CVE-2025-48057

Executive Summary

CVE-2025-48057 is a critical security vulnerability in Icinga 2, an open-source monitoring system. The flaw resides in the VerifyCertificate() function, which can be tricked into incorrectly treating a malicious certificate request as a renewal of an existing, valid certificate. This allows an unauthenticated attacker to obtain a valid certificate signed by the Icinga CA, enabling them to impersonate trusted nodes within the monitoring cluster. The vulnerability is exploitable only when Icinga 2 is compiled against OpenSSL versions older than 1.1.0. Successful exploitation can lead to full compromise of the monitoring infrastructure, including data exfiltration, lateral movement, and disruption of monitoring services.

Supply Chain & Package Security

This vulnerability is not directly related to a package manager (NPM, PyPI, Go, Ruby) but affects the Icinga 2 binary itself, which is typically distributed via operating system package managers (e.g., apt, yum) or built from source. The supply chain risk is elevated because the exploit depends on the version of OpenSSL linked at compile time. Organizations using Icinga 2 built against an older OpenSSL (pre-1.1.0) are vulnerable even if they have updated Icinga 2 to a "patched" version that was compiled against an older OpenSSL. Therefore, verifying the build-time OpenSSL version is critical.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder. Based on the vulnerability description, a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N) - The attacker sends a malicious certificate request over the network.
• Attack Complexity (AC): Low (L) - No special conditions are required beyond the vulnerable OpenSSL version.
• Privileges Required (PR): None (N) - No authentication is needed to send the request.
• User Interaction (UI): None (N) - No user action is required.
• Scope (S): Changed (C) - The attacker can impersonate a trusted node, affecting resources beyond the initial vulnerable component.
• Confidentiality (C): High (H) - Full access to monitoring data and potential lateral movement.
• Integrity (I): High (H) - The attacker can inject false monitoring data or commands.
• Availability (A): High (H) - The attacker can disrupt monitoring services.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H Estimated Base Score: 10.0 (Critical)

Deep Dive & Mechanics

The vulnerability is in the certificate validation logic of Icinga 2, specifically in the VerifyCertificate() function. This function is responsible for authenticating nodes joining the Icinga cluster. The intended flow is:

1. A new node sends a Certificate Signing Request (CSR) to the Icinga master.
2. The master verifies the CSR and, if valid, signs it with the CA certificate.
3. The signed certificate is returned to the requesting node.

The flaw occurs when Icinga 2 is built with OpenSSL < 1.1.0. In these older versions, the X509_verify_cert() function (or equivalent) has a subtle behavior difference. The VerifyCertificate() function can be tricked into treating a malicious CSR as a renewal request for an existing certificate. This means:

• The attacker sends a CSR that is crafted to match the subject or serial number of an already-trusted node.
• Due to the OpenSSL bug, the verification logic incorrectly believes this is a renewal and skips the full validation.
• The master then signs the attacker's public key, issuing a valid certificate for the impersonated node's identity.

The attacker now possesses a valid certificate that the Icinga cluster trusts. They can use this certificate to:

• Authenticate as the impersonated node.
• Access all monitoring data.
• Send fake check results or acknowledgments.
• Potentially execute commands on the master if the impersonated node has elevated privileges.

Vulnerable vs. Patched Code

The vulnerable code is in the VerifyCertificate() function. While the exact source code is not publicly available in the CVE description, the logic can be inferred.

Vulnerable Code (Conceptual):

bool VerifyCertificate(X509* cert, X509* ca_cert) {
    // OpenSSL < 1.1.0 behavior
    if (X509_verify_cert(cert, ca_cert) == 0) {
        // Check if it's a renewal
        if (IsRenewalRequest(cert)) {
            return true; // BUG: Returns true even if cert is malicious
        }
        return false;
    }
    return true;
}

Patched Code (Conceptual):

bool VerifyCertificate(X509* cert, X509* ca_cert) {
    // OpenSSL >= 1.1.0 or fixed logic
    if (X509_verify_cert(cert, ca_cert) == 0) {
        // Renewal requests must still pass full validation
        if (IsRenewalRequest(cert) && X509_verify_cert(cert, ca_cert) == 1) {
            return true;
        }
        return false;
    }
    return true;
}

The patch ensures that even renewal requests undergo proper certificate chain validation, preventing the bypass.

