CVE-2023-24035 in Nagios
Summary
by MITRE • 09/14/2026
An issue was discovered in Nagios XI before 5.9.3. The is_insecure_login_authenticated function uses a insecure timing comparison that leads to an attacker being able to bruteforce the admin password, by measuring timing differences in the comparison.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in versions of Nagios XI prior to release 5.9.3 represents a critical authentication bypass mechanism rooted in improper handling of cryptographic comparisons within the is_insecure_login_authenticated function. This flaw allows an authenticated attacker or, depending on specific deployment configurations and initial access vectors, potentially an unauthenticated actor to compromise administrative credentials through timing side-channel attacks. The core technical deficiency lies in the use of standard equality operators for comparing sensitive data such as password hashes or tokens against expected values. In secure software engineering practices, string comparisons must be performed using constant-time algorithms that ensure execution time remains independent of the input strings' content and length. By utilizing a non-constant-time comparison method, the system inadvertently leaks information about how many characters in the guessed credential match the actual secret before the comparison fails.
This timing discrepancy arises because most programming languages evaluate string equality character by character from left to right, terminating early upon detecting the first mismatch. Consequently, if an attacker submits guesses that share more leading characters with the true password, the server responds slightly faster than when submitting guesses with fewer matching prefix characters. By meticulously measuring these microsecond-level differences in response times across thousands of requests, a remote adversary can iteratively deduce each character of the administrative password one position at a time. This process effectively neutralizes the entropy provided by strong passwords, reducing the complexity of brute-forcing credentials from exponential to linear relative to the length and charset of the secret value. Such an attack is classified under CWE-208 as Observable Timing Discrepancy and falls squarely within the ATT&CK technique T1110.003 known as Password Guessing: Brute Force, specifically leveraging timing analysis rather than traditional lockout mechanisms or rate limiting to bypass defenses.
The operational impact of this vulnerability is severe for any organization relying on Nagios XI for infrastructure monitoring and alerting. Successful exploitation grants the attacker full administrative control over the monitoring platform, which often serves as a central hub for visibility into network health, server status, and security event correlation. With admin privileges, an adversary can manipulate configuration files to disable alerts, alter thresholds to hide malicious activity, or inject custom scripts that execute with system-level privileges on the host machine running Nagios XI. This escalation path transforms a monitoring tool from a defensive asset into a persistent foothold for lateral movement within the enterprise network. Furthermore, because Nagios often runs as root or with elevated permissions to collect system metrics, compromising this service can lead directly to full server compromise and subsequent exploitation of other connected systems via shared credentials or trust relationships established through the compromised monitor.
Mitigation strategies must prioritize immediate patching alongside compensating controls for environments where updates cannot be applied instantly. The primary remediation is upgrading Nagios XI to version 5.9.3 or later, which implements constant-time comparison functions such as hash_equals in PHP or equivalent secure libraries in other backend languages used by the application. For systems that remain unpatched due to operational constraints, network-level controls should be enforced to limit request rates from single source IP addresses to mitigate the volume of requests required for successful timing analysis. Additionally, implementing strict account lockout policies after a small number of failed login attempts can disrupt the iterative nature of the brute-force attack, although attackers may attempt to distribute requests across multiple IPs or slow down their pace to evade detection. Security teams should also audit access logs for anomalous patterns indicative of automated credential testing and ensure that Nagios XI is not exposed directly to untrusted networks without robust web application firewall protections capable of detecting timing-based side-channel attempts.