CVE-2026-71888

CVE-2026-71888 is a high-severity vulnerability with a CVSS 4.0 base score of 8.7. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-354.

Key facts

Description

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).

Frequently asked questions

What is CVE-2026-71888?
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).
How severe is CVE-2026-71888?
CVE-2026-71888 has a CVSS 4.0 base score of 8.7, rated high severity.
Is CVE-2026-71888 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (1st percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-71888?
Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2026-71888 published?
CVE-2026-71888 was published on 2026-10-03 and last updated on 2026-10-06.

References

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