---
id: "CVE-2026-126839"
page: "https://infrastack.ai/live/CVE-2026-126839"
kind: "vulnerability"
aliases: ["CVE-2026-126839"]
title: "BSON deserializer: type allow-list bypass lets unlisted classes be built from input"
severity: "medium"
cvss: 4.2
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["bson", "deserialization", "package"]
not_affected_if: ["no-loosely-typed-deserialization", "trusted-input-only"]
published_at: "2026-11-13T00:20:13+00:00"
first_seen_at: "2026-11-13T00:41:06+00:00"
updated_at: "2026-11-13T00:41:24+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-126839", "first_seen_at": "2026-11-13T00:41:06+00:00"}]
exploit_sources: []
---

# BSON deserializer: type allow-list bypass lets unlisted classes be built from input

Link for people: https://infrastack.ai/live/CVE-2026-126839 (this .md is for agents)

The BSON serialization component's type-restriction check identifies compiler-generated types with an overly loose heuristic, so it can be bypassed. An attacker who can supply BSON that the application deserializes into a loosely typed member can get types outside the allow-list constructed and populated with attacker-chosen values, causing unexpected state and downstream errors.

_Summary, check and fix written by a model from the sources; labelled as such._

## Timeline

- 2026-11-13T00:41:06+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-126839

## From the sources

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.

---

_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_