CVE-2026-76070

Executive Summary

CVE-2026-76070 is a critical, unauthenticated stack-based buffer overflow vulnerability (CWE-121) affecting the Netis NC63 router firmware through version V3.0.0.3327. The flaw resides in the login handler of the /bin/netis.cgi CGI binary, which is executed by the Boa web server with root privileges. An unauthenticated remote attacker can trigger the overflow by submitting an oversized Base64-encoded password, exploiting a custom Base64 decoder that fails to validate output length against a fixed-size stack buffer. Successful exploitation results in remote code execution (RCE) with root privileges, allowing full device compromise. The vulnerability carries a CVSS score of 9.8 (Critical) and currently has no patch available, no known public exploit code, and is not listed on CISA's Known Exploited Vulnerabilities (KEV) catalog. Given the severity and the unpatched status, immediate mitigation actions are strongly advised for any exposed Netis NC63 devices.

Technical Analysis

The vulnerability is a classic stack-based buffer overflow (CWE-121) located in the login authentication component of the Netis NC63 router firmware. The NVD description identifies the vulnerable code path as the login handler within the /bin/netis.cgi binary. This CGI executable is spawned by the Boa web server, which is configured to run CGI processes with root privileges — a common but dangerous design decision in embedded networking devices that dramatically escalates the impact of any memory corruption vulnerability.

Mechanism: The overflow is triggered by the handling of the password field during the login process. The application receives a Base64-encoded password string from the remote client. This string is processed by a custom Base64 decoder implemented within the CGI binary. The flaw is that this custom decoder does not perform output length validation against the size of the destination buffer, which is a fixed-size stack-allocated array. When the decoder writes the decoded bytes into the stack buffer, an oversized input leads to a write past the buffer boundary. This overwrites adjacent stack memory, including the saved return address (and potentially other saved registers and stack canaries if present — though the description implies they are absent or bypassable).

Attack Surface: The attack surface is the web-based management interface exposed on port 80 (HTTP) by the Boa web server. The login handler is reachable without authentication, as the overflow occurs during the authentication check rather than after it. This makes the vulnerability trivially exploitable by any network-reachable attacker, with no credentials or prior access required.

Code-Level Perspective (inferred from vulnerability class): While specific function names beyond /bin/netis.cgi and the login handler are not provided in the NVD description, the flaw class implies the following structure: a function reads the HTTP POST/GET parameter containing the Base64 password, passes it to a custom Base64 decoding routine, and the routine writes to a stack buffer declared with a fixed size (e.g., char decoded[64]). The decoder likely iterates over the Base64 input in 4-character blocks, producing 3 bytes of output per block, but fails to check whether the cumulative output size exceeds the buffer capacity. An attacker providing a Base64 string that decodes to more bytes than the buffer can hold causes the stack corruption.

Lack of Hardening: The absence of stack canaries, ASLR, or non-executable stack protections is implied by the assertion that attackers can "overwrite saved stack state" to achieve RCE. On MIPS-based routers (common for Netis devices), a simple return-address overwrite with a jump to shellcode placed on the stack is a well-established exploitation technique.

Exploitation Scenarios

The attack chain for CVE-2026-76070 is inferred from the vulnerability description and class. No public PoC exists at the time of writing; the following scenario is based on standard exploitation of stack overflows in embedded web servers.

Step 1: Reconnaissance / Target Identification The attacker identifies Internet-facing Netis NC63 devices. This is achievable via Shodan/Censys searches using filters such as title:"Netis" or product:"Netis" to locate the default web login page. Alternatively, network scans of common router IP ranges for open port 80 with the Netis login banner can be used. The attacker confirms the target is running a vulnerable firmware version by fingerprinting the login page or by attempting benign requests to the login handler.

Step 2: Triggering the Flaw Class The attacker crafts a legitimate HTTP request to the login endpoint of the web interface, targeting the /bin/netis.cgi login handler. The request must include a Base64-encoded password parameter that, when decoded, exceeds the size of the stack buffer. The attacker does not need valid credentials — the request is designed to fail authentication but trigger the overflow during the password processing/decoding stage before any authentication result is returned.

