What is CVE-2026-71892?

CVE-2026-71892 is CMS key-transport recipient key-size validation never runs for RFC 9709 HKDF-derived keys. The supplied enrichment data classifies it as Unclassified mapped to CWE-697 CWE-697.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-71892. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-71892 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

2 public PoC repositories reported; 0 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-71892 using CVSS 6.9, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

mediumpublic exploit

CVE-2026-71892

CVE-2026-71892 — CMS key-transport recipient key-size validation never runs for RFC 9709 HKDF-derived keys

In Bouncy Castle for Java before 1.86, the opt-in key-size validation on CMS key-transport recipients, org.bouncycastle.cms.jcajce.JceKeyTransRecipient.setKeySizeValidation(true), never ran for a message using RFC 9709 content-encryption key derivation (id-alg-cek-hkdf-sha256). The branch that should have selected the actual content-encryption algorithm carried in the key derivation AlgorithmIdentifier's parameters compared the encrypted-key byte array against the id-alg-cek-hkdf-sha256 object identifier, a comparison between a byte array and an ASN1ObjectIdentifier that is false for every possible input, so the check fell through to a key-size lookup on the outer wrapper OID. That OID identifies a key-derivation construction rather than a cipher and has no registered key size, so the size comparison was skipped entirely. A key-transport EnvelopedData or AuthEnvelopedData whose transported, HKDF-derived content-encryption key did not match the key size of the advertised content-encryption algorithm was therefore accepted even with validation explicitly enabled, silently defeating the only mechanism the API offers for enforcing recovered key size. The recipient now dispatches on the content-encryption AlgorithmIdentifier's algorithm OID, so validation checks the recovered key against the inner content-encryption algorithm. Messages with a matching key size, non-HKDF messages, and recipients that do not enable validation are unaffected. This issue also affects Bouncy Castle for Java FIPS (BC-FJA) before bcpkix-fips 2.0.13 (2.0.X series) and 2.1.13 (2.1.X series).

Published Oct 3, 2026Updated Oct 3, 2026Sources: NVD, Legion of the Bouncy Castle Inc. (CNA), GitHub, GitHub Security Advisory, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Exploit code

public exploit, none observed

EPSS

ahead of 0% of scored CVEs

Affects

Legion of the Bouncy Castle Inc.

3 products listed

CVSS base

6.9

medium

Remediation

The vendor's own words where we have them.

Upgrade BC-JAVA to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Legion of the Bouncy Castle Inc.	BC-JAVA	1.78 to < 1.86	Vulnerable
Legion of the Bouncy Castle Inc.	BC-FJA	2.0.7 to < 2.0.13	Vulnerable
Legion of the Bouncy Castle Inc.	BC-FJA	2.1.0 to < 2.1.13	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Scope

Attack Complexity

Low

Attack Vector

Network

Privileges Req

None

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
6.9	CVSS 4.0	medium	—	—	Legion of the Bouncy Castle Inc. (CNA)

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/U:Amber

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-697 · Incorrect Comparison

Varies by Context::

Weakness & attack patterns

CWE-697

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-3 · Using Leading 'Ghost' Character Sequences to Bypass Input FiltersCAPEC-8 · Buffer Overflow in an API CallCAPEC-41 · Using Meta-characters in E-mail Headers to Inject Malicious PayloadsCAPEC-92 · Forced Integer OverflowCAPEC-47 · Buffer Overflow via Parameter ExpansionCAPEC-52 · Embedding NULL BytesCAPEC-7 · Blind SQL InjectionCAPEC-79 · Using Slashes in Alternate EncodingCAPEC-64 · Using Slashes and URL Encoding Combined to Bypass Validation LogicCAPEC-9 · Buffer Overflow in Local Command-Line UtilitiesCAPEC-78 · Using Escaped Slashes in Alternate EncodingCAPEC-15 · Command Delimiters

T1027Obfuscated Files or Information

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Prioritize edge telemetry for network-reachable BC-JAVA, BC-FJA.
Monitor for scanner or exploit-pattern traffic after 2 public PoC repositories were reported.

web/proxy/WAF logsapplication logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 6.9 (AV:N)
Oct 3, 2026 · NVD
Initial · CVE published
Oct 3, 2026 · NVD
Added · GHSA-x42w-4gqh-qrvm published (medium)
Oct 3, 2026 · GitHub Security Advisory
CVE published by MITRE.
Oct 3, 2026 · SOCRadar CTI

References

4 on the record

github.com/bcgit/bc-java/commit/be0a7d925c818ef2f338bcec55178b9b6336dfc3
Exploit, Third Party Advisory, NIST
github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071892
Exploit, Third Party Advisory, NIST
nvd.nist.gov/vuln/detail/CVE-2026-71892
NVD
github.com/advisories/GHSA-x42w-4gqh-qrvm
Exploit, Third Party Advisory

Elsewhere on this site

Legion of the Bouncy Castle Inc.every CVE for this vendor
Unclassifiedsame class
CWE-697other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-71892 is CMS key-transport recipient key-size validation never runs for RFC 9709 HKDF-derived keys, a medium vulnerability affecting BC-JAVA, BC-FJA from Legion of the Bouncy Castle Inc.. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-71892. Public exploit evidence is: 2 public PoC repositories reported; 0 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 6.9 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.