CVE-2026-84971: Improper handling of an unexpected value size in the decryption path of a client-side encryption library can c

Routine

MEDIUM 6.5UPGRADE

CVE-2026-84971 is a medium-severity flaw (CVSS 6.5) affecting libmongocrypt. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Improper handling of an unexpected value size in the decryption path of a client-side encryption library can cause a failed internal check that terminates the process using the library. A party able to place a suitably formed encrypted value where an application will decrypt it, or able to control the responses the application receives, may cause that application to stop running.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.3% (top 83%)
CISA exploited list	No
Weakness	CWE-617
Published	2026-09-03
Last modified	2026-09-17

Publisher (CNA): MongoDB, Inc. · Vendor: mongodb

Last updated 2026-09-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-03 15:17 UTCPublished CVE record published
2026-09-17 13:59 UTCUpdated Record last modified

mongodb libmongocrypt UPGRADE

Affected: >= 1.7.0 and < 1.20.4

Fixed in: 1.20.4

Safe to downgrade to: a release before 1.7.0

1. Check installed version

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

2. UPGRADE to 1.20.4 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libmongocrypt

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libmongocrypt

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 1.7.0 (Debian / Ubuntu)

apt-cache madison libmongocrypt sudo apt install libmongocrypt=<VERSION_BEFORE_1.7.0> sudo apt-mark hold libmongocrypt

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

dnf --showduplicates list libmongocrypt sudo dnf downgrade libmongocrypt-<VERSION_BEFORE_1.7.0> sudo dnf versionlock add libmongocrypt

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-2026-84971?

libmongocrypt: upgrade to 1.20.4. Exact commands for each OS are on this page.

Is CVE-2026-84971 being exploited?

CVE-2026-84971 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-2026-84971 affect?

CVE-2026-84971 affects libmongocrypt. Severity is medium (CVSS 6.5). Improper handling of an unexpected value size in the decryption path of a client-side encryption library can cause a failed internal check that terminates the process using the library. A party able to place a suitably formed encrypted valu

How urgent is CVE-2026-84971?

Priority 27/100. No elevated risk signals yet.

Software pages

MongoDB vulnerabilities

Related CVEs

CVE-2026-84962 MEDIUM 4.2 UPGRADE An unauthorized user with key vault write access may cause an authorized client to issue arbitrary authenticat
CVE-2026-81523 MEDIUM 4.4 UPGRADE A missing input-validation issue in MongoDB libmongocrypt's automatic-encryption context setup allows a caller

References

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