CVE-2026-34873: An issue was discovered in Mbed TLS 3.5.0 through 4.0.0

This month

CRITICAL 9.1REMOTEUPGRADE

CVE-2026-34873 is a critical-severity flaw (CVSS 9.1) affecting mbed tls. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An issue was discovered in Mbed TLS 3.5.0 through 4.0.0. Client impersonation can occur while resuming a TLS 1.3 session.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 85%)
CISA exploited list	No
Weakness	CWE-287
Published	2026-04-01
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: trustedfirmware

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-01 21:17 UTCPublished CVE record published
2026-06-17 10:39 UTCUpdated Record last modified

trustedfirmware mbed tls UPGRADE

Affected: >= 3.5.0 and < 3.6.6 · >= 4.0.0 and < 4.1.0

Fixed in: 3.6.6, 4.1.0

Safe to downgrade to: a release before 3.5.0

1. Check installed version

apt-cache policy mbed-tls || rpm -q mbed-tls || mbed-tls --version

2. UPGRADE to 3.6.6 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade mbed-tls

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mbed-tls

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 3.5.0 (Debian / Ubuntu)

apt-cache madison mbed-tls sudo apt install mbed-tls=<VERSION_BEFORE_3.5.0> sudo apt-mark hold mbed-tls

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

dnf --showduplicates list mbed-tls sudo dnf downgrade mbed-tls-<VERSION_BEFORE_3.5.0> sudo dnf versionlock add mbed-tls

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-2026-34873?

mbed tls: upgrade to 3.6.6, 4.1.0. Exact commands for each OS are on this page.

Is CVE-2026-34873 being exploited?

CVE-2026-34873 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-2026-34873 affect?

CVE-2026-34873 affects mbed tls. Severity is critical (CVSS 9.1). An issue was discovered in Mbed TLS 3.5.0 through 4.0.0. Client impersonation can occur while resuming a TLS 1.3 session.

How urgent is CVE-2026-34873?

Priority 41/100. Critical severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-34877 CRITICAL 9.8 UPGRADE DOWNGRADE An issue was discovered in Mbed TLS versions from 2.19.0 up to 3.6.5, Mbed TLS 4.0.0
CVE-2026-34876 HIGH 7.5 UPGRADE An issue was discovered in Mbed TLS 3.x before 3.6.6
CVE-2026-34875 CRITICAL 9.8 UPGRADE UPGRADE An issue was discovered in Mbed TLS through 3.6.5 and TF-PSA-Crypto 1.0.0
CVE-2026-34874 HIGH 7.5 UPGRADE An issue was discovered in Mbed TLS through 3.6.5 and 4.x through 4.0.0
CVE-2026-34872 CRITICAL 9.1 UPGRADE DOWNGRADE An issue was discovered in Mbed TLS 3.5.x and 3.6.x through 3.6.5 and TF-PSA-Crypto 1.0

References

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