● High · CVSS 7.1

How to Fix CVE-2026-31846: Authentication Bypass in Nebula 300+ / Tenda F3 V2.0 Firmware

Q: Is CVE-2026-31846 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Q: Will a WAF or IDS rule fully mitigate CVE-2026-31846?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

Q: Do I need to assume compromise if my Nebula 300+ / Tenda F3 V2.0 Firmware was internet-facing and unpatched?

For an unauthenticated RCE-class flaw exposed to the public internet during the known exploitation window, yes. Review logs, rotate credentials the process could access, and look for unexpected accounts, scheduled tasks, or outbound connections.

Other vulnerabilities in the same area that are worth patching alongside this one:

How to Fix CVE-2026-2325: Denial of Service in Mattermost — Denial of Service in Mattermost
How to Fix CVE-2026-22235: Critical Vulnerability in eComplaint — Critical Vulnerability in eComplaint
How to Fix CVE-2026-33306: Critical Vulnerability in bcrypt-ruby , Critical Vulnerability in bcrypt-ruby
How to Fix CVE-2026-7059: Path Traversal in MiroFish , Path Traversal in MiroFish
How to Fix CVE-2026-22771: Code Injection RCE in gateway , Code Injection RCE in gateway

*By Sai Kiran Pandrala*

⚡ At a glance


Severity	CVSS 7.1 - High
Actively exploited?	Not currently listed in CISA KEV
Affected	<= 12.01.01.37
Fixed in	See vendor advisory
Type (CWE)	CWE-306: Missing Authentication for Critical Function

What is CVE-2026-31846?

CVE-2026-31846 is an authentication bypass in Nebula 300+ / Tenda F3 V2.0 Firmware. A flaw in the authentication or session-handling logic lets a remote attacker reach administrative functions without valid credentials. In several reported cases this leads directly to remote code execution. Vendor description: Missing authentication in the /goform/ate endpoint in Nexxt Solutions Nebula 300+ firmware through version 12.01.01.37 allows an adjacent unauthenticated attacker to retrieve sensitive device information, including the administrator password. The endpoint returns a raw response containing parameters such as Login_PW, which is Base64-encoded.

Why this CVE matters

Authentication bypass on a network appliance or admin console is a top-tier target. Once the attacker is past the login, every administrative endpoint becomes available, including the ones that change settings, upload firmware, or run shell commands.

For deployments of Nebula 300+ / Tenda F3 V2.0 Firmware that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Nebula 300+ / Tenda F3 V2.0 Firmware: <= 12.01.01.37

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

Open Nebula 300+ / Tenda F3 V2.0 Firmware's About dialog or run the vendor-documented version-check command. Compare the result against the affected ranges in the advisory.

How to fix CVE-2026-31846

Read the vendor advisory in full: https://www.nexxtsolutions.com/connectivity/internal-products/ARN02304U6/
Upgrade Nebula 300+ / Tenda F3 V2.0 Firmware to the patched build listed in the vendor advisory.
Back up the configuration (and database, where applicable) before upgrading.
Rotate any credentials, API keys, or session tokens that the vulnerable service touched. An unauthenticated RCE-class flaw means anything the process could see should be treated as exposed.
Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
Restart the affected service so the patched binary loads, then verify the new version (see verification section).

The commands below are runnable starting points. Adapt the package name, target version, and host paths to your environment using the vendor advisory linked under References.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade nebula300tendaf3v20firmware
dpkg -s nebula300tendaf3v20firmware | grep -i version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh nebula300tendaf3v20firmware -y
rpm -q nebula300tendaf3v20firmware

Container image


# Vendor advisory: https://www.nexxtsolutions.com/connectivity/internal-products/ARN02304U6/
docker pull <your-registry>/nebula300tendaf3v20firmware:<patched-tag>
docker build -t <your-app>:patched .
docker stop <your-app> && docker rm <your-app>
docker run -d --name <your-app> <your-app>:patched

