CVE-2026-17507: Bouncycastle Bc-Java Security Vulnerability (CVSS 8.7)

CVE-2026-17507 affects Bouncycastle Bc-Java, a product from Bouncycastle. Before acting, confirm where Bouncycastle Bc-Java runs in your estate, who owns it and whether it is reachable from the internet, because exposure decides real risk more than the score alone.

In plain terms: 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.

Based on its CVSS vector, the flaw can be reached over the network, needs no login, and needs no user interaction. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.