What is CVE-2026-71888?

CVE-2026-71888 is CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent. The supplied enrichment data classifies it as Unclassified mapped to CWE-354 CWE-354.

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-2026-71888. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-71888 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-2026-71888 using CVSS 8.7, 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.

highpublic exploit

CVE-2026-71888

CVE-2026-71888 — CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent

In Bouncy Castle for Java before 1.86, the streaming CMS AuthenticatedData parser accepted a message whose digestAlgorithm and authAttrs fields disagreed about whether authenticated attributes were present. RFC 5652 sec. 9.1 pairs the two, requiring that authAttrs be present whenever digestAlgorithm is, and sec. 9.2 makes the MAC cover the DER encoding of authAttrs when they are present and the eContent OCTET STRING directly when they are not. CMSAuthenticatedDataParser has to choose between those two in its constructor, before it can reach authAttrs, which comes later in the SEQUENCE, so it chose on digestAlgorithm alone: for a message with digestAlgorithm absent but authAttrs present it verified the content MAC and then returned the attributes through getAuthAttrs() as though they had been authenticated, when the MAC had never covered them. An attacker able to modify a message in transit could insert an authenticated attribute, such as an RFC 2634 ESSSecurityLabel, into an otherwise valid message while holding neither the key-encryption key nor the content-MAC key, and an application taking an authorization, routing or labelling decision from those attributes would act on attacker-chosen values. The content itself remained MAC-bound. asn1.cms.AuthenticatedData now rejects the mismatched pairing when parsing and CMSAuthenticatedDataParser cross-checks the two fields once authAttrs is read. This is a variant of CVE-2026-59642, which bound the content to the MAC for messages that legitimately carry authAttrs, and which does not address this case. This issue also affects Bouncy Castle for Java LTS before 2.73.13, and Bouncy Castle for Java FIPS (BC-FJA) before bcpkix-fips 1.0.13 (1.0.X series), 2.0.13 (2.0.X series) and 2.1.13 (2.1.X series), and bcutil-fips 2.0.8 (2.0.X series) and 2.1.8 (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.

9 products listed

CVSS base

8.7

high

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	0 to < 1.86	Vulnerable
Legion of the Bouncy Castle Inc.	BC-JAVA	1.70 to < 1.86	Vulnerable
Legion of the Bouncy Castle Inc.	BC-LTS-JAVA	2.73.0 to < 2.73.13	Vulnerable
Legion of the Bouncy Castle Inc.	BC-LTS-JAVA	2.73.0 to < 2.73.13	Vulnerable
Legion of the Bouncy Castle Inc.	BC-FJA	1.0.0 to < 1.0.13	Vulnerable
Legion of the Bouncy Castle Inc.	BC-FJA	2.0.0 to < 2.0.13	Vulnerable
Legion of the Bouncy Castle Inc.	BC-FJA	2.1.0 to < 2.1.13	Vulnerable
Legion of the Bouncy Castle Inc.	BC-FJA	2.0.0 to < 2.0.8	Vulnerable
Legion of the Bouncy Castle Inc.	BC-FJA	2.1.0 to < 2.1.8	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
8.7	CVSS 4.0	high	—	—	Legion of the Bouncy Castle Inc. (CNA)

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

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-354 · Improper Validation of Integrity Check Value

Modify Application Data
Other
Hide Activities

Mitigation: Ensure that the checksums present in messages are properly checked in accordance with the protocol specification before they are parsed and used.::

Weakness & attack patterns

CWE-354

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

CAPEC-463 · Padding Oracle Crypto AttackCAPEC-145 · Checksum SpoofingCAPEC-75 · Manipulating Writeable Configuration Files

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-LTS-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 8.7 (AV:N)
Oct 3, 2026 · NVD
Initial · CVE published
Oct 3, 2026 · NVD
Added · GHSA-j453-27jp-4cph published (high)
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/dcb683b32b440f1beb1d2276a34930164281fb95
Exploit, Third Party Advisory, NIST
github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071888
Exploit, Third Party Advisory, NIST
nvd.nist.gov/vuln/detail/CVE-2026-71888
NVD
github.com/advisories/GHSA-j453-27jp-4cph
Exploit, Third Party Advisory

Elsewhere on this site

Legion of the Bouncy Castle Inc.every CVE for this vendor
Unclassifiedsame class
CWE-354other 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-2026-71888 is CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent, a high vulnerability affecting BC-JAVA, BC-LTS-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-2026-71888. 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 8.7 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.