CVE-2026-18688: MongoDB Server DoS Vulnerability

CVE-2026-18688 Overview

CVE-2026-18688 is an out-of-bounds read vulnerability [CWE-125] in MongoDB Server's aggregation framework. An authenticated user can trigger the flaw by supplying a specially formed numeric parameter to a specific aggregation pipeline stage. Exploitation causes the server to read memory outside intended bounds, resulting in a server crash and denial of service. The condition may also expose limited memory contents to the attacker.

Critical Impact
An authenticated attacker with low privileges can crash MongoDB Server over the network by submitting a crafted aggregation pipeline, disrupting database availability and potentially leaking small amounts of process memory.

Affected Products

MongoDB Server (aggregation framework component)
Refer to MongoDB JIRA Issue SERVER-129617 for affected version ranges
Deployments accepting authenticated aggregation queries from untrusted users

Discovery Timeline

2026-08-11 - CVE-2026-18688 published to the National Vulnerability Database (NVD)
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-18688

Vulnerability Analysis

The vulnerability resides in MongoDB Server's aggregation framework, the pipeline engine that transforms documents through sequential stages such as $group, $project, and $bucket. A specific pipeline stage fails to validate a numeric parameter before using it to compute a memory offset or buffer index. When the parameter is crafted outside expected bounds, the server reads memory beyond the intended allocation.

The out-of-bounds read produces two outcomes. The process typically terminates when it accesses unmapped memory, causing a denial-of-service condition for all clients connected to that MongoDB instance. In some cases the read returns adjacent process memory to the attacker, exposing a limited amount of data that may include query fragments, cached documents, or internal state.

Exploitation requires authenticated access, which restricts the attacker population but includes any user with query privileges on a targeted collection. See MongoDB JIRA Issue SERVER-129617 for tracked resolution details.

Root Cause

The root cause is missing or insufficient bounds validation on a numeric input to an aggregation pipeline stage. The code path accepts the value from the client-supplied BSON document and uses it directly in memory access calculations. Classified under [CWE-125] Out-of-Bounds Read, the flaw stems from trusting a client-controlled integer where the surrounding code assumes it falls within a valid range.

Attack Vector

The attack vector is network-based and requires authenticated, low-privilege access to the MongoDB instance. An attacker constructs an aggregation pipeline containing the vulnerable stage with a malformed numeric parameter, then submits it through any standard MongoDB driver or the mongo shell using an aggregate command. No user interaction is required. Successful exploitation crashes the mongod process and may return residual memory bytes in the error response or query output.

No verified public proof-of-concept code is available at this time. Refer to the vendor tracking issue for technical specifics.

Detection Methods for CVE-2026-18688

Indicators of Compromise

Unexpected mongod process crashes or restarts correlated with recent authenticated aggregate commands
MongoDB logs containing segmentation faults, SIGSEGV, or invariant failures following pipeline execution
Repeated aggregation queries from a single authenticated user immediately preceding server termination
Anomalous aggregation pipelines containing unusually large or negative numeric parameters in stage operators

Detection Strategies

Enable MongoDB audit logging and monitor the aggregate command with attention to parameter values in pipeline stages
Correlate database availability alerts with authenticated session activity to identify the triggering user account
Baseline normal aggregation pipeline shapes per application and alert on outliers containing unexpected numeric extremes
Review crash dumps and diagnostic logs for stack traces referencing aggregation stage handlers

Monitoring Recommendations

Forward MongoDB logs and audit events into a centralized SIEM for correlation with authentication events
Track mongod process uptime and restart counts as availability health indicators
Alert on authenticated sessions that generate parser errors or crashes within short time windows
Monitor egress from database hosts for unusual response sizes that could indicate memory content exposure

How to Mitigate CVE-2026-18688

Immediate Actions Required

Apply the MongoDB Server patch referenced in SERVER-129617 as soon as it is available for your release train
Audit MongoDB user accounts and remove aggregation privileges from accounts that do not require them
Rotate credentials for accounts with query access to sensitive collections if exploitation is suspected
Restrict network access to mongod instances to trusted application hosts only

Patch Information

MongoDB tracks remediation for this issue in SERVER-129617. Consult the MongoDB security advisories and release notes for the fixed version applicable to your deployment channel (Community, Enterprise, or Atlas), and upgrade all replica set members and shards.

Workarounds

Enforce least-privilege role-based access control so only trusted service accounts can execute aggregate commands
Place MongoDB behind a query proxy or application layer that validates aggregation pipelines before forwarding
Isolate MongoDB instances on private networks and require VPN or bastion access for administrative queries
Deploy replica sets with automated failover to reduce downtime if a primary crashes from a triggering query

bash

# Example: revoke unneeded aggregation privileges by removing broad roles
use admin
db.revokeRolesFromUser("reporting_user", [ { role: "readWrite", db: "prod" } ])
db.grantRolesToUser("reporting_user", [ { role: "read", db: "prod_reports" } ])

# Verify effective roles
db.getUser("reporting_user", { showPrivileges: true })