Medium

CVE-2026-84971 — Improper handling of an unexpected value size in the decryption path of a client-side…

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.

Published Sep 3, 2026 · Updated Oct 15, 2026 · Sources: NIST National Vulnerability Database · CNA: cna@mongodb.com

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

—

No EPSS score.

Affects

Mongodb

Libmongocrypt

CVSS base

6.5

Medium · 1 source(s)

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — jira.mongodb.org (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Mongodb Libmongocrypt).
Apply the fixed version or the patch documented under Remediation.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Mongodb	Libmongocrypt	≥ 1.7.0 and < 1.20.4	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

Low

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
7.1	CVSS 4.0	High	—	—	cna@mongodb.com via NIST National Vulnerability Database
6.5	CVSS 3.1	Medium	2.8	3.6	cna@mongodb.com via NIST National Vulnerability Database

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (score shown at the top: cna@mongodb.com)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-617

Other CVEs with this weakness →

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
Record published
Oct 15, 2026
Fix documented
Oct 15, 2026 · NIST National Vulnerability Database
CVSS changed · 6.5
Oct 15, 2026 · NIST National Vulnerability Database
Source status changed · → Analyzed
Oct 15, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 15, 2026 · NIST National Vulnerability Database
CVE published
Sep 3, 2026 · cna@mongodb.com

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 15, 2026 – Oct 15, 2026

References

1 on the record

jira.mongodb.org/browse/MONGOCRYPT-971
Vendor Advisory, Issue Tracking, Patch, NIST National Vulnerability Database

Elsewhere on this site

Mongodb
every CVE for this vendor

CWE-617
other pages naming this weakness

Related CVEs (same products)

High CVE-2026-106433 Heap corruption via duplicate masterKey fields in MongoDB libmongocrypt 7d
Medium CVE-2026-106434 Unrecognized payload acceptance in explicit decryption in MongoDB libmongocrypt 7d
Medium CVE-2026-106429 Application denial of service via malformed KMS endpoint in MongoDB libmongocrypt 7d
Medium CVE-2026-84962 An unauthorized user with key vault write access may cause an authorized client to issue… Sep 3, 2026
Medium CVE-2026-81523 Cross-tenant database retargeting via dot/NUL injection in namespace strings in libmongocrypt Aug 27, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No FIRST EPSS score.
No CISA SSVC assessment.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-84971 is "Improper handling of an unexpected value size in the decryption path of a client-side…", rated Medium. It affects Libmongocrypt (Mongodb); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 6.5 describes technical severity. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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