Overview
A medium-severity vulnerability (CVE-2026-1642) has been identified in NGINX OSS and NGINX Plus that could allow attackers in a man-in-the-middle position to inject plaintext data into encrypted TLS responses when NGINX is configured to proxy to upstream TLS servers.
Vulnerability Details
| Attribute | Value |
|---|---|
| CVE ID | CVE-2026-1642 |
| Vendor | F5/NGINX |
| Products | NGINX OSS, NGINX Plus |
| CVSS Score | 5.9 (Medium) |
| Attack Vector | Network (MITM position required) |
| Complexity | High |
Technical Analysis
Vulnerable Configuration
The vulnerability affects NGINX instances configured to proxy HTTPS traffic to upstream TLS servers:
# Vulnerable configuration example
upstream backend {
server backend.example.com:443;
}
server {
listen 443 ssl;
location / {
proxy_pass https://backend;
proxy_ssl_verify off; # Increases risk
}
}Attack Scenario
┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐
│ Client │────►│ NGINX │────►│ Attacker │────►│ Backend │
│ │◄────│ Proxy │◄────│ (MITM) │◄────│ Server │
└──────────┘ └──────────┘ └──────────┘ └──────────┘
│ │
│ Injected │
│ Plaintext │
│◄───────────────┘
Exploitation Requirements
For successful exploitation, an attacker must:
- Be in a network position to intercept traffic
- Target NGINX-to-upstream TLS connections
- Inject data during the TLS handshake or session
Affected Versions
| Product | Affected Versions | Fixed Version |
|---|---|---|
| NGINX OSS | < 1.27.4 | 1.27.4 |
| NGINX Plus | R32 and earlier | R33 |
Mitigation Steps
1. Update NGINX
# For Debian/Ubuntu
sudo apt update
sudo apt upgrade nginx
# For RHEL/CentOS
sudo yum update nginx
# For Docker
docker pull nginx:1.27.42. Enable Strict TLS Verification
# Secure configuration
upstream backend {
server backend.example.com:443;
}
server {
listen 443 ssl;
location / {
proxy_pass https://backend;
# Enable certificate verification
proxy_ssl_verify on;
proxy_ssl_verify_depth 2;
proxy_ssl_trusted_certificate /etc/nginx/certs/ca-bundle.crt;
# Enforce TLS 1.2+
proxy_ssl_protocols TLSv1.2 TLSv1.3;
# Use strong ciphers
proxy_ssl_ciphers 'ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256';
# Enable SNI
proxy_ssl_server_name on;
proxy_ssl_name backend.example.com;
}
}3. Network Segmentation
Reduce MITM risk by:
- Using dedicated backend networks
- Implementing network encryption (IPsec, WireGuard)
- Deploying in private VPCs
Detection
Check NGINX Version
nginx -v
# nginx version: nginx/1.27.4 (safe)
# nginx version: nginx/1.27.3 (vulnerable)Audit Proxy Configurations
# Find all proxy_pass directives with HTTPS
grep -r "proxy_pass https" /etc/nginx/Log Analysis
Monitor for anomalous upstream responses:
# Enhanced logging for upstream connections
log_format upstream_log '$remote_addr - $upstream_addr '
'[$time_local] "$request" $status '
'$upstream_response_time $upstream_status';
access_log /var/log/nginx/upstream.log upstream_log;Risk Assessment
Who Is Most At Risk?
| Environment | Risk Level | Reason |
|---|---|---|
| Cloud with private networking | Low | Limited MITM opportunity |
| On-premises with segmentation | Low | Controlled network |
| Shared hosting | Medium | Multiple tenants |
| Public internet backends | High | Exposed to network attacks |
Mitigating Factors
- Requires MITM position (not trivial)
- Only affects upstream TLS connections
- Certificate verification reduces risk
- Modern network architectures limit exposure
Timeline
| Date | Event |
|---|---|
| January 2026 | Vulnerability discovered |
| January 30, 2026 | NGINX notified |
| February 4, 2026 | Patches released |
| February 5, 2026 | Public disclosure |
References
Related Articles
- Securing NGINX as a Reverse Proxy
- TLS Best Practices for Web Servers