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How the Vulnerability Works
CVE-2026-58253 resides in the NATS Server, a high‑performance cloud‑native messaging system. The flaw is triggered when the administrator enables the `no_auth_user` configuration option. This option is designed to allow unauthenticated client connections to operate under a predefined user account, simplifying deployments where full authentication is not required for every client.
Internally, the NATS Server uses a parser fast‑path optimization to handle ordinary client connections efficiently. This fast path assumes that the connection is a standard client and applies the `no_auth_user` credentials without performing the full CONNECT authentication handshake. However, the vulnerability arises because this same parser fast path is inadvertently applied to route and leafnode listeners – two connection types used for inter‑server communication in a NATS cluster.
When a route or leafnode listener is exposed, an unauthenticated attacker with network access to that listener can connect to it before the inter‑server CONNECT authentication flow completes. Because the fast path treats the connection as a regular client, it grants the attacker the privileges associated with the `no_auth_user` account. In a typical setup, route and leafnode connections are expected to authenticate with shared secrets or certificates and are granted elevated privileges (e.g., the ability to forward messages, manage subscriptions, or access system accounts). By bypassing this authentication, the attacker effectively impersonates a trusted peer and gains those same high‑level privileges.
The attack requires that:
- The `no_auth_user` directive is configured in the NATS configuration file.
- At least one route or leafnode listener is enabled and reachable from the attacker’s network.
- The attacker can send a maliciously crafted connection request that triggers the fast path before the server enforces the proper CONNECT authentication.
This flaw is classified as an Improper Authentication issue (CWE‑287) and stems from an Incorrect Behavior Order (CWE‑551) – the server authorizes the connection based on the fast‑path assumption before fully parsing and validating the CONNECT message. The vulnerability affects all NATS Server versions prior to 2.14.0, 2.12.7, and 2.11.16. The fixes in these releases ensure that the parser fast path is only applied to ordinary client connections and never to route or leafnode listeners, thereby restoring the required authentication checks for inter‑server links.
DailyCVE Form:
Platform: NATS Server
Version: <2.14.0
Vulnerability: Auth Bypass
Severity: Medium
date: 2026-07-08
Prediction: 2026-04-30
Analytics – What Undercode Say
The following commands and code snippets help administrators detect vulnerable configurations and assess exposure.
Check current NATS configuration for `no_auth_user`:
grep -i "no_auth_user" /etc/nats-server.conf
List active listeners (including route/leafnode ports):
netstat -tulpn | grep nats-server
Test for the bypass using a simple Go client (PoC snippet):
package main
import (
"fmt"
"github.com/nats-io/nats.go"
)
func main() {
// Connect to a route/leafnode listener (e.g., port 6222 for routes)
nc, err := nats.Connect("nats://<attacker-ip>:6222")
if err != nil {
fmt.Println("Connection failed:", err)
return
}
defer nc.Close()
// If connection succeeds without credentials, the bypass is possible.
fmt.Println("Connected as unauthenticated peer – vulnerable!")
}
Monitor logs for unexpected route/leafnode connections:
tail -f /var/log/nats-server.log | grep -E "route|leafnode"
Exploit
An attacker can exploit this vulnerability by:
- Identifying a publicly accessible route (default port 6222) or leafnode (default port 7422) listener.
- Sending a plain TCP connection to that port without performing the expected CONNECT authentication.
- The server’s fast path assigns the `no_auth_user` identity, granting the attacker the privileges of that user.
- Once connected, the attacker can publish/subscribe to any subject permitted by the `no_auth_user` account, potentially including system subjects (e.g.,
$SYS.>), leading to message interception, data tampering, or cluster disruption.
Proof‑of‑Concept (Python) – raw socket connection:
import socket
target = ("<attacker-ip>", 6222) route port
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(target)
Send a minimal "INFO" message to trigger fast path
sock.send(b"INFO {}\r\n")
response = sock.recv(1024)
print("Received:", response)
sock.close()
Protection
- Upgrade to NATS Server versions 2.14.0, 2.12.7, or 2.11.16 (or later) immediately.
- If upgrading is not possible, disable `no_auth_user` and enforce explicit authentication for all clients.
- Restrict network access to route and leafnode listeners using firewall rules – allow only trusted IP addresses (e.g., cluster peers) to connect to ports 6222 and 7422.
- Enable TLS with client certificate verification for inter‑server connections, ensuring that even if the fast path is triggered, the certificate validation fails.
- Monitor logs for any unexpected connections to route/leafnode ports and set up alerts for anomalous authentication attempts.
Impact
- Confidentiality: Low – the attacker may read messages flowing through the cluster if the `no_auth_user` has subscribe permissions.
- Integrity: High – the attacker can publish arbitrary messages, modify stream data, or inject false information into the system.
- Availability: Low – while not directly causing DoS, the attacker could disrupt message routing or exhaust resources.
- Scope: Changed – the compromise of a single route/leafnode listener can affect the entire cluster, as the attacker operates with trusted peer privileges.
- Overall Risk: Medium to High – in multi‑tenant environments, this flaw can lead to cross‑tenant data exposure and complete loss of trust in message authenticity.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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