Step 3: Payload / Execution Method (Vulnerability-Class Level) The malicious payload consists of a Base64 string that decodes to (a) padding bytes to reach the saved return address on the stack, and (b) a return address value pointing to attacker-controlled shellcode. The shellcode itself is typically embedded within the same decoded payload buffer. On successful exploitation, the overwritten return address causes the CPU to jump to the shellcode. Given the root privileges of the Boa/CGI environment, the shellcode executes with full root privileges. At the vulnerability-class level, the shellcode would likely be designed to:

• Bind a remote shell (e.g., on a high port) or establish a reverse shell connection to the attacker's machine.
• Alternatively, it could patch the firmware's authentication logic to grant persistent administrative access, or disable security features such as firewall rules.

Step 4: Post-Exploitation Impact With root shell access, the attacker gains complete control of the router. Post-exploitation actions include:

• Credential Theft: Reading configuration files (e.g., /etc/passwd, router config) to harvest Wi-Fi passwords, ISP credentials, and any saved VPN or administrative credentials.
• Network Pivoting: Using the compromised router as a pivot point to attack internal network hosts behind the NAT, intercepting traffic via ARP spoofing, or setting up port forwards.
• Persistence: Installing a persistent backdoor (e.g., a modified firmware image, a small binary that survives reboot, or altering the startup scripts).
• Botnet Recruitment: Adding the device to a botnet for DDoS attacks or as a proxy for malicious traffic, given the typically high-bandwidth, always-on nature of routers.
• Data Exfiltration: Since the router processes all network traffic, the attacker can perform on-the-fly packet capture to exfiltrate sensitive data transmitted across the local network.

Note: No public PoC is available; this attack chain is inferred from the vulnerability class. The described steps reflect the standard exploitation methodology for stack overflow vulnerabilities in unauthenticated web interfaces.

Impact Assessment

Business Impact: Given the CVSS vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, the vulnerability is trivially exploitable over the network without authentication or user interaction, and it compromises confidentiality, integrity, and availability to a high degree. This translates to a criticality rating of 9.8, categorizing it as a "Critical" severity issue.

Affected Systems: Netis NC63 router firmware through version V3.0.0.3327 is affected. The exact hardware revisions are not specified, but the NC63 is a widely deployed SOHO (Small Office/Home Office) router. Given the long lifecycle of such devices and the lack of automatic update mechanisms, many deployed units are likely vulnerable, even if newer firmware versions exist.

Blast Radius:

• Direct Device Compromise: Full root-level code execution on the router hardware.
• Network Compromise: The router is a gateway device; compromise allows interception, modification, and redirection of all traffic flowing through it. This affects all devices on the local network, whether on the LAN, guest Wi-Fi, or behind the NAT.
• Data Confidentiality: Credentials stored on the device (Wi-Fi passwords, PPPoE credentials, DDNS keys) are exposed, potentially allowing further intrusion into the ISP account or adjacent network segments.
• Availability: Attacker can brick the device or disrupt network connectivity entirely.
• Lateral Movement: The router becomes a launchpad for attacks against internal hosts that are otherwise protected from the Internet.

Exploitability Factors: The lack of authentication requirement, network reachability, and the complexity of the flaw (a simple length-check omission) make this a highly reliable and low-skill exploit. The custom Base64 decoder issue suggests the same vulnerable code pattern might be replicated across other Netis models or firmware versions, although only the NC63 is confirmed affected.

Detection Guidance

Detection of this vulnerability is challenging due to the lack of a known specific payload. The following rules are designed to detect anomalous requests to the login handler that may indicate an attempt to exploit the overflow.

