CVE-2025-55740: nginx-defender Auth Bypass Vulnerability

CVE-2025-55740 Overview

CVE-2025-55740 affects nginx-defender, a Web Application Firewall (WAF) and threat detection system for web infrastructure. The vulnerability stems from hardcoded default credentials shipped in example configuration files. The config.yaml file contains default_password: "change_me_please", and docker-compose.yml sets GF_SECURITY_ADMIN_PASSWORD=admin123. Deployments that retain these defaults expose administrative interfaces to any attacker with network access. Successful authentication grants administrative control and allows bypass of the WAF's own security protections. The maintainers addressed the issue in nginx-defender v1.5.0. This weakness is categorized as [CWE-1392] Use of Default Credentials.

Critical Impact
Attackers with network reachability to an unmodified nginx-defender deployment can log in with published default credentials and disable WAF protections.

Affected Products

nginx-defender versions prior to v1.5.0
Deployments using shipped config.yaml without credential changes
Deployments using shipped docker-compose.yml without credential changes

Discovery Timeline

2025-08-19 - CVE-2025-55740 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-55740

Vulnerability Analysis

nginx-defender ships with example configuration artifacts intended for quick evaluation. These artifacts embed static, publicly documented credentials. The config.yaml file declares default_password: "change_me_please" for administrative access. The bundled docker-compose.yml sets the Grafana admin secret to GF_SECURITY_ADMIN_PASSWORD=admin123. Operators who deploy these files without modification expose management interfaces protected by known secrets. The exposure is not a code defect but a configuration and packaging weakness that violates secure-by-default principles.

An attacker who reaches the administrative endpoint over the network can authenticate directly. Once authenticated, the attacker can modify WAF rules, disable identification logic, exfiltrate telemetry, or pivot into monitored applications. Because nginx-defender sits in front of protected web services, compromising it undermines every downstream control it enforces.

Root Cause

The root cause is the presence of hardcoded default credentials in reference configuration files distributed with the product. The software does not force credential rotation on first launch and does not fail closed when defaults remain in place. Users following quick-start instructions therefore inherit predictable secrets.

Attack Vector

Exploitation requires network access to the nginx-defender administrative interface and does not require prior authentication or user interaction. An attacker submits the documented default credentials to the login endpoint. The vulnerability is exploitable remotely and requires no specialized tooling. Refer to the GitHub Security Advisory GHSA-pr72-8fxw-xx22 for vendor-supplied technical details.

Detection Methods for CVE-2025-55740

Indicators of Compromise

Successful administrative logins from unexpected source IP addresses to nginx-defender or its bundled Grafana instance
Presence of the strings change_me_please or admin123 in deployed config.yaml or docker-compose.yml files
Unauthorized modifications to WAF rulesets, allow-lists, or logging configuration
New administrative sessions occurring outside operator maintenance windows

Detection Strategies

Scan configuration repositories and running containers for the default credential strings shipped with nginx-defender
Audit authentication logs for the built-in administrative account and correlate against approved change tickets
Compare current WAF rule state against a known-good baseline to identify tampering
Enumerate exposed nginx-defender management ports on internal and perimeter networks

Monitoring Recommendations

Alert on any successful login to the nginx-defender admin interface from non-administrative subnets
Monitor egress traffic from the WAF host for anomalous outbound connections that may indicate post-compromise activity
Track configuration file integrity using file integrity monitoring on config.yaml and docker-compose.yml
Log and review Grafana administrative access events tied to the GF_SECURITY_ADMIN_PASSWORD account

How to Mitigate CVE-2025-55740

Immediate Actions Required

Upgrade nginx-defender to v1.5.0 or later, which addresses the default credential issue
Rotate the administrative password and the GF_SECURITY_ADMIN_PASSWORD value on every existing deployment
Restrict network access to the administrative interface using firewall rules or a management VLAN
Review authentication logs for prior logins using the default credentials and investigate any matches

Patch Information

The maintainers resolved the issue in nginx-defender v1.5.0. Details are published in the GitHub Security Advisory GHSA-pr72-8fxw-xx22. Operators should pull the updated release, replace shipped example files, and confirm that no artifact in their deployment pipeline reintroduces the default values.

Workarounds

Replace default_password: "change_me_please" in config.yaml with a unique, high-entropy secret before first startup
Override GF_SECURITY_ADMIN_PASSWORD in docker-compose.yml or via environment variables managed by a secrets store
Bind the administrative listener to a management interface unreachable from untrusted networks
Require multi-factor authentication or reverse-proxy authentication in front of the admin endpoint where supported

bash

# Configuration example
# Generate a strong replacement secret and inject it at deploy time
export ADMIN_PASSWORD="$(openssl rand -base64 32)"
export GF_SECURITY_ADMIN_PASSWORD="$(openssl rand -base64 32)"

# Verify no default credentials remain in configuration
grep -RniE 'change_me_please|admin123' ./config.yaml ./docker-compose.yml \
  && echo "Default credentials detected - remediate before deployment" \
  || echo "No default credentials found"