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TI-2026-65795 Aliases CVE-2026-17507GHSA-x7w3-c4mv-whm4

MLS membership checks compare a uint32 leaf_index as signed, admitting an out-of-range sender

Priority MEDIUM 44CWE-195Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Attack requirements
None
Impact type
—
Confidentiality · integrity · availability
None · None · High
Fix status
Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA.

Summary

In Bouncy Castle for Java before 1.86, the MLS implementation (org.bouncycastle.mls) holds RFC 9420's uint32 leaf_index in a signed int, so a wire value with the top bit set decodes to a negative number. That is a legitimate encoding rather than malformed input, and it must still decode, since the MLS interop test vectors round-trip the full range. GroupKeySet.SecretTree.hasLeaf and Group.validateRemove compared the decoded value directly against the tree's leaf count, and a signed comparison treats any negative int as less than a positive bound, so an out-of-range sender passed the membership check. In the hasLeaf case the SenderData of an unprotected PrivateMessage could then drive LeafIndex.directPath through NodeIndex.parent() arithmetic that never reaches the tree root, growing the resulting node list without bound until the JVM exhausted its heap. A single small message from any current group member could therefore deny service to every other member of the group. Both comparisons now interpret the value as unsigned via Integer.toUnsignedLong, rejecting an out-of-range sender however it was encoded; well-formed leaf indices are unaffected.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.0042 36th percentileScored 2026-11-07
Fix Fixed BC-JAVA (Legion of the Bouncy Castle Inc.): 1.86

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
Yes
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
Legion of the Bouncy Castle Inc.BC-JAVA1.73 to before 1.861.86Patch availableCNA

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:bouncycastle:bc-java:*:*:*:*:*:*:*:*1.73 to before 1.86VulnerableLegion of the Bouncy Castle Inc.

Remediation

  • Fix · CNA BC-JAVA (Legion of the Bouncy Castle Inc.) Fixed in 1.86

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

  • Release ·

    Legion of the Bouncy Castle BC-JAVA 1.86

    3 records Highest priority Priority MEDIUM 1 MEDIUM, 2 LOW

    Other records, highest priority first

    • TI-2026-65797 NTRU leaks private key information by reducing secret values with a non-constant-time integer divisionPriority LOW 35
    • TI-2026-65796 Password-based KDF cost parameters honoured unbounded from untrusted input across the remaining PBE entry pointsPriority LOW 24
    Show all 3 records →

Vendor assessments

Red Hat

Red Hat's page for this CVE →

Not affected: 1 product
  • Red Hat

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-17507
  2. 2026-10-03 Alias added: GHSA-x7w3-c4mv-whm4
  3. 2026-10-15 CVSS score added · NVD

References

Patch 2

  • TI-2018-15967 Legion of the Bouncy Castle Legion of the Bouncy Castle Java Cryptography APIs 1.58 up to… CVSS 3.1CRITICAL 9.8
  • TI-2017-09736 BouncyCastle JCE TLS Bleichenbacher/ROBOT CVSS 3.0HIGH 7.5
  • TI-2026-42545 MTI/A0 DH agreement exponentiates unvalidated peer value CVSS 4.0CRITICAL 9.3
  • TI-2026-42533 JSSE hostname verifier CN-fallback enabled by default despite documented opt-in CVSS 4.0CRITICAL 9.3
  • TI-2026-41816 Stapled OCSP response accepted without binding to the checked certificate CVSS 4.0CRITICAL 9.3
  • TI-2026-07701 Name Constraints bypass via trailing dot in rfc822Name and URI CVSS 4.0CRITICAL 9.3
See all 52 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences