CVE-2026-93758: MongoDB Mongoid Auth Bypass Vulnerability

CVE-2026-93758 Overview

CVE-2026-93758 is an insecure direct object reference (IDOR) vulnerability in the Mongoid object-document mapper (ODM) developed by MongoDB. The flaw resides in the nested attributes handling logic, where record identifiers submitted by a user are processed without enforcing ownership or scoping restrictions. An authenticated user with basic application privileges can supply the identifier of a record belonging to another user. Mongoid then loads that record, updates it, and links it to the attacker's own account. The result is unauthorized disclosure and modification of data across tenant or user boundaries. The weakness is classified under CWE-639: Authorization Bypass Through User-Controlled Key.

Critical Impact
Authenticated attackers can read and modify records owned by other users of any Ruby application that relies on Mongoid nested attributes for form input processing.

Affected Products

MongoDB Mongoid ODM (Ruby)
MongoDB Mongoid version 9.1.0
Ruby applications using Mongoid accepts_nested_attributes_for with user-controlled identifiers

Discovery Timeline

2026-09-18 - CVE-2026-93758 published to the National Vulnerability Database
2026-09-25 - Last updated in NVD database

Technical Details for CVE-2026-93758

Vulnerability Analysis

Mongoid is the official Ruby ODM for MongoDB and provides the accepts_nested_attributes_for mechanism to persist associated documents through a single form submission. When a nested payload includes an _id (or id) key, Mongoid resolves that identifier to an existing associated document and applies the submitted attribute updates.

The vulnerable code path performs the identifier lookup without applying the parent association's ownership scope. Consequently, any authenticated user who can submit a nested attributes payload can substitute an arbitrary document identifier. Mongoid retrieves the referenced document, applies the attacker-supplied attribute changes, and re-links the document to the attacker's parent record.

This bypasses application-level authorization checks that assume nested identifiers refer only to records the user already owns. The impact includes confidentiality loss for the disclosed record and integrity loss where attributes are overwritten.

Root Cause

The root cause is missing authorization validation on user-controlled document identifiers passed through nested attributes. Mongoid trusts the identifier supplied in the request body and treats the lookup as a trusted internal operation. No scoping constraint is applied to ensure the referenced document belongs to the parent association owned by the current user.

Attack Vector

Exploitation requires network access to the target application and low-privilege authenticated credentials. No user interaction is required. An attacker submits a standard update request to any endpoint that permits nested attribute writes. Within the nested payload, the attacker replaces the legitimate _id value with the identifier of a victim's document. The server-side controller passes the payload to Mongoid, which loads the victim's record, applies the writes, and returns success. Refer to the MongoDB Jira Issue MONGOID-5992 for the authoritative technical description.

Detection Methods for CVE-2026-93758

Indicators of Compromise

Application logs showing nested attribute update requests where the submitted child _id does not appear in the requesting user's prior read history.
MongoDB audit log entries showing update operations against documents whose _id values were never previously accessed by the acting user session.
Unexpected reassignment of foreign key or association fields linking one user's records to another user's parent document.

Detection Strategies

Enable MongoDB server audit logging and correlate document identifiers written during nested update requests against the authenticated session's prior document access set.
Instrument Mongoid callbacks (before_update, before_save) on associated models to log the acting current_user and the loaded document owner, and alert on mismatches.
Deploy application-layer request inspection to flag POST or PATCH bodies containing nested _id fields that reference documents outside the caller's tenant scope.

Monitoring Recommendations

Track spikes in update_one and find_and_modify operations initiated by low-privilege user roles.
Monitor for the sudden appearance of many-to-one relationship changes where child documents are reassigned to new parents.
Alert on repeated 4xx-free requests to nested attribute endpoints from a single authenticated principal enumerating sequential or high-entropy _id values.

How to Mitigate CVE-2026-93758

Immediate Actions Required

Inventory all Ruby applications using Mongoid 9.1.0 and identify controllers that permit nested attributes for associated documents.
Restrict which parameters are permitted through Rails strong parameters, and strip _id and id keys from nested attribute payloads unless strictly required.
Add explicit authorization checks in controllers that verify every nested identifier resolves to a document owned by current_user before invoking Mongoid persistence methods.

Patch Information

MongoDB tracks the fix under MongoDB Jira Issue MONGOID-5992. Upgrade to a Mongoid release that incorporates the ownership-scoping fix once available and verify the patched version through the vendor advisory.

Workarounds

Override accepts_nested_attributes_for behavior in affected models to reject nested payloads containing _id values that fall outside the parent association's existing collection.
Introduce an authorization framework such as Pundit or CanCanCan and enforce record-level scopes on every nested document lookup.
Disable nested attribute writes on high-sensitivity models and require dedicated authorized endpoints for updating associated records.

bash

# Configuration example: strip nested _id keys in a Rails controller
# to prevent identifier substitution until a patched Mongoid is deployed
def safe_params
  params.require(:parent).permit(
    :name,
    children_attributes: [:name, :value]  # note: :id intentionally omitted
  )
end