CVE-2026-1848: Proxy Protocol DoS Vulnerability

CVE-2026-1848 Overview

CVE-2026-1848 is a resource exhaustion vulnerability affecting MongoDB Server where connections received from the proxy port may not count towards total accepted connections. This flaw can result in server crashes when the total number of connections exceeds available resources. The vulnerability specifically impacts connections accepted from the proxy port while pending the proxy protocol header.

Critical Impact

Attackers can exploit this connection counting flaw to exhaust server resources, leading to denial of service conditions that crash the MongoDB server and disrupt database availability.

Affected Products

• MongoDB Server (versions with proxy port functionality)

Discovery Timeline

• 2026-02-10 - CVE-2026-1848 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-1848

Vulnerability Analysis

This vulnerability is classified as CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue lies in how MongoDB Server handles connection counting for its proxy port. When connections arrive through the proxy port and are waiting for the proxy protocol header, they are not properly counted against the server's total connection limit.

This accounting discrepancy creates a dangerous condition where the actual number of connections can silently exceed the configured maximum. Since the server believes it has capacity for more connections based on its faulty count, it continues accepting new connections until system resources are completely exhausted, ultimately resulting in a server crash.

Root Cause

The root cause stems from improper resource allocation tracking in the MongoDB connection handling logic. Connections that arrive via the proxy port enter a pending state while awaiting the proxy protocol header. During this pending state, the connection counting mechanism fails to include these connections in the total count. This creates a blind spot in resource management, allowing unbounded connection accumulation that bypasses the server's protective connection limits.

Attack Vector

An attacker with network access to the MongoDB proxy port can exploit this vulnerability by initiating numerous connections without completing the proxy protocol handshake. Since these pending connections are not counted:

1. The attacker opens connections to the proxy port in rapid succession
2. Each connection enters the pending state awaiting the proxy protocol header
3. The server's connection counter does not increment for these pending connections
4. The attacker continues opening connections until server resources are exhausted
5. The server crashes due to resource exhaustion, causing denial of service

This attack requires no authentication and can be executed remotely from any network position with access to the proxy port. For additional technical details, refer to the MongoDB Jira Issue SERVER-114695.

Detection Methods for CVE-2026-1848

Indicators of Compromise

• Unexpected MongoDB server crashes or restarts without apparent cause
• Discrepancy between reported connection counts and actual system connection metrics
• High number of connections in pending/incomplete state on the proxy port
• System resource exhaustion (memory, file descriptors) preceding server crashes

Detection Strategies

• Monitor system-level connection counts to the proxy port independently of MongoDB's internal metrics
• Implement alerting when system connection counts significantly exceed MongoDB's reported connection count
• Track proxy protocol handshake completion rates to identify anomalous connection patterns
• Deploy network-level monitoring to detect connection flooding attempts against the proxy port

Monitoring Recommendations

• Enable verbose logging for proxy port connection handling to capture pending connection states
• Configure alerts for sudden increases in pending connections that do not complete handshakes
• Monitor system resource utilization (file descriptors, memory) as early warning indicators
• Implement rate limiting at the network perimeter for connections to the MongoDB proxy port

How to Mitigate CVE-2026-1848

Immediate Actions Required

• Review the MongoDB Jira Issue SERVER-114695 for vendor-specific guidance and patch availability
• Restrict network access to the MongoDB proxy port using firewall rules to trusted sources only
• Consider disabling the proxy port functionality if it is not required for your deployment
• Implement connection rate limiting at the network layer to prevent connection flooding

Patch Information

Consult the MongoDB Jira Issue SERVER-114695 for official patch information and updated MongoDB Server versions that address this vulnerability. Apply the vendor-recommended security update as soon as it becomes available.

Workarounds

• Disable proxy port functionality if not operationally required by your environment
• Implement strict firewall rules to limit proxy port access to known, trusted IP addresses only
• Deploy a reverse proxy or load balancer with connection limits in front of the MongoDB proxy port
• Configure operating system-level connection limits as an additional protective layer

For environments where the proxy port must remain enabled, network segmentation and strict access controls provide the most effective interim protection until an official patch is applied.