High 8.7Published Oct 2, 2026

CVE-2026-17507MLS membership checks compare a uint32 leaf_index as signed, admitting an out-of-range sender

Updated under an hour ago (2026-10-19 09:20 UTC)

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It affects Legion of the Bouncy Castle Inc. BC-JAVA and 1 other product. Add what you run to find out whether that includes you.

EPSS puts the chance of exploitation in the next 30 days at 0.34%, higher than 26% of scored CVEs.

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Every CVE for Legion of the Bouncy Castle Inc. BC-JAVA

Exploited in the last 30 days

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.

CVSS: 8.7 HIGH (v4.0, CNA)CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/U:Amber
Weakness (CWE): CWE-195
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): 0.34% (percentile 26)
Assigned by: bcorg
NVD analysis: Analyzed
Record last updated: Oct 2, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Legion of the Bouncy Castle Inc. BC-JAVA legion_of_the_bouncy_castle:bc_java (from the name)	from 1.73 before 1.86	CNA record
bouncycastle bc-java bouncycastle:bc-java	from 1.73 before 1.86	NVD

References

https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9017507vendor-advisory, patch
https://github.com/bcgit/bc-java/commit/1eb83471eb01727bfdb944a5a46355cc77d55f44patch

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.