CVE-2023-24035

Executive Summary

CVE-2023-24035 is a timing-side-channel vulnerability (CWE-208, Observable Timing Discrepancy) in Nagios XI prior to version 5.9.3. The is_insecure_login_authenticated function performs an insecure (non-constant-time) comparison that leaks information through measurable timing differences, allowing an attacker to gradually recover the admin password through timing-guided brute force. Because the vulnerable check is reachable over the network with low privileges, successful exploitation can result in recovery of valid administrative credentials and, through them, disclosure or manipulation of the monitoring platform's data.

Technical Analysis

The vulnerability resides in the Nagios XI authentication code path associated with the is_insecure_login_authenticated function. Per the NVD description, this function "uses a[an] insecure timing comparison that leads to an attacker being able to bruteforce the admin password, by measuring timing differences in the comparison." This is an instance of CWE-208: Observable Timing Discrepancy.

In a typical non-constant-time string comparison, the function iterates over the characters of the attacker-supplied value and the stored secret and returns (or exits the loop) as soon as a mismatch is found. The duration of the comparison therefore depends on the number of leading characters that match: a candidate sharing a longer prefix with the secret takes measurably longer to reject. While cryptographically robust authentication normally uses constant-time comparison (or, better, hashed values with one-way processing), the timing-observable comparison here converts a password-verification oracle into an incremental-guessing oracle. The attacker is no longer searching the full password space blind; they can reduce an exponentially large space to a linear, character-by-character search, with a small number of timing samples needed per candidate character to average out network and scheduler noise.

Key points that follow from the description and CVSS vector (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:N/A:N, score 3.5):

• Attack Vector is Network (AV:N) — the comparison is reachable remotely.
• Attack Complexity is High (AC:H), consistent with timing measurement requiring repeated, statistically refined observations over an unreliable network path.
• Privileges Required is Low (PR:L) — the attacker can reach the code path with minimal or no authenticated footing (e.g., any user able to invoke the login flow).
• User Interaction is None (UI:N).
• Scope is Changed (S:C) — exploitation of the authentication component impacts resources beyond it (the administration function of the platform; the vector explicitly declares a scope change, reflecting an authorization/authentication component affecting the wider application).
• Confidentiality impact is Low (C:L), and Integrity/Availability are None (I:N/A:N) — consistent with recovery of a single secret (the admin password) that then grants read-oriented reach, rather than a direct RCE or DoS primitive.

No vendor advisory, patch artifact, researcher blog, PoC, Nuclei template, or exploit code exists in the provided data. The name is_insecure_login_authenticated and the timing-comparison behavior are the only implementation-level details available; all other specifics must not be fabricated.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred from the CWE-208 vulnerability class and the CVSS vector, using only the one named function from the NVD description.

1. Reconnaissance / target identification. The attacker identifies a network-reachable Nagios XI instance running a version earlier than 5.9.3. Passive fingerprinting (web UI title, product banners) or Shodan-style enumeration matches monitored Nagios XI deployments. Because the flaw is in the authentication check rather than in a post-auth feature, the attacker only needs a stable network path to the login endpoint — no session or administrative account is needed.

2. Triggering the specific flaw class. The attacker repeatedly invokes the login/authentication flow such that the is_insecure_login_authenticated function evaluates attacker-controlled password candidates against the stored admin secret. Each request causes the function to perform its non-constant-time comparison; the attacker records the response latency for each candidate. The distinguishing factor is not the response body but the elapsed time attributable to the comparison: candidates whose leading characters agree with the secret consume marginally more comparison time before the function rejects them.

3. Payload/execution at the vulnerability-class level. The attacker builds a timing oracle: for each position in the password, they submit a set of candidate characters while holding the already-guessed prefix fixed, collect many samples per candidate, and apply statistical filtering (median/trimmed mean, outlier rejection) to suppress network jitter. The candidate whose timing distribution is measurably slower is treated as correct, and the prefix is extended. This reduces the search from brute force over the full password space to a per-character, timing-guided recovery. Because authentication is not required and no user interaction is involved (UI:N), the oracle can be exercised in bulk from a single host or, more effectively for noise reduction, from a distributed vantage point.

4. Post-exploitation impact. Recovery of the administrator password yields administrative access to the Nagios XI platform. The CVSS vector assigns Scope:Changed and Confidentiality:Low with no Integrity/Availability impact, so the expected result is unauthorized disclosure and visibility into the monitored environment — host/service inventories, performance and availability metrics, trap/inform data, and configuration views — rather than direct integrity or availability compromise. However, administrative access to a monitoring server frequently represents a high-value pivot: the platform normally holds credentials, notification configurations, and topology data for a large slice of the monitored estate. This secondary expansion is a function of the credentials obtained, not of a documented vulnerability primitive, and should be treated as an operational risk rather than a claim from the CVE data.

Impact Assessment

Business impact centers on compromise of administrative credentials for the enterprise monitoring platform. With AV:N and PR:L, the attack is feasible remotely without user interaction, but AC:H and the 3.5 base score correctly frame it as a high-skill, statistically demanding attack rather than a drive-by exploit. The Changed Scope subscore reflects that the vulnerable authentication component impacts resources beyond itself: successfully guessing the admin secret grants access to the broader Nagios XI application, not merely to the comparison function.

Blast radius is determined by how widely Nagios XI administrative credentials are reused or how much environment data the platform holds. Affected systems are network-reachable Nagios XI deployments before 5.9.3. Because the vulnerability enables credential recovery — not memory disclosure, code execution, or denial of service — the direct, CVE-attributable impact is limited to Confidentiality:Low. Integrity and Availability are unaffected directly; any integrity or availability impact arises only from subsequent use of recovered credentials and is therefore outside the scope of this CVE.

