Threat Intelligence Report: CVE-2026-77004

Executive Summary

CVE-2026-77004 is a command injection vulnerability in the Comfast CF-N1-S wireless router firmware version 2.6.0.1. The flaw resides in the sprintf function of the /cgi-bin/mbox-config endpoint when the sn argument is processed with method SET and section ptest_sn. A remote authenticated attacker can inject arbitrary operating system commands, leading to full device compromise. With a CVSS score of 7.4, a published public exploit, and no vendor patch available, this vulnerability poses an immediate risk to exposed Comfast devices.

Technical Analysis

Vulnerability Mechanism

The vulnerability is a classic command injection flaw (CWE-74 — Improper Neutralization of Special Elements in Output Used by a Downstream Component). The NVD description identifies the root cause as improper use of the C library function sprintf in the file /cgi-bin/mbox-config.

The sprintf function is a formatted string output function that writes formatted data to a string buffer without bounds checking. In this specific case, the vulnerability is not a buffer overflow per se — the injection vector is the untrusted content of the sn argument being passed through sprintf and eventually reaching a system shell execution context (likely via system(), popen(), or exec() calls downstream — though the NVD description does not confirm which exact sink is used).

The flaw class is as follows:

1. Input handling: The CGI script /cgi-bin/mbox-config receives an HTTP request with method parameter method=SET and section parameter section=ptest_sn.
2. Data flow: The value of the sn argument is read from the HTTP request and passed to sprintf without sanitization.
3. Sink: The formatted result is used in a context where shell metacharacters (e.g., ;, &&, |, backticks, $(command)) are interpreted by the operating system shell.
4. Impact: An attacker injects commands that execute with the privileges of the CGI process — typically root on embedded router platforms.

Attack Surface

The /cgi-bin/mbox-config endpoint is part of the device's web management interface. The attack vector attributes in the CVSS vector indicate:

• AV:N — Network attack vector; the endpoint is remotely accessible.
• AC:L — Low attack complexity; no special conditions are required.
• PR:L — Requires low-privilege authentication. The attacker must have a valid account on the router's web interface (often default credentials such as admin/admin are not changed by home users).
• UI:N — No user interaction required.
• S:C — Scope changed; the compromised component (web interface) can affect resources beyond its security scope, meaning the injected command executes outside the web application sandbox.

Affected Component

• Device: Comfast CF-N1-S (wireless router)
• Firmware: 2.6.0.1
• Entry Point: POST/GET to /cgi-bin/mbox-config with method=SET&section=ptest_sn&sn=<malicious_payload>
• Vulnerable Function: sprintf (C standard library)

Attack Vector Summary

The attacker crafts an HTTP request to the device's web interface, supplying a malicious value in the sn parameter. Because the device uses sprintf to build a system command string with this parameter unsanitized, shell metacharacters in the sn value are interpreted by the OS shell, executing arbitrary commands on the device.

Exploitation Scenarios

Prerequisites

• Network access to the device's management interface (port 80/443 typically).
• Valid low-privilege credentials (factory default or user-provided).
• Public exploit is available (per NVD description).

Attack Chain Steps

Step 1 — Reconnaissance / Target Identification

Attackers scan for Comfast CF-N1-S devices exposed to the internet. Common techniques include:

• Shodan/Censys queries for HTTP response fingerprints containing "Comfast" or router model identifiers.
• Banner grabbing on common admin ports (TCP 80, 443, 8080) to identify the device.
• Default credential testing (admin/admin, root/root, etc.) to establish low-privilege access.

Step 2 — Triggering the Vulnerability

The attacker sends an HTTP request to the vulnerable endpoint:

GET /cgi-bin/mbox-config?method=SET&section=ptest_sn&sn=test HTTP/1.1
Host: <target>
Authorization: Basic <base64-credentials>

Because authentication is required (PR:L), the request must include valid session credentials.

Step 3 — Payload Execution

The attacker modifies the sn parameter to include shell metacharacters. At the vulnerability-class level, the payload pattern is:

sn=test;command_to_execute
sn=test && command_to_execute
sn=$(command_to_execute)
sn=`command_to_execute`

For example, a payload like sn=test;reboot would cause the device to reboot, demonstrating command execution. More common exploitation would use a reverse shell payload or command to download and execute malware.

