CVE-2026-93763: MongoDB Mongoid Information Disclosure Flaw

CVE-2026-93763 Overview

CVE-2026-93763 affects MongoDB Mongoid, the Ruby object-document mapper (ODM) for MongoDB. The vulnerability stems from a protection mechanism failure in the ODM's encryption configuration generation. Fields that an application declares for client-side field-level encryption (CSFLE) can be written and retained in cleartext. No error or warning is raised at write time. Any party with ordinary read access to the database can retrieve values intended to be protected from that party, leading to unintended disclosure of sensitive information. The issue is tracked as CWE-312: Cleartext Storage of Sensitive Information.

Critical Impact
Sensitive application fields marked for CSFLE are silently persisted as plaintext, exposing them to any authenticated database reader.

Affected Products

MongoDB Mongoid (Ruby ODM)
MongoDB Mongoid version 9.1.0
Applications using Mongoid-generated CSFLE configuration

Discovery Timeline

2026-09-18 - CVE-2026-93763 published to the National Vulnerability Database
2026-09-24 - Last updated in NVD database

Technical Details for CVE-2026-93763

Vulnerability Analysis

Mongoid provides declarative field-level encryption through model annotations. Developers mark specific fields as encrypted, and Mongoid generates a CSFLE schema that the MongoDB driver uses to encrypt values before they leave the client. The vulnerability breaks this contract. Under specific configuration-generation paths, the schema Mongoid produces omits encryption metadata for fields the application declared as encrypted. The driver then treats those fields as ordinary attributes and writes them without transformation.

The failure is silent. Neither the ODM nor the driver signals that an encrypted field is being persisted in cleartext. Applications continue to operate normally, and reads via the same encrypted client return values that appear correct. Only a direct read outside of the CSFLE-aware application path reveals that the stored data is plaintext.

Because the disclosure requires only read access to the database, the impact scales with the number of principals holding routine query permissions, including operators, analytics jobs, backup systems, and any compromised credential.

Root Cause

The root cause is a protection mechanism failure in Mongoid's encryption configuration generator. The generator fails to emit encryption directives for declared fields under certain model or inheritance configurations. The result is a mismatch between the developer's declared intent and the CSFLE schema handed to the driver [CWE-312].

Attack Vector

Exploitation does not require an active attack. A party with low-privilege authenticated read access to the affected collection can query the field directly and retrieve cleartext values. The CVSS 4.0 vector indicates a network-reachable attack path with low attack complexity, low required privileges, and no user interaction, with confidentiality impact only.

See the MongoDB Issue Tracker entry MONGOID-5984 for the vendor's technical description.

Detection Methods for CVE-2026-93763

Indicators of Compromise

Fields declared with encrypt: true (or equivalent Mongoid encryption directives) returning readable string or numeric values when queried with a non-CSFLE client.
CSFLE schema documents generated by Mongoid that omit encryptMetadata or per-field encrypt entries for models that declare encrypted attributes.
Backup exports or mongodump output containing plaintext values for fields expected to appear as BSON binary subtype 6 (encrypted payload).

Detection Strategies

Audit generated CSFLE schemas against the application's Mongoid model definitions and flag any declared encrypted field missing from the schema.
Run a non-CSFLE read against a representative sample of documents and confirm that fields declared for encryption are not returned as readable cleartext.
Inspect BSON types for encrypted fields at the collection level; expected type is binary subtype 6, not string or numeric.

Monitoring Recommendations

Log and alert on database access by identities that should not have read access to sensitive collections.
Track schema-generation output during application deploys and diff against the previous version to catch silent encryption regressions.
Monitor Mongoid and MongoDB Ruby driver versions across services to detect drift onto affected releases.

How to Mitigate CVE-2026-93763

Immediate Actions Required

Inventory all Ruby services using Mongoid and identify those relying on client-side field-level encryption.
Verify at the collection level whether previously written data is stored in cleartext, and treat exposed records as disclosed to any principal that held read access.
Rotate secrets, tokens, and credentials that may have been stored in the affected fields.
Upgrade Mongoid to a fixed release once available from MongoDB; consult the MongoDB Issue Tracker entry MONGOID-5984 for the current fixed version.

Patch Information

MongoDB tracks the fix under MONGOID-5984. Applications running Mongoid 9.1.0 and earlier releases in the affected range should upgrade to the patched version identified in that ticket. After upgrade, regenerate the CSFLE schema and re-encrypt any records that were persisted in cleartext.

Workarounds

Restrict database read privileges to the minimum set of principals required, reducing the population that can observe cleartext values.
Enforce CSFLE at the driver level by supplying an explicit schemaMap rather than relying solely on Mongoid's generated configuration.
Re-encrypt affected collections by reading each document through a CSFLE-aware client and writing it back after confirming the corrected schema is in effect.
Apply MongoDB queryable encryption or field-level access controls as a defense-in-depth layer until the ODM patch is deployed.

bash

# Verify a field is stored as encrypted BSON binary subtype 6
mongosh "$MONGO_URI" --quiet --eval '
  db.getSiblingDB("app").users.aggregate([
    { $project: { field_type: { $type: "$ssn" } } },
    { $limit: 5 }
  ]).forEach(printjson)
'
# Expected output for encrypted fields: { field_type: "binData" }
# Cleartext indicator: { field_type: "string" } or "int"