CVE-2026-18697: MongoDB Server DoS Vulnerability

CVE-2026-18697 Overview

CVE-2026-18697 is a denial of service vulnerability in the MongoDB Server aggregation framework. An unauthenticated remote attacker can submit a specially formed aggregation command that causes the mongos router process to terminate unexpectedly. The failure disrupts all client connections routed through the affected instance and breaks query routing across sharded clusters.

The issue is tracked under [CWE-617: Reachable Assertion] and requires no authentication, no user interaction, and low attack complexity over the network. MongoDB tracks the fix in the internal ticket SERVER-130110.

Critical Impact
An unauthenticated attacker with network access to a mongos endpoint can crash the router process, producing an availability outage for all clients routed through the affected instance.

Affected Products

MongoDB Server (aggregation framework, mongos router component)
Sharded MongoDB deployments exposing mongos endpoints to untrusted networks
See the MongoDB Server Issue Tracking entry for authoritative version ranges

Discovery Timeline

2026-08-11 - CVE-2026-18697 published to NVD
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-18697

Vulnerability Analysis

The defect lives in MongoDB Server's aggregation framework, the pipeline engine that processes aggregate commands across sharded collections. When mongos parses a specially crafted aggregation command, an internal invariant or assertion is reached that the process cannot recover from. The router terminates rather than returning a controlled error to the client.

Because mongos fronts all client traffic to a sharded cluster, its termination has an outsized blast radius. Every session routed through the crashed router is severed, and any driver that is not configured for automatic failover to another mongos will surface connection errors to application code. The vulnerability affects availability only; there is no confirmed impact to confidentiality or integrity of stored data.

Root Cause

The root cause is a reachable assertion [CWE-617] in aggregation command handling. Input that is syntactically parseable but semantically malformed drives the pipeline into a state the code treats as impossible, triggering process termination instead of returning a query error. Details of the specific pipeline stage or operator are tracked privately in SERVER-130110.

Attack Vector

An attacker sends a single crafted aggregate command to any reachable mongos listener, typically TCP 27017. No credentials are required, so any environment that exposes mongos to untrusted networks, including misconfigured cloud security groups or shared internal networks, is reachable. The attack does not require prior knowledge of database names, collections, or cluster topology.

No public proof-of-concept is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See the MongoDB Server Issue Tracking entry for vendor-supplied technical detail.

Detection Methods for CVE-2026-18697

Indicators of Compromise

Unexpected termination of mongos processes without a preceding shutdown command in mongod.log or mongos.log
Assertion or invariant failure entries in MongoDB logs immediately before process exit
Bursts of client connection resets and driver-side NotPrimaryNoSecondaryOk or connection pool errors correlated with router restarts
Inbound aggregate commands from unauthenticated or unexpected source addresses in cluster audit logs

Detection Strategies

Alert on mongos process restarts and non-zero exit codes reported by systemd, Kubernetes liveness probes, or the MongoDB Ops Manager agent
Parse MongoDB logs for invariant, fassert, or terminate called strings emitted near aggregation command handlers
Baseline aggregation command volume per source IP and flag anomalous senders, particularly unauthenticated sessions
Correlate router crashes with recent client command payloads captured through MongoDB profiler or free monitoring

Monitoring Recommendations

Forward mongos and mongod logs to a centralized analytics pipeline and retain assertion traces for post-incident review
Monitor cluster health metrics such as mongos uptime, connection counts, and command failure rates through Prometheus exporters or MongoDB Cloud Manager
Track network exposure of port 27017 and alert when mongos becomes reachable from outside the intended trust boundary

How to Mitigate CVE-2026-18697

Immediate Actions Required

Restrict network access to mongos endpoints so only application tiers and administrative jump hosts can reach TCP 27017
Enforce authentication on all MongoDB deployments and disable any anonymous or --noauth listeners
Inventory sharded clusters and identify every mongos instance exposed beyond its intended trust boundary
Prepare a rolling restart plan so patched mongos binaries can be deployed without cluster-wide downtime

Patch Information

MongoDB tracks the fix under SERVER-130110. Consult the MongoDB security advisories page and release notes for the specific fixed versions applicable to your release train, then upgrade all mongos instances in the affected clusters. Restart configuration servers and shard members as required by the vendor upgrade path.

Workarounds

Place mongos behind a firewall or service mesh that permits connections only from vetted client identities
Require TLS and SCRAM or x.509 authentication on every mongos listener to remove the unauthenticated attack surface
Configure MongoDB drivers with multiple mongos hosts so a single router crash triggers automatic failover instead of an application outage
Rate-limit aggregate commands at an application proxy where feasible until patched binaries are deployed

bash

# Example: restrict mongos to internal application subnet using iptables
iptables -A INPUT -p tcp --dport 27017 -s 10.0.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 27017 -j DROP

# Example: enforce authentication and TLS in /etc/mongos.conf
net:
  bindIp: 10.0.20.15
  port: 27017
  tls:
    mode: requireTLS
    certificateKeyFile: /etc/ssl/mongos.pem
security:
  authorization: enabled