CVE-2026-93764 in Mongoid
Summary
by MITRE • 09/18/2026
Mongoid may omit encryption rules for fields declared on embedded models when generating the client-side field-level encryption schema. Applications that enable this feature can therefore store values intended to be encrypted in readable form, with no error or warning. A party with routine read access to the database, a backup, or the underlying data files may then see data that was meant to remain unreadable outside the application.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability described involves a critical flaw in Mongoid's implementation of client-side field-level encryption when applied to embedded models within MongoDB documents. In modern applications utilizing document databases like MongoDB, it is common practice to embed related data structures directly within parent documents rather than normalizing them into separate collections. While this approach improves read performance and simplifies transactional integrity for certain use cases, it introduces complexity in how security policies are propagated across nested or embedded schemas. Mongoid provides a mechanism for defining encryption rules at the model level, ensuring that specific fields containing sensitive information such as personally identifiable data, financial records, or authentication credentials are encrypted before being persisted to the database storage engine. This process relies on generating a client-side schema definition that instructs the application driver which fields require encryption and decryption operations during write and read cycles respectively.
The core technical flaw lies in how Mongoid processes these schemas for embedded models. When an application declares encryption rules on fields belonging to an embedded model, the library fails to correctly include those specific field definitions within the generated client-side schema used by the MongoDB driver. This omission occurs silently without raising any exceptions or warnings during initialization or data persistence operations. Consequently, the encryption engine remains unaware of these fields and treats them as standard plaintext attributes. The result is that sensitive data intended for protection via envelope encryption or similar mechanisms is written to the database in its original readable form. This behavior creates a false sense of security where developers believe their most sensitive data is protected by cryptographic safeguards while it is actually exposed in clear text within the storage layer.
The operational impact of this vulnerability is severe and potentially catastrophic depending on the sensitivity of the affected data. Since the encryption rules are omitted, any entity with routine read access to the database can view unencrypted values that were meant to remain confidential. This includes not only direct attackers but also legitimate system administrators, backup operators, or cloud service providers who manage the underlying infrastructure. If an attacker gains access through a separate vulnerability such as SQL injection in another part of the application, they do not need to reverse-engineer encryption keys because no encryption was applied at all. Furthermore, backups and snapshots taken during routine maintenance cycles will also contain this unencrypted data, extending the window of exposure beyond just live database instances. The lack of error messages exacerbates the risk by allowing organizations to remain unaware of the misconfiguration until a breach occurs or an audit reveals discrepancies between expected security postures and actual storage states.
This issue aligns with CWE-319 which describes cleartext transmission of sensitive information, although in this context it is more accurately categorized under CWE-209 where an error condition leads to exposure of sensitive data due to improper handling or configuration errors. From a threat modeling perspective using the MITRE ATT&CK framework, this vulnerability facilitates Data from Information Repositories techniques such as T1005 which involves collecting data directly from local system sources like databases. The absence of encryption means that lateral movement by an attacker who has compromised application credentials can lead to immediate exfiltration of high-value assets without triggering alerts associated with decryption attempts or key usage anomalies.
Mitigation strategies must address both the technical defect and broader security hygiene practices. Organizations relying on Mongoid for field-level encryption should immediately audit their embedded model definitions to ensure that all sensitive fields are explicitly declared within parent models where possible, as this may bypass the bug depending on version specifics. Upgrading to a patched version of Mongoid is essential if available from the vendor, ensuring that the schema generation logic correctly propagates encryption rules for nested structures. Additionally, implementing database activity monitoring tools can help detect anomalous access patterns or unexpected plaintext data in fields previously expected to be encrypted. Developers should also adopt defense-in-depth principles by encrypting sensitive data at the application layer using robust libraries before passing it to the ORM, thereby reducing reliance on framework-level security features that may have implementation gaps. Regular penetration testing and code reviews focusing on data handling flows are critical to identifying such configuration drifts before they lead to compliance violations or data breaches under regulations like GDPR or HIPAA.