CVE-2026-8686: Missing bounds validation in the MQTT v5.0 property parser in coreMQTT before 5.0.1 allows an MQTT broker to c

Routine

HIGH 7.5REMOTEDOWNGRADE

CVE-2026-8686 is a high-severity flaw (CVSS 7.5) affecting coremqtt. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Missing bounds validation in the MQTT v5.0 property parser in coreMQTT before 5.0.1 allows an MQTT broker to cause a denial of service by sending a crafted packet. To remediate this issue, users should upgrade to v5.0.1.

Why this priority

High severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

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 67%)
CISA exploited list	No
Weakness	CWE-125
Published	2026-05-15
Last modified	2026-10-25

Publisher (CNA): AMZN · Vendor: freertos

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-15 19:17 UTCPublished CVE record published
2026-10-25 23:10 UTCUpdated Record last modified

freertos coremqtt DOWNGRADE

Affected: 5.0.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 5.0.0

1. Check installed version

apt-cache policy coremqtt || rpm -q coremqtt || coremqtt --version

2. No fixed release: DOWNGRADE to a release before 5.0.0 (Debian / Ubuntu)

apt-cache madison coremqtt sudo apt install coremqtt=<VERSION_BEFORE_5.0.0> sudo apt-mark hold coremqtt

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list coremqtt sudo dnf downgrade coremqtt-<VERSION_BEFORE_5.0.0> sudo dnf versionlock add coremqtt

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-2026-8686?

coremqtt: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-8686 being exploited?

CVE-2026-8686 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-2026-8686 affect?

CVE-2026-8686 affects coremqtt. Severity is high (CVSS 7.5). Missing bounds validation in the MQTT v5.0 property parser in coreMQTT before 5.0.1 allows an MQTT broker to cause a denial of service by sending a crafted packet. To remediate this issue, users should upgrade to v5.0.1.

How urgent is CVE-2026-8686?

Priority 39/100. High severity; Remotely exploitable, no login, no user action

References

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