Step 4 — Post-Exploitation Impact

Once command execution is achieved (typically as root on embedded devices), the attacker can:

• Establish persistent reverse shells or bind shells.
• Extract configuration files containing WiFi passphrases, PPPoE credentials, or administrative hashes.
• Modify device DNS settings to redirect traffic (DNS hijacking for phishing or ad injection).
• Add the device to a botnet (Mirai-style) for DDoS operations.
• Pivot to the internal network the device bridges.

Exploitation Notes

The NVD description states: "The exploit has been published and may be used." This indicates that functional exploit code is publicly accessible, lowering the skill barrier for attackers. However, no specific PoC code or Nuclei template is available in this report's data set, so the precise payload structure cannot be quoted beyond the vulnerability class level.

Impact Assessment

CVSS Breakdown Context

• Severity Score: 7.4 (High)
• Base Vector: AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:L

The scope change (S:C) is significant — the web interface compromise transcends into the operating system layer. While individual CIA impacts are rated Low, the cumulative effect on a network gateway device is disproportionate:

• Confidentiality (Low): Access to device configuration and credentials.
• Integrity (Low): Modification of device settings, DNS, firewall rules.
• Availability (Low): Device reboot, DoS via resource exhaustion, or firmware corruption.

Business Impact

Regulatory Considerations

• For organizations using these devices in a business context, this vulnerability may trigger breach notification obligations if sensitive customer data traverses compromised networks.
• PCI-DSS environments where these routers handle cardholder data would face compliance violations.

Detection Guidance

Sigma Rule

title: Comfast CF-N1-S Command Injection via mbox-config sn Parameter
id: 9e2a4c42-8a9d-4f04-9b39-2f1c6f8a335d
status: experimental
description: Detects command injection attempts against Comfast CF-N1-S router /cgi-bin/mbox-config endpoint by identifying shell metacharacters in the sn parameter.
logsource:
  product: web
  service: http
detection:
  selection:
    cs-uri-path:
      - '/cgi-bin/mbox-config'
    cs-uri-query:
      - '*method=SET*'
      - '*section=ptest_sn*'
    cs-uri-query|contains:
      - 'sn=*;*'
      - 'sn=*|*'
      - 'sn=*&&*'
      - 'sn=*`*'
      - 'sn=*$(*'
  condition: selection
falsepositives:
  - Legitimate administration using unusual characters in serial number field (very unlikely)
level: critical

YARA Rule

rule Comfast_CFN1S_SN_CommandInjection
{
    meta:
        author = "Valtersit Threat Intel"
        description = "Detects HTTP payloads targeting Comfast CF-N1-S mbox-config command injection"
        cve = "CVE-2026-77004"
        date = "2026-01-01"
        severity = "high"
        reference = "NVD: CVE-2026-77004"

    strings:
        $endpoint = "/cgi-bin/mbox-config" ascii nocase
        $method_set = "method=SET" ascii nocase
        $section_ptest = "section=ptest_sn" ascii nocase
        $shell_semi = "sn=" ascii nocase
        $meta_semicolon = ";"
        $meta_pipe = "|"
        $meta_and = "&&"
        $meta_backtick = "`"
        $meta_dollar = "$("
        $meta_newline = "%0a" ascii nocase

    condition:
        all of them and 1 of ($meta_*)
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (
    msg:"ET EXPLOIT Comfast CF-N1-S Command Injection Attempt (CVE-2026-77004)";
    flow:to_server,established;
    content:"/cgi-bin/mbox-config"; http_uri; nocase;
    content:"method=SET"; http_uri; nocase;
    content:"section=ptest_sn"; http_uri; nocase;
    pcre:"/sn=[^&]*(?:;|\||&&|`|\$\(|%0a)/Ui";
    classtype:attempted-admin;
    sid:202677004;
    rev:1;
    metadata:created_at 2026-01-01, cve CVE-2026-77004;
)