Sigma Rule

title: Netis NC63 CGI Login Buffer Overflow Attempt
id: 0988a111-b2a2-4c4b-8d0f-1b9ac0a56789
status: experimental
description: Detects HTTP requests to Netis NC63 login endpoint with an abnormally long Base64-encoded password parameter, indicating a potential stack buffer overflow attempt.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-76070
tags:
    - attack.initial-access
    - attack.t1190
    - cve.2026-76070
logsource:
    category: webserver
    product: apache
    definition: 'Requires access logs from the Boa web server or an HTTP proxy/IPS logging URL parameters.'
detection:
    selection:
        cs-uri-path|contains: 'netis.cgi'
    keywords:
        - 'password='
    condition: selection and keywords and len(cs-uri-query) > 500
    timeframe: 5m
level: high
falsepositives:
    - Legitimate configuration changes where a long (but < 500 char) password is set via the web interface.
    - Custom scripts managing the router.
fields:
    - c-ip
    - cs-uri-query
    - sc-status

Note: The threshold of 500 characters may need tuning based on the actual legitimate maximum password length of the device (typically 63 chars for ASCII or ~128 bytes for Base64). A more precise rule would decode the Base64 parameter and check length > 256 bytes.

YARA Rule

rule Netis_CGI_Base64_Overflow_Payload
{
    meta:
        author = "ValterSec Research"
        description = "Detects strings indicative of a Netis NC63 login buffer overflow attempt: an embedded Base64 blob containing NOP sled or shellcode patterns within a netis.cgi request."
        cve = "CVE-2026-76070"
        date = "2026-05-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-76070"
    strings:
        $netis = "netis.cgi" ascii fullword
        $b64_char = /[A-Za-z0-9+\/=]{200,}/ ascii
        $mips_shell = { 24 18 06 24 01 00 02 24 }  // common MIPS shellcode bytes (not specific)
        $nop_sled = { 21 20 20 20 20 20 20 20 }  // MIPS NOP-like (0x21202020)
    condition:
        $netis and any of ($b64_char*, $mips_shell, $nop_sled) and filesize < 10KB
}

Note: This rule is a heuristic. The MIPS shellcode bytes are generic placeholders; without a public PoC, precise shellcode detection is not possible. The rule aims to catch any file (e.g., captured PCAP, HTTP body dumps) containing the vulnerable script path and a very long Base64 string.

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"CVE-2026-76070 Netis NC63 Login Buffer Overflow Attempt"; flow:to_server,established; http.uri; content:"netis.cgi"; http.request_body; content:"password="; within:255; distance:0; pcre:"/password=[A-Za-z0-9+\/=]{400,}/Ui"; classtype:attempted-admin; sid:2026076070; rev:1; reference:cve,2026-76070;)

Explanation: This rule inspects HTTP traffic to the server. It looks for a URI containing netis.cgi and a request body containing password= followed by a Base64 string of 400+ characters. The within and distance keywords ensure the password parameter is near the beginning of the body. The PCRE matches the excessively long Base64 string. Since legitimate passwords are short, this is a high-confidence indicator of an exploit attempt.

Elastic Detection (EQL)

sequence by process.entity_id with maxspan=5m
  [network where destination.port == 80 and process.name : "boa" and network.protocol == "http"]
  [network where destination.port == 80 and process.name : "boa" and network.protocol == "http"
   and message : "*netis.cgi*password=*" and message : "*password=[A-Za-z0-9+/=]{400,}*"]

Note: This EQL rule assumes a modern Linux-based IDS/agent capturing HTTP traffic on the host. The first event establishes HTTP traffic to Boa, and the second looks for the malicious pattern. In a typical deployment, this would be run on a network tap or a NDR tool sending EQL events. The message field would need to be populated by the data source (e.g., Zeek logs, packet capture parser). A more robust implementation would use Kibana's https://www.elastic.co/guide/en/security/current/rules-ui-create.html with a custom query on HTTP body fields.

Splunk SPL Query

index=proxy OR index=web sourcetype="access_combined_wcookie" OR sourcetype="stream:http" 
| where match(cs_uri_query, "netis\.cgi") OR match(form_data, "netis\.cgi") OR match(http_request_body, "netis\.cgi")
| eval password_field = coalesce(form_data, http_request_body) 
| where match(password_field, "password=[A-Za-z0-9+\/=]{400,}")
| table _time, src_ip, dest_ip, uri, http_method, user_agent, password_field
| stats count by src_ip, dest_ip, uri
| sort - count

Explanation: This query searches web proxy or network logs for requests containing netis.cgi. It then inspects the request body (either form_data or http_request_body fields, depending on the data source) for the password= parameter followed by a Base64 string of 400+ characters. It aggregates results by source IP to identify potential scanners/attackers. Tune the 400-character threshold based on your environment.

