CVE-2026-90688

Executive Summary

CVE-2026-90688 is a stack-based buffer overflow (CWE-119) in Tenda W20E firmware version 15.11.0.61068_1546_841_CN_TDC. The flaw resides in the formIPMacBindAdd function, part of the router's HTTP handler component, and is triggered by improper validation of the IPMacBindRule argument. A remote attacker with valid credentials (PR:L) can send a crafted HTTP request to cause memory corruption, resulting in a denial-of-service condition (CVSS 6.5, A:H). No public exploit, PoC, or patch exists at the time of writing.

Technical Analysis

The vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and manifests as a stack-based buffer overflow. The affected component is the HTTP Handler of the Tenda W20E, specifically the formIPMacBindAdd function. This function processes IP/MAC binding rules — a common feature on SOHO and SMB routers that maps static IP addresses to hardware MAC addresses to enforce network access policies.

Per the NVD description, the vulnerability is reached via manipulation of the IPMacBindRule argument. When a request containing an oversized or malformed value for this argument is processed, the data is copied into a fixed-size stack buffer without adequate bounds checking. Because the buffer resides on the stack, overflow can overwrite adjacent stack memory, including saved return addresses and control data.

The attack is network-reachable (AV:N) and has low attack complexity (AC:L), but requires the attacker to be authenticated (PR:L). This means the HTTP handler route that invokes formIPMacBindAdd is presumably behind some form of authentication gate — likely the web management interface login. No user interaction is required (UI:N). While scope is unchanged (S:U) and there is no stated confidentiality or integrity impact (C:N/I:N), the availability impact is rated High (A:H), consistent with a crash-and-reboot or watchdog-reset pattern typical of embedded router buffer overflows.

At the code level, the exact buffer size, copy mechanism (strcpy, sprintf, memcpy), and input validation routines are not disclosed in the NVD description. What is derivable from the data is the vulnerability class: a fixed-length stack buffer receiving unvalidated attacker-controlled input via the IPMacBindRule HTTP parameter. The specific HTTP endpoint path, request method (GET vs. POST), and content-type are not stated in the provided intelligence and should not be assumed. Security teams should treat the formIPMacBindAdd handler as the target and enumerate the router's IP/MAC binding configuration interface to locate the corresponding endpoint.

The absence of a CVE description beyond the function and product details limits precise code-level reconstruction. Analysts should rely on the function name semantics — formIPMacBindAdd implies a web form handler that adds a binding rule — to identify the attack surface, but should not fabricate parameter names beyond IPMacBindRule, which is explicitly named.

Exploitation Scenarios

No public PoC or exploit code is available, and no Nuclei template has been published. The following attack chain is inferred strictly from the vulnerability class and the disclosed function/argument names.

1. Reconnaissance / Target Identification. The attacker scans for internet- or LAN-exposed Tenda W20E management interfaces. The router's web admin panel is typically reachable on TCP/80 or TCP/443. Shodan or similar services can identify targets by product fingerprint. Because the CVSS vector indicates PR:L, the attacker must first possess valid management credentials — this may be obtained via default-credential testing, credential stuffing, or prior compromise of a lower-privileged account.

2. Authentication and Interface Mapping. Once authenticated, the attacker navigates to the IP/MAC binding management section of the web UI, which corresponds to the formIPMacBindAdd handler. The attacker identifies the request that submits a new binding rule and observes the IPMacBindRule parameter being transmitted.

3. Triggering the Overflow. The attacker crafts a request where the IPMacBindRule argument contains an oversized value designed to exceed the destination stack buffer. Because the flaw is a stack-based buffer overflow, the payload class is a long, uncontrolled string rather than a format-string or integer-overflow payload. Depending on the exact copy routine and stack layout (undisclosed), the overflow may overwrite saved registers, a saved return address, or a structured exception handler address.

4. Impact and Post-Exploitation. With N/I/C impacts rated None and A:H, the primary demonstrable effect is a crash — the router process handling the HTTP request, possibly the entire web server or device, becomes unavailable. A watchdog reset may cause a reboot loop if the malicious request is replayed. Depending on mitigations present in the firmware (stack canaries, NX, ASLR — all unstated), the overflow could theoretically be leveraged for code execution; however, the CVSS scoring and lack of disclosed exploitability suggest the realistic impact is denial of service. An attacker who can achieve code execution could persist on the device, pivot into the LAN, or intercept traffic, but no such capability is claimed by the CVE data.

Impact Assessment

The affected product is the Tenda W20E, a wireless router aimed at small business and branch-office deployments. Because the W20E sits at the network edge, its availability is directly tied to business connectivity. Successful exploitation causes a denial-of-service condition — the device becomes unresponsive or reboots, interrupting internet access, VPN connectivity, and any dependent services for all downstream users.

The CVSS v3.1 base score is 6.5 (Medium), derived from AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H. The score is moderated by the authentication requirement (PR:L) and the lack of confidentiality or integrity impact in the official scoring. However, in real deployments the blast radius can be significant: a small business with a single W20E loses its entire network perimeter. Repeated exploitation can prevent recovery. If the overflow proves exploitable for code execution in practice, the CIA triad impact could escalate far beyond the scored availability-only impact, and the score would understate risk.