Elastic Detection

// Elastic EQL query for HTTP endpoint logs (Packetbeat or Filebeat HTTP module)
sequence by source.ip
  [ network where network.protocol == "http" and
    http.request.path == "/cgi-bin/mbox-config" and
    http.request.body like "*method=SET*" and
    http.request.body like "*section=ptest_sn*" and
    (http.request.body like "*sn=*;*" or
     http.request.body like "*sn=*|*" or
     http.request.body like "*sn=*&&*" or
     http.request.body like "*sn=*`*" or
     http.request.body like "*sn=*$*(*")
  ]

Splunk SPL Query

index=web sourcetype=access_combined OR sourcetype=pan:traffic
| where uri_path="/cgi-bin/mbox-config"
| where uri_query LIKE "%method=SET%" AND uri_query LIKE "%section=ptest_sn%"
| regex uri_query="sn=[^&]*(;|\||&&|`|\$\(|%0a)"
| stats count by src_ip, dest_ip, uri, _time
| sort - _time

Wazuh OSSEC Rule

<group name="web,exploit,cve_2026_77004,">
  <rule id="331088" level="15">
    <decoded_as>json</decoded_as>
    <field name="url">/cgi-bin/mbox-config</field>
    <match>method=SET|section=ptest_sn</match>
    <regex>sn=[^&]*(;|\||&&|`|\$\(|%0a)</regex>
    <description>Comfast CF-N1-S Command Injection Attempt (CVE-2026-77004)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>command_injection,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence Assessment

UNPATCHED — 0/4 confidence. No patch evidence from any tracked source. Vendor has not released a firmware update. Affected version list is empty (exact version 2.6.0.1 confirmed vulnerable).

1. Immediate Mitigations (No Patch Available)

2. Patch/Upgrade Steps

• Contact Comfast support for firmware patch status. As of this report, no fixed version exists.
• Isolate vulnerable devices until a patched firmware (2.6.0.2 or later) is released.
• Monitor vendor channels (Comfast official website, firmware update portal) for CVE-2026-77004 remediation.
• If a patched firmware becomes available, test in a lab environment before deploying to production devices.

3. Long-Term Hardening

1. Replace vulnerable hardware: If CF-N1-S devices are mission-critical, consider replacing them with alternatives from vendors with active security support.
2. Implement network-level controls:
   • Use an IDS/IPS (Suricata/Snort) with the rule above to block exploit attempts in transit.
   • Apply egress filtering to prevent reverse-shell callbacks from compromised devices.
3. Configuration management:
   • Maintain an asset inventory of all network devices.
   • Establish a firmware update cadence and track CVEs affecting your device fleet.
4. Access control:
   • Move management interfaces to dedicated VLANs.
   • Use centralized authentication (RADIUS/TACACS+) if the device supports it.
5. Compensating controls:
   • Deploy a WAF in front of any exposed device management interfaces.
   • Use Honeypot / canary credentials to detect credential-stuffing attempts.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-77004
2. CWE-74: https://cwe.mitre.org/data/definitions/74.html — Improper Neutralization of Special Elements in Output Used by a Downstream Component
3. CVSS Calculator: https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:L
4. MITRE ATT&CK — T1190: https://attack.mitre.org/techniques/T1190/ — Exploit Public-Facing Application

Exploit and Threat Actor Evidence

• The NVD description confirms: "The exploit has been published and may be used." — indicating public exploit code exists.
• No known APT campaigns specifically targeting this CVE have been identified at the time of writing.
• Given the device class (small business/home routers), exploitation is likely conducted by:
  • IoT botnet operators (Mirai variants) for DDoS recruitment.
  • Credential-stuffing / opportunistic attackers scanning for exposed management interfaces.
  • Local network attackers who obtain low-privilege credentials through other means.

CISA KEV

Not listed on CISA Known Exploited Vulnerabilities catalog at the time of this report. However, the public exploit availability increases the likelihood of inclusion in the future.

Report generated by Valtersit Threat Intelligence. This report is for defensive security purposes only. Authorized testing of systems requires explicit permission from system owners.