---
id: CVE-2026-126839
title: >-
  Deserialization allow-list bypass via anonymous-type misdetection in BSON
  object deserialization
summary: >-
  A deserialization type-restriction control in the BSON serialization component
  can be bypassed because the check used to recognize a class of
  compiler-generated types relies on an overly permissive heuristic rather than
  a precise identif…
severity: medium
cvss: 4.2
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L'
cvssSource: cna
cwe:
  - CWE-502
vendor: MongoDB
product: C# Driver
affected:
  - c_driver >= 2.21.0 < 3.14.0
published: '2026-11-12'
updated: '2026-11-12'
sourceUpdated: '2026-11-12T23:58:16.490Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-126839'
references:
  - url: 'https://jira.mongodb.org/browse/CSHARP-6169'
tags:
  - cve.org
ingestedAt: '2026-11-13T00:18:16.515Z'
---

## Overview

A deserialization type-restriction control in the BSON serialization component can be bypassed because the check used to recognize a class of compiler-generated types relies on an overly permissive heuristic rather than a precise identification. An unauthorized party able to supply BSON content that an application later deserializes into a loosely typed member can cause types outside the intended allow-list to be constructed and populated with unauthorized party-supplied values inside the consuming application. This can lead to unexpected object state and errors in downstream application logic. No elevated privileges beyond the ability to place unauthorized party-influenced content in the deserialization path are required.

## Affected

- `c_driver >= 2.21.0 < 3.14.0`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.