CVE-2026-17507: In Bouncy Castle for Java before 1.86, the MLS implementation (org.bouncycastle.mls) holds RFC 9420's uint32 l

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-17507 is a high-severity flaw (CVSS 7.5) affecting bc-java. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 64%)
CISA exploited list	No
Weakness	CWE-195
Published	2026-10-02
Last modified	2026-10-14

Publisher (CNA): bcorg · Vendor: bouncycastle

Last updated 2026-10-14 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-10-02 08:17 UTCPublished CVE record published
2026-10-14 18:59 UTCUpdated Record last modified

bouncycastle bc-java UPGRADE

Affected: >= 1.73 and < 1.86

Fixed in: 1.86

Safe to downgrade to: a release before 1.73

1. Check installed version

apt-cache policy bc-java || rpm -q bc-java || bc-java --version

2. UPGRADE to 1.86 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade bc-java

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade bc-java

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 1.73 (Debian / Ubuntu)

apt-cache madison bc-java sudo apt install bc-java=<VERSION_BEFORE_1.73> sudo apt-mark hold bc-java

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list bc-java sudo dnf downgrade bc-java-<VERSION_BEFORE_1.73> sudo dnf versionlock add bc-java

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-17507?

bc-java: upgrade to 1.86. Exact commands for each OS are on this page.

Is CVE-2026-17507 being exploited?

CVE-2026-17507 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-17507 affect?

CVE-2026-17507 affects bc-java. Severity is high (CVSS 7.5). 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

How urgent is CVE-2026-17507?

Priority 40/100. High severity; Remotely exploitable, no login, no user action

Software pages

Java vulnerabilities

Related CVEs

CVE-2026-59644 HIGH 7.5 UPGRADE In Bouncy Castle for Java before 1.85, MLS hash-ratchet honours arbitrary 32-bit generation counter from sende
CVE-2026-12852 HIGH 7.5 UPGRADE In Bouncy Castle for Java before 1.85, MLS wire decoder allocates attacker-declared opaque length before bound
CVE-2026-59652 MEDIUM 6.5 UPGRADE In Bouncy Castle for Java before 1.85, LDAP filter injection in legacy jdk1.4 LDAPStoreHelper.

References

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