CVE-2026-18693: MongoDB Server DoS Vulnerability

CVE-2026-18693 Overview

CVE-2026-18693 affects MongoDB Server's handling of timeseries collections. An authenticated user with write privileges can cause an internal data structure to become inconsistent through crafted document insertions. A subsequent insert into the affected bucket triggers the server to access memory outside its intended bounds. The result can be a server crash, exposure of limited memory contents, or memory corruption. The issue is classified under CWE-787 (Out-of-Bounds Write).

Critical Impact
An authenticated attacker with write access can crash the MongoDB server, disclose adjacent memory, or corrupt server memory by inserting crafted documents into a timeseries collection.

Affected Products

MongoDB Server (timeseries collections feature)
Deployments exposing write privileges to untrusted or lower-trust users
Applications relying on timeseries buckets for ingestion

Discovery Timeline

2026-08-11 - CVE-2026-18693 published to the National Vulnerability Database (NVD)
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-18693

Vulnerability Analysis

MongoDB timeseries collections group documents into internal buckets to optimize storage and query performance. CVE-2026-18693 arises when specific document insertions leave a bucket's internal metadata inconsistent with its stored measurements. The server does not reconcile this drift before the next write operation.

On a subsequent insert into the same bucket, the write path calculates offsets against stale or invalid structural assumptions. The engine then writes or reads beyond the intended memory region. Depending on adjacent heap contents, this yields a process crash, leakage of small amounts of memory, or corruption of live server state.

The flaw requires network access to the MongoDB endpoint and valid credentials with write privileges on at least one timeseries collection. No user interaction is needed once the attacker holds the required role.

Root Cause

The root cause is an out-of-bounds write [CWE-787] in the timeseries bucket handling logic. The internal data structure that tracks bucket layout can be driven into an inconsistent state by particular document shapes or sequences. Subsequent writes trust that structure and access memory outside the allocated bucket buffer.

Attack Vector

The attack vector is network-based and requires low privileges: any authenticated principal with write access to a targeted timeseries collection. The attacker sends an initial insert that establishes the inconsistent bucket state, followed by a triggering insert that forces the out-of-bounds access.

No verified public exploit is available. Technical tracking details are maintained in the MongoDB Server Issue Tracking system.

Detection Methods for CVE-2026-18693

Indicators of Compromise

Unexpected mongod process crashes or restarts with segmentation faults referenced in server logs
Assertion failures or fatal errors tied to timeseries bucket code paths
Anomalous insert patterns from a single authenticated principal against timeseries collections
Sudden growth in write errors or invariant failures reported by the storage engine

Detection Strategies

Alert on mongod termination signals (SIGSEGV, SIGABRT) and correlate with the last executed operations from the profiler or slow query log
Enable the database profiler at level 1 or higher on timeseries collections and review inserts that immediately precede crashes
Monitor authentication audit logs for accounts with write access to timeseries collections and baseline their normal insert volumes

Monitoring Recommendations

Ship mongod logs to a centralized logging or SIEM platform and create detections for repeated fatal assertions
Track process uptime and restart counts per replica set member as a health signal
Instrument application-layer telemetry to capture insert payload shapes that trigger server errors

How to Mitigate CVE-2026-18693

Immediate Actions Required

Inventory all MongoDB deployments and identify clusters using timeseries collections
Restrict write privileges on timeseries collections to the minimum set of trusted service accounts
Rotate credentials for any account with broad write access that is not strictly required
Isolate MongoDB endpoints behind network controls so only application tier hosts can reach the wire protocol

Patch Information

Apply the fixed MongoDB Server release referenced in the MongoDB Server Issue Tracking entry SERVER-129994. Follow the standard rolling upgrade procedure for replica sets and sharded clusters to avoid downtime. Validate the upgrade in a staging environment before production rollout.

Workarounds

Avoid granting write privileges on timeseries collections to untrusted or multi-tenant principals until patching is complete
Route timeseries inserts through an application-layer validator that enforces schema and rejects unexpected field shapes
Enable authentication and role-based access control on every MongoDB deployment, including development clusters
Ensure replica set redundancy so a single crashed mongod does not cause a service outage while patching proceeds

bash

# Example: restrict a role to read-only on a timeseries database until patched
mongosh --eval '
  use admin;
  db.updateRole("appIngest", {
    privileges: [
      { resource: { db: "metrics", collection: "" }, actions: [ "find" ] }
    ],
    roles: []
  });
'