CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent

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).

Scores

EPSS (exploitation in 30 days)

0.11% · top 99% of all CVEs (6 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: yes · Technical impact: partial

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 8.7.

Weakness

• CWE-354 Improper Validation of Integrity Check Value (opens in a new tab)All about CWE-354 and its CVEs

Affected products

• Legion of the Bouncy Castle Inc. · BC-JAVA

  • affected < 1.86

• Legion of the Bouncy Castle Inc. · BC-JAVA

  • affected 1.70 – < 1.86

• Legion of the Bouncy Castle Inc. · BC-LTS-JAVA

  • affected 2.73.0 – < 2.73.13

• Legion of the Bouncy Castle Inc. · BC-LTS-JAVA

  • affected 2.73.0 – < 2.73.13

• Legion of the Bouncy Castle Inc. · BC-FJA

  • affected 1.0.0 – < 1.0.13
  • affected 2.0.0 – < 2.0.13
  • affected 2.1.0 – < 2.1.13

• Legion of the Bouncy Castle Inc. · BC-FJA

  • affected 2.0.0 – < 2.0.8
  • affected 2.1.0 – < 2.1.8

References (2)

• https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071888 (opens in a new tab)vendor-advisory
• https://github.com/bcgit/bc-java/commit/dcb683b32b440f1beb1d2276a34930164281fb95 (opens in a new tab)patch

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisories (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-71888](https://trackcve.app/badge/CVE-2026-71888.svg)](https://trackcve.app/cve/CVE-2026-71888)

Record last updated 5 Oct 2026, 16:52 UTC.