CVE-2026-71888 CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent
Attackers can inject unauthenticated attributes into CMS messages, causing applications to act on attacker-controlled values for authorization or routing decisions.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-71888?
CVE-2026-71888 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. BC-FJA, published on 3 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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).
Weakness: CWE-354
Which versions are affected by CVE-2026-71888?
How do I fix CVE-2026-71888?
Upgrade to a fixed version.
Is CVE-2026-71888 being exploited?
There is no known exploitation of CVE-2026-71888 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-EM43415, CLEANSTART-2026-PP90634, DEBIAN-CVE-2026-71888, GHSA-j453-27jp-4cph, OESA-2026-4243, OESA-2026-4244 and 4 more.