Remote code execution via generic-type allow-list bypass in BSON object deserialization

published 2026-11-13 2d ago modified 2026-11-13 yesterday also EUVD-2026-124807

cvss: 7.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 22%
public exploits: 0; none known

A type allow-list bypass in the BSON serialization library of the MongoDB C# Driver allows a type-restriction check to be circumvented when polymorphic data is deserialized into loosely typed members. An unauthorized party who can supply or plant BSON/JSON data that the application later deserializes can cause the application process to instantiate .NET types outside the intended allow-list and populate them with unauthorized party-controlled values. In applications that contain a suitable gadget type, this can lead to arbitrary code execution in the context of the application, with corresponding loss of confidentiality, integrity, and availability. No special privileges beyond the ability to introduce data the application reads are required.

Patch / fix

fixed in 1

MongoDB C# Driver 3.14.0

Is CVE-2026-126982 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

Check with Fencer

Exploit prediction

0.30% chance of exploitation in the next 30 days, higher than 22% of all scored CVEs (FIRST EPSS, 2026-11-14).

CVSS scores

assigned by	version	score	vector
CNA	4.0	7.7	CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
CNA	3.1	7.5	CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
CNA	4.0	7.7	CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Affected products

MongoDB C# Driver

Classification

Weakness

CWE-502 Deserialization of Untrusted Data

References

jira.mongodb.org /browse/CSHARP-6160

Sources

CVE List CVE-2026-126982
NVD CVE-2026-126982

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.