CVE-2026-9740: MongoDB Server DoS Vulnerability

CVE-2026-9740 Overview

CVE-2026-9740 is a denial-of-service vulnerability in MongoDB Server's BSON validation logic. An unauthenticated remote attacker can crash the mongod process by sending a specially crafted message. The flaw resides in how the BSON validator handles nested binary data structures, where mutual recursion between validation functions resets internal depth tracking. This allows uncontrolled recursion that exhausts the call stack and terminates the database process. The issue is classified under CWE-674: Uncontrolled Recursion.

Critical Impact

Unauthenticated network attackers can repeatedly crash MongoDB Server instances, causing service outages and database availability loss.

Affected Products

• MongoDB Server (see MongoDB Server Issue Tracking for affected versions)

Discovery Timeline

• 2026-06-09 - CVE-2026-9740 published to NVD
• 2026-06-10 - Last updated in NVD database

Technical Details for CVE-2026-9740

Vulnerability Analysis

The vulnerability exists in the BSON (Binary JSON) validation routines used by MongoDB Server to verify incoming wire protocol messages. When the validator encounters nested binary data structures, it transfers control between multiple validation functions through mutual recursion. Each re-entry into a paired validation function resets the depth counter that is intended to prevent runaway recursion.

An attacker who crafts a BSON payload with sufficiently deep nesting of binary subtype fields can drive the validator into unbounded recursion. The recursion consumes the thread stack until the operating system terminates the mongod process. Because validation occurs before authentication completes, no credentials are required to trigger the crash.

Root Cause

The root cause is improper depth tracking across mutually recursive validation functions. A single depth counter scoped to one function does not account for control transfers into a peer validator. Each cross-function call effectively starts a fresh depth measurement while the actual call stack continues to grow. This pattern matches CWE-674: Uncontrolled Recursion.

Attack Vector

Attackers exploit this flaw over the network by connecting to any exposed MongoDB listener and sending a malformed BSON document. The attack requires no authentication and no user interaction. A single crafted message is sufficient to terminate the process, and repeated messages produce a sustained denial of service against the database tier.

No verified proof-of-concept code has been published. See the MongoDB Server Issue Tracking entry for additional technical context.

Detection Methods for CVE-2026-9740

Indicators of Compromise

• Unexpected mongod process termination or repeated automatic restarts under supervisor control such as systemd or Kubernetes
• Stack overflow or SIGSEGV entries correlated with BSON parsing in MongoDB logs immediately before crash
• Spikes in inbound connections to MongoDB ports (default 27017) followed by abrupt session resets

Detection Strategies

• Monitor MongoDB server logs for abnormal termination signatures and assertion failures originating from BSON validation code paths
• Inspect wire protocol traffic for BSON messages containing deeply nested binary subtype elements that exceed normal application depth
• Correlate connection patterns from unauthenticated sources with subsequent process restarts to identify exploitation attempts

Monitoring Recommendations

• Alert on mongod process uptime resets and crash loops across replica set members
• Track replica set elections and primary step-downs that occur without operator action, as these can indicate node-level crashes
• Enable network telemetry on database-tier subnets to capture source IPs sending malformed wire protocol messages

How to Mitigate CVE-2026-9740

Immediate Actions Required

• Restrict network access to MongoDB listeners using firewall rules so only trusted application hosts can reach port 27017
• Apply the fixed MongoDB Server release referenced in the MongoDB Server Issue Tracking advisory as soon as it is available for your deployment channel
• Audit MongoDB deployments for instances exposed to the public internet and remove direct exposure

Patch Information

Refer to the official MongoDB Server Issue Tracking entry for affected version ranges and fixed builds. Upgrade all replica set members and sharded cluster components to the patched release using a rolling restart to preserve availability during remediation.

Workarounds

• Place MongoDB instances behind a network policy or VPC security group that allows only application-tier source addresses
• Require TLS client authentication on MongoDB listeners to filter unauthenticated connection attempts at the transport layer
• Run mongod under a process supervisor configured for fast automatic restart to reduce outage duration if exploitation occurs

# Example iptables rule restricting MongoDB to a trusted application subnet
iptables -A INPUT -p tcp --dport 27017 -s 10.0.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 27017 -j DROP