CVE-2026-18692: MongoDB Server Use-After-Free Vulnerability

CVE-2026-18692 Overview

CVE-2026-18692 is a use-after-free vulnerability [CWE-416] in MongoDB Server's handling of timeseries bucket lifecycle operations. An authenticated user with write privileges can trigger an internal reference to be used after the underlying memory has been freed. Subsequent operations against the affected reference can crash the server process or, under specific conditions, execute unintended code within the mongod process context.

The vulnerability is tracked in MongoDB's issue tracker as SERVER-129887. It affects deployments that expose write access to authenticated users who can interact with timeseries collections.

Critical Impact
Authenticated attackers with write privileges can crash MongoDB Server instances or potentially achieve arbitrary code execution through memory corruption in timeseries bucket handling.

Affected Products

MongoDB Server (see SERVER-129887 for affected version ranges)
Deployments using timeseries collections
Instances exposing write privileges to authenticated users

Discovery Timeline

2026-08-11 - CVE-2026-18692 published to NVD
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-18692

Vulnerability Analysis

The flaw resides in MongoDB Server's timeseries bucket lifecycle management. Timeseries collections group documents into internal buckets to optimize storage and query performance. The server maintains references to these bucket objects across operations.

When specific sequences of write operations occur, the bucket's backing memory can be released while a live reference to it still exists. Subsequent operations dereference this dangling pointer, producing undefined behavior. In the observed case, the server process terminates with a crash, causing denial of service. Under attacker-controlled heap conditions, the freed memory may be reallocated with attacker-influenced content, opening a path to code execution inside the mongod process.

The issue is classified as CWE-416 (Use After Free) and is network-reachable via the standard MongoDB wire protocol, requiring low-privilege authenticated access.

Root Cause

The root cause is a lifetime management error in the timeseries bucket subsystem. Code paths that free bucket memory do not invalidate or synchronize with concurrent code paths that continue to hold and use references to the same bucket. This results in a stale reference being dereferenced after the associated allocation has been returned to the allocator.

Attack Vector

Exploitation requires network access to the MongoDB instance and valid credentials with write privileges on a timeseries collection. The attacker issues a crafted sequence of operations against a timeseries collection to force the server into the vulnerable state. No user interaction is required. Full technical details are tracked in the MongoDB Jira Issue SERVER-129887.

Detection Methods for CVE-2026-18692

Indicators of Compromise

Unexpected mongod process crashes or restarts with fatal signal entries in server logs
Assertion failures or invariant violations referencing timeseries bucket code paths
Anomalous write patterns from a single authenticated principal against timeseries collections
Sudden spikes in connection resets from clients following writes to timeseries namespaces

Detection Strategies

Monitor mongod logs for crash signatures, Invariant failure, SIGSEGV, and stack traces referencing timeseries components
Alert on abnormal frequencies of insert, update, or delete operations targeting timeseries collections by low-privileged accounts
Correlate authentication events with subsequent server termination or replica set failovers

Monitoring Recommendations

Enable MongoDB audit logging for write operations against timeseries collections and forward logs to a centralized platform
Track replica set state changes and unplanned primary step-downs that follow write bursts
Baseline expected write volumes per role and alert on deviations that could indicate exploitation attempts

How to Mitigate CVE-2026-18692

Immediate Actions Required

Upgrade MongoDB Server to a fixed release as listed in MongoDB Jira Issue SERVER-129887
Audit database roles and revoke write privileges from accounts that do not require them
Restrict network exposure of mongod to trusted application tiers only
Enable authentication and enforce least-privilege role assignments on all deployments

Patch Information

MongoDB has tracked the fix under SERVER-129887. Consult the referenced Jira issue for the specific server versions containing the patched timeseries bucket lifecycle handling and apply the corresponding upgrade path for community or enterprise editions.

Workarounds

Limit access to timeseries collections to trusted, application-level service accounts
Place MongoDB behind a network segmentation boundary to block direct client access from untrusted zones
Deploy replica sets so an induced crash on one node triggers automatic failover while patching proceeds
Where feasible, avoid provisioning new timeseries collections until the upgrade is complete

bash

# Example: revoke write privileges from a non-essential user in the mongo shell
use admin
db.revokeRolesFromUser("reporting_user", [
  { role: "readWrite", db: "metrics" }
])
db.grantRolesToUser("reporting_user", [
  { role: "read", db: "metrics" }
])