Wazuh OSSEC Rule

<group name="netis,">
  <rule id="100760" level="15">
    <decoded>netis.cgi</decoded>
    <description>CVE-2026-76070: Netis NC63 login buffer overflow attempt via oversized Base64 password.</description>
    <field name="url">netis\.cgi</field>
    <field name="data">password=[A-Za-z0-9+\/=]{400,}</field>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>attack.initial-access,</group>
  </rule>
</group>

Note: This rule is designed for the Wazuh manager to parse Apache/Boa access logs. The decoded and field elements expect the log to be in a key-value format. If your Boa logs do not include the query string by default, you will need to configure logging to include %q (query string). The threshold of 400 characters is heuristic.

Mitigation & Patch Guidance

Patch Confidence: 0/4 — This vulnerability is UNPATCHED. No vendor advisory, firmware update, or patch of any kind has been identified from Netis or any other tracked source. The vendor has not acknowledged the issue publicly as of this report. Given this status, the following recommendations are prioritized accordingly.

1. Immediate Mitigations (Minimize Exposure):

• Limit Network Exposure: Immediately restrict access to the web management interface (TCP/80) to only trusted administrative IPs via firewall rules. If remote management is not essential, disable it entirely. This reduces the attack surface from the entire Internet to specific IPs.
• Disable Remote Management: If the router supports remote management over WAN, disable it. The Boa web server should only be listening on the LAN interface.
• Change Default/Weak Credentials: While the vulnerability does not require valid credentials to trigger, enforcing strong, unique credentials prevents trivial post-exploitation credential reuse if the attacker's shellcode includes a credential harvesting step.
• Network Segmentation: Place the router's management interface on a dedicated VLAN or restrict it from the general LAN traffic.
• IDS/IPS Deployment: Deploy the detection rules provided above on your network monitoring infrastructure to alert on exploit attempts.

2. Patch/Upgrade Steps (Long-Term Fix):

• Vendor Monitoring: Monitor Netis's official website and support channels for any firmware release that addresses CVE-2026-76070. As of this report, no such release exists.
• Firmware Update: If and when a patch is released, update all affected NC63 devices to the patched firmware version. Test the firmware in a non-production environment first.
• Device Replacement: If the vendor does not provide a patch within a reasonable timeframe, plan for the eventual replacement of affected Netis NC63 routers with alternatives from vendors with a demonstrated commitment to security patching. The device is EOL (end-of-life) risk given the lack of patching.

3. Long-Term Hardening:

• Network Inventory: Maintain an accurate inventory of all network devices, including routers, switches, and IoT, to facilitate rapid identification of affected hardware.
• Vendor Security Posture: When purchasing new networking equipment, prioritize vendors with a track record of timely security updates and public vulnerability disclosure.
• Configuration Baseline: Establish a configuration baseline for all routers, including disabling unnecessary services (e.g., remote management, UPnP, WPS) and enforcing least-privilege access.
• Logging & Monitoring: Ensure that web server and network logs for router management interfaces are centrally collected and retained for a minimum of 90 days to enable post-incident investigation.
• Incident Response Plan: Include compromised-router scenarios in incident response playbooks, as device compromise can be a silent entry point into a network.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-76070
2. CWE-121 Stack-based Buffer Overflow: https://cwe.mitre.org/data/definitions/121.html
3. Boa Web Server Project (for context on CGI privileges): https://www.boa.org/
4. CISA Known Exploited Vulnerabilities Catalog (to confirm not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
5. FIRST.org EPSS (for future scoring): https://www.first.org/epss/

Exploit-in-the-Wild Evidence: None. At the time of this report, there is no public evidence of exploitation in the wild, no PoC code, and no threat actor campaigns referencing this CVE. The vulnerability is considered high-risk due to its critical severity, ease of exploitation, and the prevalence of exposed Netis devices. The lack of a patch and the absence of public exploit code suggest the vulnerability was recently disclosed (or withheld by the researcher) and may be exploited in the future, particularly if a PoC emerges. Monitoring for the Netis NC63 login pattern on your network is strongly recommended.