CVE-2026-34876: An issue was discovered in Mbed TLS 3.x before 3.6.6

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-34876 is a high-severity flaw (CVSS 7.5) 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.x before 3.6.6. An out-of-bounds read vulnerability in mbedtls_ccm_finish() in library/ccm.c allows attackers to obtain adjacent CCM context data via invocation of the multipart CCM API with an oversized tag_len parameter. This is caused by missing validation of the tag_len parameter against the size of the internal 16-byte authentication buffer. The issue affects the public multipart CCM API in Mbed TLS 3.x,

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 67%)
CISA exploited list	No
Weakness	CWE-125
Published	2026-04-02
Last modified	2026-07-24

Publisher (CNA): MITRE · Vendor: trustedfirmware

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-02 16:16 UTCPublished CVE record published
2026-07-24 21:10 UTCUpdated Record last modified

trustedfirmware mbed tls UPGRADE

Affected: >= 3.1.0 and < 3.6.6

Fixed in: 3.6.6

Safe to downgrade to: a release before 3.1.0

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 3.1.0 (Debian / Ubuntu)

apt-cache madison mbed-tls sudo apt install mbed-tls=<VERSION_BEFORE_3.1.0> 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_3.1.0> 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-2026-34876?

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

Is CVE-2026-34876 being exploited?

CVE-2026-34876 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-2026-34876 affect?

CVE-2026-34876 affects mbed tls. Severity is high (CVSS 7.5). An issue was discovered in Mbed TLS 3.x before 3.6.6. An out-of-bounds read vulnerability in mbedtls_ccm_finish() in library/ccm.c allows attackers to obtain adjacent CCM context data via invocation of the multipart CCM API with an oversize

How urgent is CVE-2026-34876?

Priority 39/100. High 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-2026-34875 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-2026-34873 CRITICAL 9.1 UPGRADE An issue was discovered in Mbed TLS 3.5.0 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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