CMS key-transport recipient key-size validation never runs for RFC 9709 HKDF-derived keys

Description

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).

Scores

EPSS (exploitation in 30 days)

0.17% · top 93% 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: Medium 6.9.

Weakness

• CWE-697 Incorrect Comparison (opens in a new tab)All about CWE-697 and its CVEs

Affected products

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

  • affected 1.78 – < 1.86

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

  • affected 2.0.7 – < 2.0.13
  • affected 2.1.0 – < 2.1.13

References (2)

• https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071892 (opens in a new tab)vendor-advisory
• https://github.com/bcgit/bc-java/commit/be0a7d925c818ef2f338bcec55178b9b6336dfc3 (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-2026-71892](https://trackcve.app/badge/CVE-2026-71892.svg)](https://trackcve.app/cve/CVE-2026-71892)

Record last updated 5 Oct 2026, 16:58 UTC.