Exploitation & Lab Setup

Red Team (Attacker)

Lab Setup:

1. Environment: Two VMs: Attacker (Kali Linux) and Target (Ubuntu 20.04 with Icinga 2.14.5 compiled against OpenSSL 1.0.2).
2. Target Configuration: Icinga 2 master with default CA and a trusted node (e.g., node1.example.com).
3. Tools: OpenSSL CLI, custom Python script to craft malicious CSR.

Exploitation Steps:

1. Reconnaissance: Identify the Icinga master IP and port (default 5665). Use Shodan or Nmap to find vulnerable instances.
2. Craft Malicious CSR:
   • Extract the subject of a trusted node's certificate (e.g., CN=node1.example.com).
   • Generate a new RSA key pair.
   • Create a CSR with the same subject but a different public key.
   • Set the CSR to appear as a renewal (e.g., by including a specific extension or serial number pattern).
3. Send CSR to Master:

   openssl req -new -key attacker.key -out malicious.csr -subj "/CN=node1.example.com"
   # Modify CSR to include renewal flag (conceptual)
   # Use custom script to send CSR to Icinga master
   python3 send_csr.py --target 192.168.1.100 --csr malicious.csr
   

4. Receive Signed Certificate: The master returns a signed certificate for node1.example.com.
5. Impersonate Node: Use the signed certificate and attacker's private key to authenticate to the master.

   icinga2 node setup --cn node1.example.com --cert signed.crt --key attacker.key
   

6. Access Monitoring Data: Once authenticated, the attacker can query the Icinga API or execute commands.

Blue Team (Defender)

Detection & Response:

1. Monitor Certificate Requests: Log all CSR submissions. Look for requests with subjects matching existing nodes but from different IP addresses.
2. Check OpenSSL Version: Verify the OpenSSL version used to compile Icinga 2.

   ldd /usr/sbin/icinga2 | grep ssl
   

3. Audit Certificate Logs: Review Icinga logs for unusual certificate renewals.

   grep -i "certificate" /var/log/icinga2/icinga2.log
   

4. Network Segmentation: Restrict access to the Icinga master port (5665) to only trusted IPs.
5. Harden OpenSSL: Upgrade to OpenSSL 1.1.0 or later, or recompile Icinga 2 against a patched OpenSSL.

Indicators of Compromise (IOCs)

• Network: Unexpected certificate signing requests (CSRs) from unknown IPs.
• Logs: Icinga log entries showing certificate renewals for nodes that did not initiate them.
• Certificates: Multiple valid certificates with the same subject (CN) but different public keys.
• Processes: Unauthorized icinga2 processes or connections from unexpected nodes.
• Files: New certificate files in /var/lib/icinga2/certs/ with suspicious timestamps.

Detection Engineering

YARA Rule

rule CVE_2025_48057_Icinga_Cert_Bypass {
    meta:
        description = "Detects Icinga 2 certificate validation bypass (CVE-2025-48057)"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-48057"
    strings:
        $s1 = "VerifyCertificate" ascii wide nocase
        $s2 = "X509_verify_cert" ascii wide nocase
        $s3 = "renewal" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: Icinga 2 Certificate Validation Bypass
id: cve-2025-48057
status: experimental
description: Detects suspicious certificate renewal requests in Icinga 2 logs
author: Valters IT
logsource:
    product: icinga
    service: icinga2
detection:
    selection:
        EventID: 2000  # Example event ID for certificate operations
        Message: "*certificate renewal*"
    condition: selection
falsepositives:
    - Legitimate certificate renewals from known nodes
level: high

Suricata Rule

alert tcp any any -> any 5665 (msg:"CVE-2025-48057 Icinga 2 Certificate Bypass Attempt"; flow:to_server; content:"|30 82|"; depth:2; content:"|02 01 00|"; within:10; content:"|30 82|"; distance:2; within:100; reference:cve,2025-48057; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Patch: Upgrade Icinga 2 to versions 2.12.12, 2.13.12, or 2.14.6 or later.
2. OpenSSL Upgrade: Ensure Icinga 2 is compiled against OpenSSL 1.1.0 or later. If using a package manager, verify the linked OpenSSL version.
3. Recompile: If patching is not possible, recompile Icinga 2 from source with a patched OpenSSL.
4. Network Controls: Restrict access to the Icinga master port (5665) using firewall rules.
5. Certificate Monitoring: Implement monitoring for unexpected certificate renewals.
6. Incident Response: If exploitation is suspected, revoke all certificates and reissue them after patching.

Valters IT