A flaw has been found in Tenda A21 1

CVE-2026-2874 is a stack-based buffer overflow in the Tenda A21 web management path /goform/fast_setting_wifi_set, where a crafted ssid value can overflow a fixed 64-byte stack buffer. The public write-up ties it to firmware V1.0.0.0, and Tenda's download pages only expose that same V1.0.0.0 build for A21; one regional support page also labels the product end of life. In plain English: if an attacker can reach the extender's admin interface, they may be able to crash it or potentially execute code on the device.

The vendor-style 8.8 HIGH score is technically defensible in a lab because memory corruption on a network appliance can be ugly, but it overstates enterprise urgency. The real friction is attacker position: this device is a consumer Wi-Fi range extender whose management interface is typically reached locally during setup, not a broadly internet-exposed enterprise platform. Add the very low EPSS, no KEV listing, and only a public issue-level PoC rather than mature exploit tooling, and this lands as MEDIUM for most enterprise patch queues.

Why this verdict

• Downward pressure: requires management-plane reachability — this is not a drive-by browser bug; the attacker must reach the extender admin interface first.
• Downward pressure: product population is narrow — Tenda A21 is a consumer Wi-Fi range extender, not a mainstream enterprise platform across 10,000-host fleets.
• Downward pressure: exploitation evidence is weak — public disclosure and a PoC exist, but there is no KEV listing, no confirmed campaign reporting, and EPSS is very low.
• Residual risk keeps it above LOW — if you do have these devices in branches, guest networks, or shadow IT, a memory-corruption bug on a network appliance can still enable local traffic manipulation or a foothold.

Why not higher?

It is not HIGH because the attack path is narrower than the CVSS score implies. The prerequisite of reaching a lightly managed consumer-device admin UI sharply limits exposed population, and there is no strong evidence of broad operational exploitation. It is not CRITICAL because there is no KEV signal, no verified internet-scale exposure, and no evidence that exploitation is trivial and reliable across real deployments.

Why not lower?

It is not LOW because this is still a remotely reachable memory-corruption flaw on a network device, with a public PoC and plausible code-execution impact. If an A21 is present on a branch, guest, or lab segment, compromise can affect traffic integrity and create a meaningful local foothold.

1. Block admin access from untrusted networks — Restrict the A21 management UI to a dedicated management subnet or a directly connected staging host only. For a MEDIUM verdict there is no mitigation SLA, but this is the highest-value containment step whenever these devices are discovered.
2. Retire or replace A21 units — Vendor pages reviewed do not show a fixed firmware, and one regional support page marks the product end-of-life. Use the 365-day remediation window for MEDIUM findings to remove or replace any A21 found in enterprise use.
3. Move extenders off sensitive segments — If removal is not immediate, place discovered A21 devices on guest/IoT-style VLANs with no management path into core infrastructure. This reduces the blast radius while you work through the MEDIUM-severity remediation window.
4. Watch for crashes and config drift — Because exploitation may present first as web UI instability or reboots rather than polished RCE, collect whatever syslog, DHCP, DNS, and network behavior you can from segments using these devices. This is a cheap way to spot abuse on gear that cannot run EDR.

Run this from an auditor workstation that can reach the suspected Tenda device over HTTP. Invoke it as python3 check_tenda_a21_cve_2026_2874.py http://192.168.0.254 or python3 check_tenda_a21_cve_2026_2874.py http://re.tenda.cn; no admin rights are required, but network reachability to the device is.

#!/usr/bin/env python3
# check_tenda_a21_cve_2026_2874.py
# Identify likely Tenda A21 devices and flag firmware 1.0.0.0 as VULNERABLE.
# Exit codes: 0=PATCHED, 1=VULNERABLE, 2=UNKNOWN, 3=USAGE/ERROR

import re
import sys
import ssl
import urllib.request
import urllib.error
from urllib.parse import urljoin

TIMEOUT = 5
PAGES = [
    "/",
    "/index.html",
    "/login.html",
    "/main.html",
    "/status.html",
    "/overview.html",
]

A21_RE = re.compile(r"\bTenda\b.*\bA21\b|\bA21\b.*\bTenda\b", re.I | re.S)
VER_RE = re.compile(r"\bV?1\.0\.0\.0\b")
ANY_VER_RE = re.compile(r"\bV?(\d+\.\d+\.\d+\.\d+)\b")

ctx = ssl.create_default_context()
ctx.check_hostname = False
ctx.verify_mode = ssl.CERT_NONE


def fetch(url):
    req = urllib.request.Request(url, headers={"User-Agent": "noisgate-a21-check/1.0"})
    with urllib.request.urlopen(req, timeout=TIMEOUT, context=ctx) as resp:
        body = resp.read(1024 * 512)
        ctype = resp.headers.get("Content-Type", "")
        server = resp.headers.get("Server", "")
        return body.decode("utf-8", errors="ignore"), ctype, server


def normalize_base(base):
    if not base.startswith("http://") and not base.startswith("https://"):
        base = "http://" + base
    return base.rstrip("/") + "/"


def main():
    if len(sys.argv) != 2:
        print("UNKNOWN - usage: python3 check_tenda_a21_cve_2026_2874.py <base_url_or_ip>")
        sys.exit(3)

    base = normalize_base(sys.argv[1])
    found_a21 = False
    found_vuln_ver = False
    found_other_ver = None
    errors = []

    for path in PAGES:
        url = urljoin(base, path.lstrip("/"))
        try:
            text, ctype, server = fetch(url)
            haystack = "\n".join([text, ctype, server])
            if A21_RE.search(haystack):
                found_a21 = True
            if VER_RE.search(haystack):
                found_vuln_ver = True
            else:
                m = ANY_VER_RE.search(haystack)
                if m:
                    found_other_ver = m.group(1)
        except (urllib.error.URLError, urllib.error.HTTPError, TimeoutError, ssl.SSLError) as e:
            errors.append(f"{url}: {e}")
            continue
        except Exception as e:
            errors.append(f"{url}: {e}")
            continue

    if found_a21 and found_vuln_ver:
        print("VULNERABLE - Tenda A21 detected with firmware 1.0.0.0 markers")
        sys.exit(1)

    if found_a21 and found_other_ver and found_other_ver != "1.0.0.0":
        print(f"PATCHED - Tenda A21 detected with non-vulnerable version marker {found_other_ver}")
        sys.exit(0)

    if found_a21:
        print("UNKNOWN - Tenda A21 detected but firmware version could not be confirmed")
        sys.exit(2)

    if errors:
        print("UNKNOWN - target reachable status/version not confirmed; sample error: " + errors[0])
    else:
        print("UNKNOWN - no Tenda A21 markers found")
    sys.exit(2)


if __name__ == "__main__":
    main()

Sources

1. NVD CVE-2026-2874
2. GitHub PoC / technical write-up by QIU-DIE
3. Tenda Global A21 help/download page
4. Tenda firmware download detail for A21 V1.0.0.0
5. Tenda Russia A21 support page showing end-of-life label
6. CISA Known Exploited Vulnerabilities Catalog
7. OSV entry for CVE-2026-2874
8. INCIBE summary for CVE-2026-2874