CVE-2026-126839: MongoDB BSON deserialization type-restriction bypass

CVE-2026-126839 · Severity: medium · CVSS 4.2 · Published 2026-11-13

MongoDB's BSON serialization component uses an imprecise method to restrict which types can be deserialized, allowing an attacker to bypass this control and instantiate unauthorized types. An attacker who can inject malicious BSON data into an application can cause it to create objects of unintended types with attacker-controlled values, potentially leading to application errors or unexpected behavior.

Vendors: MongoDB.

Quick answers

Is CVE-2026-126839 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-126839 being exploited in the wild.
How severe is CVE-2026-126839?
Medium severity, with a CVSS score of 4.2, published 13 November 2026.
Which products and versions are affected?
MongoDB BSON.
Is there a fix for CVE-2026-126839?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-126839 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-126839?
Check whether you run MongoDB BSON. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The BSON deserialization type-restriction control relies on a heuristic to identify compiler-generated types, which is overly permissive and can be bypassed. An unauthenticated attacker able to supply BSON content destined for deserialization into loosely typed members can construct types outside the allow-list and populate them with arbitrary values. This results in unexpected object state and downstream logic errors with no privilege escalation required.

Affected products

  • MongoDB BSON

Timeline

  • 2026-11-13: disclosed

References

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