UTC

CVSS 4.0 base score

8.7High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.1% chance of exploitation in 30 days – higher than 7% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.85, 2.73.12

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/U:Amber

Base metrics

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: High
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Supplemental metrics

Provider Urgency: Amber

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.
Provider Urgency: Amber: Provider has assessed the impact of this vulnerability as having a moderate urgency.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 3 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 1 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-834p-whmm-rg5r

What to do

Upgrade BC-JAVA and Legion of the Bouncy Castle Inc./bcprov to 1.85; BC-LTS-JAVA and Legion of the Bouncy Castle Inc./bcprov-lts8on to 2.73.12.

Affected software

Product	Affected	Fixed in
Legion of the Bouncy Castle Inc.BC-JAVA	Affected <1.85	Fixed in 1.85
Legion of the Bouncy Castle Inc.BC-LTS-JAVA	Affected >=2.73.0 <2.73.12	Fixed in 2.73.12
MavenLegion of the Bouncy Castle Inc./bcprov`pkg:maven/Legion of the Bouncy Castle Inc./bcprov`Other threats	Affected <1.85	Fixed in 1.85
MavenLegion of the Bouncy Castle Inc./bcprov-lts8on`pkg:maven/Legion of the Bouncy Castle Inc./bcprov-lts8on`Other threats	Affected >=2.73.0 <2.73.12	Fixed in 2.73.12

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

If you cannot patch yet, users should consider avoiding the use of IESEngine stream-mode MAC or apply alternative cryptographic methods that do not rely on the vulnerable KDF split. Monitoring vendor communications for updates is recommended.

Description

A vulnerability exists in Bouncy Castle for Java before version 1.85 and in the LTS version before 2.73.12, where the IESEngine stream-mode MAC can be forged due to a length-dependent key derivation function (KDF) split. This issue is tracked as CVE-2026-12816 and is classified under CWE-354 (Improper Validation of Integrity Check Value).

Impact

The vulnerability allows an attacker to forge message authentication codes (MACs) in the IESEngine stream mode, compromising message integrity. This could lead to undetected tampering of messages protected by this cryptographic primitive. There is no indication of privilege escalation or confidentiality loss directly from this issue. No known exploits in the wild have been reported at this time.

Summary

CVE-2026-12816 describes a cryptographic vulnerability in Bouncy Castle for Java versions prior to 1.85 and LTS versions before 2.73.12. The IESEngine component, which uses stream-mode MACs, is susceptible to forgery because the key derivation function splits keys based on message length in a way that can be exploited. This weakness undermines the integrity guarantees of the MAC, potentially allowing an attacker to forge valid MACs without knowledge of the secret key.

CWE-354Improper Validation of Integrity Check Value (opens in a new tab)
Base weaknessLikelihood of exploit: Medium
The product does not validate or incorrectly validates the integrity check values or "checksums" of a message.
Implementation
The protocol specification describes the algorithm used for calculating the checksum. It is then a matter of implementing the calculation and verifying that the calculated checksum and the received checksum match.
OWASP Cheat Sheets
XML Security (opens in a new tab)
REST Security (opens in a new tab)
JSON Web Token for Java (opens in a new tab)
OWASP ASVS 4.0.3
V11.1.7 (opens in a new tab)
V13.3.2 (opens in a new tab)
V3.5.3 (opens in a new tab)
Attack patterns
Checksum Spoofing (opens in a new tab)
Padding Oracle Crypto Attack (opens in a new tab)
Manipulating Writeable Configuration Files (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 3 Aug 2026, 21:41 UTC

WeaknessesCWE-354

Technical details

References

Community reviews

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