There is no CISA KEV listing, no EPSS high score (0.00397, 33.5th percentile), and no known threat-actor campaign. The immediate business risk is operational disruption rather than data breach, but the vulnerable class (memory corruption in an edge device) warrants prioritization because weaponization could raise the impact tier.

Detection Guidance

Sigma Rule

title: Tenda W20E formIPMacBindAdd Stack Buffer Overflow Attempt
id: 9f3c2b7e-1a4d-4e8a-9c6f-3d2e1f0a7b5c
status: experimental
description: Detects HTTP requests targeting the Tenda W20E formIPMacBindAdd handler with an oversized IPMacBindRule argument indicative of a stack buffer overflow attempt (CVE-2026-90688).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-90688
author: Valters IT Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1190
  - cve.2026.90688
logsource:
  category: webserver
detection:
  selection_endpoint:
    cs-uri-query|contains:
      - 'formIPMacBindAdd'
      - 'IPMacBindRule'
  selection_oversized:
    cs-uri-query|re: 'IPMacBindRule=[^&]{256,}'
  condition: selection_endpoint and selection_oversized
falsepositives:
  - Legitimate administrative configuration of unusually long MAC/IP binding comments
level: high

YARA Rule

rule CVE_2026_90688_Tenda_W20E_IPMacBind_BOF
{
    meta:
        description = "Detects oversized IPMacBindRule values in HTTP request artifacts targeting Tenda W20E formIPMacBindAdd (CVE-2026-90688)"
        author = "Valters IT Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-90688"
        date = "2026-01-01"
        severity = "high"

    strings:
        $handler = "formIPMacBindAdd" ascii wide nocase
        $param   = "IPMacBindRule" ascii wide nocase
        $longval = /IPMacBindRule=[A-Za-z0-9%]{256,}/

    condition:
        any of ($handler, $param) and $longval
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-90688 Tenda W20E formIPMacBindAdd Stack Buffer Overflow Attempt"; flow:established,to_server; http.uri; content:"formIPMacBindAdd"; nocase; http.uri; content:"IPMacBindRule"; nocase; pcre:"/IPMacBindRule=[^&\s]{256,}/i"; reference:cve,2026-90688; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-90688; classtype:attempted-admin; sid:202690688; rev:1;)

Elastic Detection

sequence by host.name
  [ network where http.request.method != null and
    stringContains(http.request.body, "IPMacBindRule") and
    length(http.request.body) > 512 ]
  [ network where stringContains(http.request.body, "formIPMacBindAdd") ]

Splunk SPL Query

index=web sourcetype=access_combined OR sourcetype=stream:http
| eval uri=coalesce(uri, uri_path, "")
| where match(uri, "(?i)formIPMacBindAdd") AND match(uri, "(?i)IPMacBindRule")
| rex field=uri "(?i)IPMacBindRule=(?<bindrule>[^&]+)"
| eval bindrule_len=len(bindrule)
| where bindrule_len > 255
| stats count min(_time) as first_seen max(_time) as last_seen values(src_ip) as sources by dest, bindrule_len
| sort - bindrule_len

Wazuh OSSEC Rule

<group name="web,router,cve-2026-90688,">
  <rule id="100688" level="12">
    <if_sid>31100</if_sid>
    <match>formIPMacBindAdd</match>
    <description>CVE-2026-90688: Tenda W20E IP/MAC binding handler accessed</description>
  </rule>
  <rule id="100689" level="14">
    <if_sid>100688</if_sid>
    <regex>IPMacBindRule=\S{256,}</regex>
    <description>CVE-2026-90688: Oversized IPMacBindRule detected - possible stack buffer overflow attempt</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No vendor fix, advisory, or workaround is available from any tracked source for firmware version 15.11.0.61068_1546_841_CN_TDC.

1) Immediate mitigations

• Restrict management interface access: bind the web admin panel to an internal management VLAN and block TCP/80 and TCP/443 from untrusted networks.
• Enforce strong, unique administrative credentials; eliminate default passwords. The PR:L prerequisite means credential hygiene directly reduces exposure.
• Place the W20E behind a firewall that filters inbound access to the management UI; disable WAN-side remote management if enabled.
• Monitor for the detection signatures above; an oversized IPMacBindRule request is the primary indicator.

2) Patch / upgrade steps

• No patch exists. Monitor Tenda's official support channels and the NVD entry for firmware updates. If Tenda releases a fixed firmware, upgrade before reconnecting the device to an untrusted network.
• Consider hardware replacement or substitution with a supported, actively maintained router if the W20E is end-of-support or receives no security updates.

3) Long-term hardening

• Inventory all edge devices and track firmware versions against known CVEs; unpatched SOHO routers are a recurring entry point.
• Isolate IoT/SOHO router management planes from production networks.
• Apply defense-in-depth: network segmentation, egress filtering, and centralized logging to detect anomalous HTTP activity against management endpoints.
• Validate embedded-device input handling during procurement; prefer vendors with a coordinated vulnerability disclosure and patch history.

References

• NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-90688
• CWE-119 Information: https://cwe.mitre.org/data/definitions/119.html
• Tenda Product Support (monitor for firmware): https://www.tendacn.com/
• CISA Known Exploited Vulnerabilities Catalog (not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat-actor campaign, exploit-in-the-wild evidence, public PoC, or Nuclei template was identified for CVE-2026-90688 at the time of this report. EPSS remains low at 0.00397 (33.5th percentile).