CVE-2026-18707: MongoDB Server DoS Vulnerability

CVE-2026-18707 Overview

CVE-2026-18707 is a denial of service vulnerability in MongoDB Server. An authenticated user, including an account with no assigned privileges, can crash the server process by submitting a specially formed aggregation command. The flaw is classified under CWE-617 (Reachable Assertion) and results in unexpected termination of the database process.

Critical Impact
Any authenticated MongoDB user, even one with zero assigned privileges, can terminate the server process and disrupt database availability with a single crafted aggregation command.

Affected Products

MongoDB Server (see the MongoDB Issue Tracker Entry for affected version ranges)

Discovery Timeline

2026-08-11 - CVE-2026-18707 published to the National Vulnerability Database
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-18707

Vulnerability Analysis

The vulnerability resides in MongoDB Server's aggregation command processing pipeline. A specially formed aggregation command triggers an unhandled condition that terminates the mongod server process. Because the flaw only requires authenticated access with no privileges, the attack surface includes any principal capable of establishing an authenticated session against the database.

The issue impacts availability only. Confidentiality and integrity are not affected, and the attack does not enable code execution or data exfiltration. Successful exploitation forces the process to exit, disrupting service until MongoDB restarts. In replica set deployments, repeated exploitation against multiple nodes can cascade into cluster-wide unavailability.

Root Cause

The root cause is a reachable assertion or unchecked condition within the aggregation framework's command handler. When the server parses or executes the malformed aggregation pipeline, it hits an internal check that transitions the process to a fatal state rather than returning a controlled error to the client. This maps directly to CWE-617, where an attacker-controllable input reaches a program assertion designed for developer diagnostics.

Attack Vector

Exploitation requires network access to the MongoDB wire protocol and valid authentication credentials. No user interaction is required. Once authenticated, the attacker issues a single crafted aggregation command through any standard MongoDB driver or the mongosh shell. Full technical details are tracked in the MongoDB SERVER-128482 ticket.

Detection Methods for CVE-2026-18707

Indicators of Compromise

Unexpected mongod process terminations coinciding with client connections issuing aggregation commands
Fatal assertion messages or F severity entries in MongoDB logs referencing the aggregation subsystem
Repeated client reconnections from a single authenticated principal immediately preceding server crashes

Detection Strategies

Correlate MongoDB audit logs with process lifecycle events to identify authenticated sessions that trigger abnormal shutdowns
Alert on any invariant failure, fassert, or tassert log lines in the MongoDB server log stream
Baseline aggregation command patterns per user and flag sessions issuing malformed or highly unusual pipeline stages

Monitoring Recommendations

Forward MongoDB server and audit logs to a centralized SIEM for real-time analysis of crash conditions
Monitor replica set health metrics and primary elections that follow authenticated aggregation activity
Track authentication events and the associated commands issued by low-privilege or newly created accounts

How to Mitigate CVE-2026-18707

Immediate Actions Required

Review the MongoDB Issue Tracker Entry for the list of fixed versions and upgrade affected clusters accordingly
Audit MongoDB user accounts and remove any authenticated principals that do not require database access
Restrict network exposure of mongod instances to trusted application subnets only

Patch Information

MongoDB has documented the fix in issue SERVER-128482. Administrators should consult the ticket for patched release numbers and apply the upgrade to all cluster members, including secondaries and arbiters, using a rolling upgrade procedure.

Workarounds

Enforce strict authentication and rotate credentials for any accounts that do not require aggregation privileges
Place MongoDB behind a network policy that limits client access to trusted application hosts
Enable database auditing to capture the source and content of aggregation commands prior to any crash

bash

# Verify current MongoDB Server version before planning the upgrade
mongosh --eval "db.version()"

# Enable auditing to capture authenticated command activity
# Add to /etc/mongod.conf, then restart the mongod service
auditLog:
  destination: file
  format: JSON
  path: /var/log/mongodb/audit.json
  filter: '{ atype: { $in: ["authenticate", "authCheck"] } }'