CVE-2026-17507 MLS membership checks compare a uint32 leaf_index as signed, admitting an out-of-range sender
A signed integer comparison flaw in Bouncy Castle's MLS implementation allows a group member to cause a denial of service via heap exhaustion.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-17507?
CVE-2026-17507 is a high-severity vulnerability in Bouncycastle Bc-Java, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-195
Which versions are affected by CVE-2026-17507?
It affects 1 product.
Products include Bouncycastle Bc-Java.
How do I fix CVE-2026-17507?
Upgrade to a fixed version.
Is CVE-2026-17507 being exploited?
There is no known exploitation of CVE-2026-17507 in the wild and no public exploit code.
Other vulnerabilities in Bouncycastle Bc-Java
- CVE-2007-6721 Legion of the Bouncy Castle Java Cryptography API Bleichenbacher Oracle Vulnerability
- CVE-2018-1000613 Deserialization of Untrusted Data in Bouncy castle
- CVE-2026-58062 Stapled OCSP response accepted without binding to the checked certificate
- CVE-2026-59638 JSSE hostname verifier CN-fallback enabled by default despite documented opt-in
- CVE-2026-59650 MTI/A0 DH agreement exponentiates unvalidated peer value
Sources
Also known as CLEANSTART-2026-XF81157, DEBIAN-CVE-2026-17507, GHSA-x7w3-c4mv-whm4, UBUNTU-CVE-2026-17507.