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OpenAdvisories
Medium · 6.9 CVE-2026-71892 EUVD-2026-91989GHSA-x42w-4gqh-qrvm

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

Published Oct 3, 2026 · Updated Oct 5, 2026 · Assigned by bcorg

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).

Affected products

VendorProductAffectedFixed
Legion of the Bouncy Castle BC-JAVA 1.78 – < 1.86 1.86
Legion of the Bouncy Castle BC-FJA 2.0.7 – < 2.0.13, 2.1.0 – < 2.1.13 2.0.13

Fixed in Linux distributions

Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.

ReleasePackageFixed inAdvisory
Debian 14 bouncycastle 1.86-1 —
Debian 13 bouncycastle No fix yet —
Debian 12 bouncycastle No fix yet —
ReleasePackageFixed inAdvisory
Ubuntu 26.10 bouncycastle No fix yet —
Ubuntu 26.04 LTS bouncycastle No fix yet —
Show 5 more (older releases, extended support, other products)
ReleasePackageFixed inAdvisory
Ubuntu Pro 24.04 LTS bouncycastle No fix yet —
Ubuntu Pro 22.04 LTS bouncycastle No fix yet —
Ubuntu Pro 20.04 LTS bouncycastle No fix yet —
Ubuntu Pro 18.04 LTS bouncycastle No fix yet —
Ubuntu Pro 16.04 LTS bouncycastle No fix yet —
ReleasePackageFixed inAdvisory
SLES 15 SP7 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
SLES 15 SP6 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
SLES 15 SP5 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
SLES 15 SP4 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
Show 7 more (older releases, extended support, other products)
ReleasePackageFixed inAdvisory
SLES for SAP 16.0 bouncycastle 1.86-160000.1.1 SUSE-SU-2026:24461-1
SLES for SAP 15 SP6 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
SLES for SAP 15 SP5 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
SLES for SAP 15 SP4 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
SLES 16.0 bouncycastle 1.86-160000.1.1 SUSE-SU-2026:24461-1
SLE HPC 15 SP5 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
SLE HPC 15 SP4 bouncycastle 1.86-150200.3.41.1 SUSE-SU-2026:4797-1
ReleasePackageFixed inAdvisory
openSUSE Tumbleweed bouncycastle 1.86-1.1 openSUSE-SU-2026:12090-1
openSUSE Leap 16.0 bouncycastle 1.86-160000.1.1 openSUSE-SU-2026:22169-1

Advisories 5

References