EPSS is 0.00767, at the 53.8th percentile, indicating low but non-negligible predicted exploitation activity relative to all CVEs. CVE-2023-24035 is not listed in CISA KEV, and no public PoC, exploit code, or Nuclei template was identified. The realistic threat is a targeted, patient adversary rather than opportunistic mass exploitation.

Detection Guidance

Sigma Rule

title: Nagios XI Login Timing-Side-Channel Brute Force (CVE-2023-24035)
id: 8f4c2a10-3b7e-4c1d-9a2b-7e5d6c1f9a30
status: experimental
description: Detects high-volume, low-and-slow login attempts against Nagios XI that are consistent with timing-guided password recovery via the insecure comparison in is_insecure_login_authenticated (CVE-2023-24035).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2023-24035
author: valtersit.com
date: 2025-01-01
logsource:
  category: web
  product: nagios
detection:
  selection:
    cs-uri-path|contains:
      - '/nagiosxi/login.php'
      - '/nagiosxi/'
  auth_failure:
    sc-status: 200
  timeframe: 5m
  condition: selection and auth_failure | count() by src_ip > 50
level: medium
tags:
  - attack.credential_access
  - attack.t1110
  - cve.2023-24035

YARA Rule

rule CVE_2023_24035_Nagios_XI_Insecure_Login_Comparison
{
    meta:
        author = "valtersit.com"
        description = "Detects references to the Nagios XI timing-vulnerable authentication function associated with CVE-2023-24035 in source or scripts."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2023-24035"
        date = "2025-01-01"
        cve = "CVE-2023-24035"

    strings:
        $func = "is_insecure_login_authenticated" ascii wide
        $nagios = "Nagios XI" ascii wide

    condition:
        $func and $nagios
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2023-24035 Possible Nagios XI Timing-Guided Login Brute Force"; flow:to_server,established; content:"/nagiosxi/login.php"; http_uri; threshold:type both, track by_src, count 50, seconds 300; classtype:attempted-user; reference:cve,2023-24035; reference:url,nvd.nist.gov/vuln/detail/CVE-2023-24035; sid:202324035; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5m
  [any where process.name != null and http.request.method == "POST" and url.path : "/nagiosxi/login.php"] with runs=25

Splunk SPL Query

index=web sourcetype=nagios_xi OR sourcetype=access_combined
| search uri_path="/nagiosxi/login.php"
| bin _time span=5m
| stats count AS login_attempts dc(uri_query) AS distinct_queries VALUES(src_ip) AS sources by src_ip, _time
| where login_attempts > 50 AND distinct_queries > 10
| sort - login_attempts

Wazuh OSSEC Rule

<group name="nagios_xi,cve-2023-24035,web,">
  <rule id="100235" level="10" frequency="25" timeframe="300">
    <if_matched_sid>31151</if_matched_sid>
    <match>/nagiosxi/login.php</match>
    <description>CVE-2023-24035: Repeated Nagios XI login requests from same source, possible timing-side-channel brute force</description>
    <mitre>
      <id>T1110</id>
    </mitre>
    <group>authentication_failures,credential_access,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations:

• Upgrade Nagios XI to version 5.9.3 or later as directed by the NVD description. Because no vendor advisory or patch artifact could be independently confirmed through tracked sources, treat 5.9.3 as the minimum boundary and verify the current vendor release before deployment.
• Where immediate upgrade is not possible, restrict network access to the Nagios XI web interface using firewall rules, VPN, or an authenticating reverse proxy so that only trusted management networks can reach the login flow.
• Enforce account lockout / rate limiting at a layer in front of the application (reverse proxy, WAF, or authentication gateway) to collapse the number of timing samples an attacker can gather, degrading the oracle.
• Add jitter or randomized delay in front of the login endpoint through a reverse proxy to raise the noise floor that a timing attack must overcome.

2. Patch/upgrade steps:

• Confirm the currently installed Nagios XI version.
• Back up the Nagios XI configuration and database.
• Follow the vendor's documented upgrade procedure to move to 5.9.3 or later.
• After upgrading, validate that authentication now behaves normally and that the web interface is reachable only from intended sources.

3. Long-term hardening:

• Adopt constant-time comparison primitives for all secret comparisons in authentication code paths.
• Enforce strong, unique administrative passwords and rotate the Nagios XI admin credential as a precaution if compromise is suspected.
• Centralize and monitor authentication logs; alert on high-frequency or highly variant login attempts.
• Track Nagios security advisories and establish a routine patch cadence for the monitoring platform, which is a high-value target given the credentials and topology it holds.

References

• NVD — CVE-2023-24035: https://nvd.nist.gov/vuln/detail/CVE-2023-24035
• MITRE CWE-208 — Observable Timing Discrepancy: https://cwe.mitre.org/data/definitions/208.html
• MITRE ATT&CK — T1110 Brute Force: https://attack.mitre.org/techniques/T1110/
• First EPSS — model and scoring information: https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (CVE-2023-24035 not listed at time of writing): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Note on exploit-in-the-wild evidence: No public PoC, exploit code, or Nuclei template was identified for CVE-2023-24035, and the CVE is not listed in CISA KEV. No specific threat actor campaign has been attributed. The EPSS score of 0.00767 (53.8th percentile) indicates low predicted exploitation activity. The complete absence of recovered patch artifacts, vendor advisories, and public research means defenders must rely on the NVD description and the version boundary (before 5.9.3) alone.