CVE-2026-18711: MongoDB Server Use-After-Free Vulnerability

CVE-2026-18711 Overview

CVE-2026-18711 is a use-after-free vulnerability [CWE-416] in the MongoDB Server query execution engine. The flaw affects queries executed against time-series collections. An authenticated user with read and write privileges can trigger an internal reference to memory that has already been freed. Exploitation can crash the server or leak the contents of freed memory through query results.

The vulnerability was published to the National Vulnerability Database (NVD) on August 11, 2026. Because the affected code path involves normal query operations, exploitation requires only standard database credentials, not administrative rights.

Critical Impact
An authenticated user with read and write privileges can crash the MongoDB Server or read arbitrary freed memory contents returned in query results.

Affected Products

MongoDB Server (query execution engine handling time-series collections)
Specific fixed versions are tracked in MongoDB JIRA Issue SERVER-130247
Deployments that use time-series collections are in scope

Discovery Timeline

2026-08-11 - CVE-2026-18711 published to NVD
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-18711

Vulnerability Analysis

The defect resides in MongoDB Server's query execution engine when processing certain queries against time-series collections. During execution, the engine retains an internal pointer or reference to a memory region that is deallocated earlier in the query pipeline. Subsequent access to that reference constitutes a classic use-after-free condition.

Two outcomes are documented. First, dereferencing the stale pointer can corrupt process state and terminate the mongod process, producing a denial of service. Second, if the freed region has been reallocated for another purpose, its residual contents may be serialized into query results, disclosing memory that the requesting user was not authorized to read.

The attack vector is network-based. The attacker must hold valid credentials on the target instance with the ability to run queries and write documents into a time-series collection.

Root Cause

The root cause is improper lifetime management of an internal object reference held by the query executor while iterating over time-series data. The engine releases the underlying buffer before all code paths have finished using it, leaving a dangling reference that is later dereferenced.

Attack Vector

An authenticated principal with read and write privileges submits a crafted query against a time-series collection. The query shape triggers the specific execution path where the internal reference outlives its target allocation. Depending on the state of the allocator at the moment of dereference, the server crashes or returns memory contents inside result documents. See MongoDB JIRA Issue SERVER-130247 for the tracking issue.

No public proof-of-concept exploit is currently listed for this issue. The vulnerability is not present in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-18711

Indicators of Compromise

Unexpected mongod process crashes or SIGSEGV entries in server logs coinciding with time-series collection queries.
Query result documents containing fields with non-printable bytes, unexpected binary content, or values inconsistent with the stored schema.
Repeated query patterns from a single authenticated principal targeting time-series collections immediately before a crash.

Detection Strategies

Enable MongoDB audit logging and monitor operations against time-series collections, correlating query submissions with abnormal server termination.
Alert on mongod process restarts, replica set primary step-downs, and unclean shutdowns that do not correspond to operator activity.
Review the slow query log and diagnostic captures for repeated queries from the same user against time-series namespaces.

Monitoring Recommendations

Ship MongoDB logs and process telemetry into a centralized SIEM or data lake for correlation across replica set members.
Track counts of aborted operations, killed cursors, and connection resets per authenticated user to surface abuse patterns.
Baseline query volume and shape against time-series collections so anomalous query structures stand out.

How to Mitigate CVE-2026-18711

Immediate Actions Required

Inventory all MongoDB Server deployments and identify those hosting time-series collections.
Apply the fixed MongoDB Server release referenced in SERVER-130247 as soon as it is available for your branch.
Audit database role assignments and revoke readWrite privileges from principals that do not require them on time-series namespaces.
Rotate credentials for any account that had unnecessary write access to time-series collections.

Patch Information

MongoDB tracks the fix under Jira ticket SERVER-130247. Consult the ticket and the MongoDB security advisory channel for the specific fixed build numbers for each supported release train. Upgrade all replica set members and shards to a patched version and validate cluster health after the upgrade.

Workarounds

Restrict access to time-series collections to trusted service accounts until the patch is deployed.
Enforce least privilege using MongoDB's role-based access control so ad hoc users cannot issue queries against time-series namespaces.
Front the database with an application layer that validates query shapes and rejects unexpected aggregation stages from end users.
Enable automated mongod restart supervision so denial-of-service impact from a crash is minimized while the patch is rolled out.

bash

# Revoke broad readWrite from a user and grant scoped read-only access instead
mongosh "mongodb://admin@cluster.example.internal/admin" --eval '
  db.revokeRolesFromUser("app_user", [ { role: "readWrite", db: "metrics" } ]);
  db.grantRolesToUser("app_user", [ { role: "read", db: "metrics" } ]);
'