CVE-2026-18687 Overview
CVE-2026-18687 is an input validation flaw in MongoDB Server's handling of a Queryable Encryption maintenance operation. The server does not properly validate certain request parameters against the collection's encrypted field configuration before use. An authenticated user with readWrite privileges can submit a specially formed request that triggers a server crash or excessive internal writes. Exploitation results in resource exhaustion and corruption of encrypted index data. The issue is classified under [CWE-191] Integer Underflow and affects deployments using Queryable Encryption.
Critical Impact
Authenticated attackers with readWrite privileges can crash the MongoDB Server, exhaust resources, and corrupt encrypted index data across affected collections.
Affected Products
- MongoDB Server (versions using Queryable Encryption maintenance operations)
- Deployments with collections configured with encrypted field configurations
- Environments granting readWrite privileges to application or user accounts
Discovery Timeline
- 2026-08-11 - CVE-2026-18687 published to NVD
- 2026-08-11 - Last updated in NVD database
Technical Details for CVE-2026-18687
Vulnerability Analysis
The vulnerability resides in the code path handling a Queryable Encryption maintenance operation. The server accepts request parameters and uses them without validating that they are consistent with the target collection's encrypted field configuration. When mismatched parameters reach downstream logic, the server either terminates unexpectedly or enters a state that triggers excessive internal writes. Both outcomes degrade availability. The excessive-writes path can also corrupt encrypted index structures, which affects the integrity of query results over encrypted fields.
Root Cause
The root cause is missing input validation against the schema-defined encrypted field configuration [CWE-191]. Request parameters that should be checked for size, range, or structural compatibility are consumed directly by the maintenance routine. An integer underflow condition in this path enables the operation to iterate or allocate incorrectly, producing the crash or write-amplification behavior.
Attack Vector
An attacker requires network access to the MongoDB Server and authenticated credentials carrying readWrite privileges on a collection using Queryable Encryption. The attacker submits a crafted maintenance operation with parameters that do not align with the collection's encrypted field configuration. No user interaction is required. Because the operation runs server-side, the impact reaches every client relying on the affected collection. See the MongoDB Jira Issue SERVER-130628 for technical tracking details.
Detection Methods for CVE-2026-18687
Indicators of Compromise
- Unexpected mongod process crashes or restarts correlated with maintenance operations against Queryable Encryption collections.
- Sudden spikes in write operations, oplog growth, or disk I/O tied to a single authenticated user session.
- Errors or inconsistencies when querying encrypted fields, suggesting corruption of encrypted index data.
- Audit log entries showing malformed or repeated Queryable Encryption maintenance commands.
Detection Strategies
- Enable MongoDB auditing and alert on Queryable Encryption maintenance operations submitted by non-administrative accounts.
- Correlate mongod crash events with recent commands from readWrite users to identify abuse patterns.
- Baseline normal write volume per user and flag deviations that indicate write amplification.
Monitoring Recommendations
- Forward MongoDB audit logs, mongod.log, and system metrics to a centralized SIEM for correlation.
- Track process availability, replica set health, and oplog growth rate as continuous signals.
- Monitor query error rates against encrypted collections to detect index corruption early.
How to Mitigate CVE-2026-18687
Immediate Actions Required
- Upgrade MongoDB Server to the fixed release identified in the vendor advisory referenced by SERVER-130628.
- Audit role assignments and revoke readWrite privileges from accounts that do not require them on Queryable Encryption collections.
- Restrict network access to MongoDB Server so only trusted application hosts can reach the database port.
- Back up encrypted collections and verify integrity of encrypted index data before and after patching.
Patch Information
MongoDB tracks the fix under MongoDB Jira Issue SERVER-130628. Apply the corresponding patched server build for your deployed MongoDB Server branch. EPSS data as of 2026-08-13 reports a probability of 0.168% at the 6.451 percentile, but authenticated internal actors remain a relevant threat model.
Workarounds
- Limit readWrite privileges to trusted service accounts and enforce least privilege on Queryable Encryption collections.
- Disable or block execution of the affected Queryable Encryption maintenance operation for non-administrative roles until patching is complete.
- Place MongoDB behind network segmentation and require mutual TLS for client connections to reduce exposure to authenticated abuse.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
