CVE-2026-1849: MongoDB Server DOS Vulnerability

CVE-2026-1849 Overview

CVE-2026-1849 is a denial of service vulnerability affecting MongoDB Server that can cause out-of-memory failures when processing expressions that produce deeply nested documents. The vulnerability exists in recursive functions where the server fails to periodically check the depth of expressions being evaluated, allowing malicious actors to exhaust server memory resources.

Critical Impact

Authenticated attackers can crash MongoDB Server instances by sending specially crafted queries that generate deeply nested document structures, leading to memory exhaustion and denial of service conditions.

Affected Products

• MongoDB Server (specific affected versions pending vendor advisory)

Discovery Timeline

• 2026-02-10 - CVE-2026-1849 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-1849

Vulnerability Analysis

This vulnerability is classified under CWE-674 (Uncontrolled Recursion), which describes scenarios where software fails to properly control recursive function calls, potentially leading to resource exhaustion. In the context of MongoDB Server, the issue manifests when evaluating expressions that produce deeply nested document structures.

The attack requires network access and low-privilege authentication to the MongoDB instance. Once authenticated, an attacker can submit queries containing expressions designed to trigger recursive evaluation without proper depth checks. As the recursion depth increases unchecked, memory consumption grows until the server exhausts available memory and crashes.

The impact is limited to availability—there is no data confidentiality or integrity breach—however, successful exploitation can render MongoDB services completely unavailable until the server is restarted.

Root Cause

The root cause lies in MongoDB's expression evaluation engine, which processes recursive functions without implementing periodic depth validation checks. When expressions generate deeply nested documents, the server continues processing without verifying that the nesting depth remains within safe operational limits. This architectural oversight allows unbounded recursion that consumes memory proportionally to the nesting depth until resources are exhausted.

Attack Vector

The attack vector is network-based and requires low-privilege authentication to the MongoDB instance. An attacker with valid credentials can craft malicious aggregation pipeline queries or other expression-based operations that intentionally produce deeply nested output structures.

The exploitation process involves:

1. Authenticating to a vulnerable MongoDB Server instance with minimal privileges
2. Submitting a query containing expressions designed to generate deeply nested documents
3. The server processes the expression recursively without depth validation
4. Memory consumption increases with each recursion level
5. Eventually, the server exhausts available memory and crashes

For technical details on the specific expression patterns involved, see the MongoDB JIRA Issue SERVER-102364.

Detection Methods for CVE-2026-1849

Indicators of Compromise

• Abnormal memory consumption spikes in MongoDB Server processes
• Unexpected MongoDB service crashes or restarts without apparent cause
• Query logs containing unusual expressions with deeply nested output patterns
• Out-of-memory error messages in MongoDB server logs

Detection Strategies

• Monitor MongoDB server memory utilization for sudden unexplained increases
• Implement query analysis to detect expressions that may produce deeply nested documents
• Configure alerting on MongoDB process crashes and unexpected restarts
• Review authentication logs for suspicious access patterns preceding memory exhaustion events

Monitoring Recommendations

• Deploy SentinelOne Singularity platform for real-time process behavior monitoring on MongoDB hosts
• Enable MongoDB profiler to capture and analyze slow or resource-intensive queries
• Implement memory threshold alerts at 80% and 90% utilization levels
• Configure log aggregation to correlate memory events with query execution patterns

How to Mitigate CVE-2026-1849

Immediate Actions Required

• Review the MongoDB JIRA Issue SERVER-102364 for vendor guidance and patch information
• Monitor MongoDB servers for unusual memory consumption patterns
• Implement network segmentation to limit MongoDB access to trusted hosts only
• Review and restrict database user privileges to minimum necessary permissions
• Consider implementing query timeouts and resource limits where supported

Patch Information

Patch details are tracked in the MongoDB JIRA Issue SERVER-102364. Organizations should monitor this issue for official patch releases and upgrade to a fixed version as soon as one becomes available. Until a patch is applied, implement the workarounds described below to reduce exposure.

Workarounds

• Restrict network access to MongoDB instances using firewall rules and network segmentation
• Implement application-level query validation to reject potentially malicious expressions before they reach the database
• Configure resource limits at the operating system level to prevent complete system memory exhaustion
• Use MongoDB's built-in resource management features to set memory limits where available
• Temporarily disable or restrict access to features that allow arbitrary expression evaluation if not business-critical

# Example: Restrict MongoDB access to specific IP ranges using iptables
iptables -A INPUT -p tcp --dport 27017 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 27017 -j DROP

# Example: Configure MongoDB to bind only to specific interfaces
# In mongod.conf:
# net:
#   bindIp: 127.0.0.1,10.0.1.100
#   port: 27017