Key-size policy bypass via unreachable HKDF validation
Published Oct 3, 2026 · Updated Oct 3, 2026
CMS key-size validation in Bouncy Castle Java before 1.86 allows attackers to submit HKDF-derived keys that contradict the declared cipher. JceKeyTransRecipient compares an encrypted-key byte array with the HKDF object identifier, so the intended branch never selects the inner cipher and the size comparison is skipped. The bypass requires a recipient that enabled validation and processes a crafted CMS message; it also affects the FIPS Java API, but does not itself disclose plaintext.
Summary
What happened
CMS key-size validation in Bouncy Castle Java before 1.86 allows attackers to submit HKDF-derived keys that contradict the declared cipher. JceKeyTransRecipient compares an encrypted-key byte array with the HKDF object identifier, so the intended branch never selects the inner cipher and the size comparison is skipped. The bypass requires a recipient that enabled validation and processes a crafted CMS message; it also affects the FIPS Java API, but does not itself disclose plaintext.
The record
- CVE
- CVE-2026-71892
- Published
- Oct 3, 2026
- Updated
- Oct 3, 2026
- Vendor
- Legion of the Bouncy Castle Inc.
- Product
- Bouncy Castle Java
- Classifications
- Unavailable
- Attack vector
- Unavailable
- Privileges
- Unavailable
Timeline
How it unfolded
- Oct 3, 2026CVE publishedPublication date reported by the CVE source.
- Oct 3, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=1.78 <1.86
What conditions does exploitation require?
What is affected?
Published CVSS scores
No CVSS assessment is available in this record.
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Activity data is delayed. The latest report is Sep 28, 2026.
Weakness, pattern, technique
No sourced classifications are available.
Public exploit references
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Get started to see how Hinoki identifies affected software and validates exploitability in your environment.
Get started