CVE-2026-96297: MongoDB Server Use-After-Free Vulnerability

CVE-2026-96297 Overview

CVE-2026-96297 is a use-after-free vulnerability [CWE-416] in the aggregation feature of MongoDB Server. The flaw triggers when an internal aggregation stage is combined with a batch size of zero, causing the server to read from memory after release. Reused memory may contain unrelated data from the server process, leading to cross-query information disclosure. The condition can also terminate the server process unexpectedly, producing a denial-of-service outcome. Exploitation requires an authenticated user holding read privileges who issues a crafted aggregate command.

Critical Impact
Authenticated users with read privileges can retrieve unrelated in-process memory contents returned in query results, or crash the MongoDB Server process.

Affected Products

MongoDB Server (aggregation framework)
Deployments permitting authenticated users with read privileges to run aggregate commands
See MongoDB Bug Report SERVER-132946 for affected version ranges

Discovery Timeline

2026-10-13 - CVE-2026-96297 published to the National Vulnerability Database (NVD)
2026-10-15 - Last updated in NVD database

Technical Details for CVE-2026-96297

Vulnerability Analysis

The defect lives in MongoDB Server's aggregation pipeline execution path. When an internal aggregation stage runs alongside a client-supplied batch size of 0, the server dereferences a buffer whose backing memory has already been freed. The freed region may be reclaimed by other allocations before it is read, so returned documents can carry fragments of unrelated process memory. Those fragments can include other users' query results, cached document data, or internal server state. Depending on allocator state at the moment of read, the process may crash instead of returning data, producing a denial-of-service outcome for the affected mongod instance.

Root Cause

The root cause is lifetime mismanagement between an internal aggregation stage and the cursor batching logic. A batch size of 0 instructs the server to return an empty initial batch while still establishing cursor state. The internal stage releases a memory region that the batching path still expects to be live, producing the classic use-after-free pattern described by [CWE-416].

Attack Vector

An authenticated user with read privileges on any database issues an aggregate command that combines the triggering internal stage with cursor: { batchSize: 0 }. No elevated role, server reconfiguration, or network-level access beyond normal client connectivity is required. Because the leaked bytes come from the server process address space, the data returned may originate from queries executed by other sessions. The vulnerability requires network reachability to the MongoDB listener and valid credentials; see MongoDB Bug Report SERVER-132946 for additional technical detail.

Detection Methods for CVE-2026-96297

Indicators of Compromise

Unexpected mongod process crashes or restarts correlated with aggregate commands in the server log
Aggregation commands in audit logs specifying cursor: { batchSize: 0 } from accounts that do not normally run analytics workloads
Query results containing fields or document fragments that do not match the queried collection's schema

Detection Strategies

Enable MongoDB audit logging and alert on aggregate operations that set batchSize to 0, especially from service or low-privilege read accounts
Correlate mongod segmentation faults or abnormal termination events with the preceding client command stream
Baseline normal aggregation pipeline shapes per application and flag deviations that introduce uncommon internal stages

Monitoring Recommendations

Forward MongoDB audit and diagnostic logs to a centralized analytics platform for retention and query
Monitor process-level crash telemetry on database hosts and tie it to authenticated session identifiers
Track read-privileged account activity for anomalous command patterns, particularly repeated zero-batch aggregations

How to Mitigate CVE-2026-96297

Immediate Actions Required

Review the vendor advisory at MongoDB Bug Report SERVER-132946 and apply the fixed MongoDB Server release for your deployment branch
Audit role assignments and remove read privileges from accounts that do not require them
Rotate credentials for any accounts exhibiting the IoCs listed above

Patch Information

MongoDB tracks the fix under the internal ticket SERVER-132946. Consult MongoDB Bug Report SERVER-132946 for the authoritative list of fixed server versions across supported release branches, and upgrade mongod and mongos binaries to those versions.

Workarounds

Restrict which clients can issue aggregate commands by applying least-privilege roles until patching is complete
Where feasible, enforce a non-zero minimum batchSize at the application or proxy layer for aggregation requests
Isolate MongoDB listeners behind network controls so only trusted application tiers can authenticate

bash

# Example: revoke broad read access and grant a narrower role
mongosh --eval 'db.getSiblingDB("admin").revokeRolesFromUser("appUser", ["readAnyDatabase"])'
mongosh --eval 'db.getSiblingDB("admin").grantRolesToUser("appUser", [{ role: "read", db: "appData" }])'