CVE-2023-24035: Nagios XI Timing Attack Auth Bypass Flaw

CVE-2023-24035 Overview

CVE-2023-24035 is a timing side-channel vulnerability in Nagios XI versions before 5.9.3. The is_insecure_login_authenticated function performs password comparison using a non-constant-time operation. An authenticated attacker with low privileges can measure response-time differences between comparison attempts to reconstruct the administrator password character by character.

The flaw is classified under [CWE-208: Observable Timing Discrepancy]. It requires network access and elevated attack complexity, but successful exploitation compromises the confidentiality of administrator credentials for the Nagios XI monitoring platform.

Critical Impact

Attackers who successfully brute-force the admin password through timing analysis gain full administrative access to the Nagios XI monitoring infrastructure and any credentials it stores for monitored assets.

Affected Products

• Nagios XI versions prior to 5.9.3
• Nagios XI is_insecure_login_authenticated authentication path
• Deployments exposing the Nagios XI login interface to untrusted networks

Discovery Timeline

• 2026-09-14 - CVE-2023-24035 published to NVD
• 2026-09-16 - Last updated in NVD database

Technical Details for CVE-2023-24035

Vulnerability Analysis

The vulnerability resides in the is_insecure_login_authenticated function within the Nagios XI authentication workflow. The function compares a submitted credential against the stored administrator password using an insecure string comparison that short-circuits on the first mismatched byte.

Because comparison time correlates with the number of matching leading bytes, an attacker can send crafted authentication requests and record server response latencies. Aggregating enough samples reveals which candidate bytes produce measurably longer processing times, allowing incremental recovery of the credential.

The attack requires the attacker to already hold a low-privilege authenticated session on the Nagios XI instance and to reach the vulnerable code path over the network. Exploitation is non-trivial because network jitter must be filtered out through statistical sampling, which is reflected in the high attack complexity rating.

Root Cause

The root cause is the use of a byte-by-byte string comparison operator rather than a constant-time comparison routine when validating login credentials. Any comparison that returns early upon detecting a mismatch leaks information about the length of the matching prefix through execution time.

Attack Vector

An attacker authenticates to Nagios XI with a low-privilege account and repeatedly submits authentication requests against the administrator account. By measuring server response times across thousands of requests per candidate byte and applying statistical analysis to isolate the signal from network noise, the attacker recovers the administrator password one byte at a time.

No verified public exploit code is available for this issue. Vendor references are published in the Nagios XI Change Log and Nagios Security Disclosures.

Detection Methods for CVE-2023-24035

Indicators of Compromise

• High volumes of authentication requests targeting the administrator account from a single authenticated session or source IP.
• Repetitive login attempts with incrementally varying password payloads over short time windows.
• Unexpected successful administrator logins following extended periods of failed authentication activity.

Detection Strategies

• Baseline normal authentication request rates per user and alert on statistical outliers, particularly for privileged accounts.
• Correlate web server access logs with Nagios XI authentication logs to identify sustained login probing patterns.
• Deploy web application firewall rules that rate-limit repeated POST requests to the login endpoint from any single session.

Monitoring Recommendations

• Forward Nagios XI authentication and web server logs to a centralized analytics platform for behavioral analysis.
• Alert on any administrator authentication that follows a burst of failed logins from the same source.
• Track outbound connections from the Nagios XI host for signs of post-compromise credential exfiltration.

How to Mitigate CVE-2023-24035

Immediate Actions Required

• Upgrade all Nagios XI instances to version 5.9.3 or later without delay.
• Restrict network access to the Nagios XI web interface to trusted management networks only.
• Rotate the administrator password and any credentials stored within Nagios XI after patching.

Patch Information

Nagios addressed this issue in Nagios XI 5.9.3 by replacing the vulnerable comparison in is_insecure_login_authenticated with a constant-time credential validation routine. Release details are documented in the Nagios XI Change Log and the Nagios Security Disclosures page.

Workarounds

• Enforce IP allow-listing at the reverse proxy or firewall so only authorized administrator workstations can reach the login endpoint.
• Enable multi-factor authentication for all administrator accounts to prevent password-only compromise.
• Implement account lockout and rate-limiting policies for repeated failed authentication attempts.

# Example: nginx rate limit for the Nagios XI login endpoint
limit_req_zone $binary_remote_addr zone=nagios_login:10m rate=5r/m;

location /nagiosxi/login.php {
    limit_req zone=nagios_login burst=5 nodelay;
    proxy_pass http://nagios_backend;
}