# 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.

- **Severity:** High 8.7 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Oct 2026
- **Updated:** 2 Oct 2026
- **Also known as:** CLEANSTART-2026-XF81157, DEBIAN-CVE-2026-17507, GHSA-x7w3-c4mv-whm4, UBUNTU-CVE-2026-17507

## 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 (Signed to Unsigned Conversion Error)

## Which versions are affected by CVE-2026-17507?

It affects 1 product.


Products: 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](https://insights.sbomify.com/CVE-2007-6721/): Legion of the Bouncy Castle Java Cryptography API Bleichenbacher Oracle Vulnerability (critical 10.0)
- [CVE-2018-1000613](https://insights.sbomify.com/CVE-2018-1000613/): Deserialization of Untrusted Data in Bouncy castle (critical 9.8)
- [CVE-2026-58062](https://insights.sbomify.com/CVE-2026-58062/): Stapled OCSP response accepted without binding to the checked certificate (critical 9.3)
- [CVE-2026-59638](https://insights.sbomify.com/CVE-2026-59638/): JSSE hostname verifier CN-fallback enabled by default despite documented opt-in (critical 9.3)
- [CVE-2026-59650](https://insights.sbomify.com/CVE-2026-59650/): MTI/A0 DH agreement exponentiates unvalidated peer value (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-17507 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-17507 in the wild and no public exploit code.

Affected product: Bouncycastle Bc-Java. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-17507/ (Markdown: https://insights.sbomify.com/CVE-2026-17507/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-17507
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-17507
- OSV: https://osv.dev/vulnerability/CVE-2026-17507

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-17507/
Summaries are generated and may contain mistakes; the official description above is the record.