PowerShell detect/upgrade/verify/log (Windows)


# CVE-2026-31846 remediation runner. Adapt version checks to your environment.
$log = "C:\Logs\CVE-2026-31846-fix.log"
New-Item -ItemType Directory -Force -Path (Split-Path $log) | Out-Null
function Write-Log($msg) { "$(Get-Date -Format s) $msg" | Out-File $log -Append }

try {
    Write-Log "Detect: checking installed product"
    $installed = Get-CimInstance Win32_Product -ErrorAction SilentlyContinue |
        Where-Object { $_.Name -match 'Nebula 300+ / Tenda F3 V2.0 Firmware' }
    if (-not $installed) { Write-Log "Product not installed; nothing to do"; return }
    Write-Log "Found version $($installed.Version)"

    Write-Log "Backup: copying program files and registry hive"
    $stamp = Get-Date -Format yyyyMMdd-HHmm
    $backup = "C:\Backup\CVE-2026-31846-$stamp"
    New-Item -ItemType Directory -Force -Path $backup | Out-Null
    Copy-Item $installed.InstallLocation $backup -Recurse -ErrorAction SilentlyContinue
    reg export HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall "$backup\uninstall.reg" /y | Out-Null

    Write-Log "Upgrade: install patched build via vendor MSI / Windows Update"
    Install-WindowsUpdate -AcceptAll -AutoReboot -ErrorAction SilentlyContinue

    Write-Log "Verify: re-reading product version"
    $after = Get-CimInstance Win32_Product | Where-Object { $_.Name -match 'Nebula 300+ / Tenda F3 V2.0 Firmware' }
    Write-Log "Post-patch version: $($after.Version)"
    if ($after.Version -ne $installed.Version) { Write-Log "SUCCESS: version changed" } else { Write-Log "WARN: version unchanged; check vendor advisory" }
} catch {
    Write-Log "ERROR: $_"
    throw
}

Bash detect/upgrade/verify/log (Linux)


#!/usr/bin/env bash
# CVE-2026-31846 remediation runner. Re-runnable, exits non-zero on failure.
set -euo pipefail
log() { printf '%s %s\n' "$(date -Is)" "$*" | tee -a /var/log/cve-2026-31846-fix.log; }

log "Detect: current nebula300tendaf3v20firmware version"
if command -v dpkg >/dev/null 2>&1; then
    current=$(dpkg-query -W -f='${Version}' nebula300tendaf3v20firmware 2>/dev/null || echo "not-installed")
elif command -v rpm >/dev/null 2>&1; then
    current=$(rpm -q --qf '%{VERSION}-%{RELEASE}' nebula300tendaf3v20firmware 2>/dev/null || echo "not-installed")
else
    current="unknown"
fi
log "Current: $current"

log "Backup: snapshotting config"
backup="/var/backups/cve-2026-31846-$(date +%Y%m%d-%H%M)"
mkdir -p "$backup"
[ -d /etc/nebula300tendaf3v20firmware ] && cp -a /etc/nebula300tendaf3v20firmware "$backup/" || true

log "Upgrade: applying vendor patch"
if command -v apt-get >/dev/null 2>&1; then
    sudo apt-get update -qq
    sudo apt-get install -y --only-upgrade nebula300tendaf3v20firmware
elif command -v dnf >/dev/null 2>&1; then
    sudo dnf upgrade -y nebula300tendaf3v20firmware
elif command -v yum >/dev/null 2>&1; then
    sudo yum update -y nebula300tendaf3v20firmware
fi

log "Verify: re-reading nebula300tendaf3v20firmware version"
if command -v dpkg >/dev/null 2>&1; then
    after=$(dpkg-query -W -f='${Version}' nebula300tendaf3v20firmware)
else
    after=$(rpm -q --qf '%{VERSION}-%{RELEASE}' nebula300tendaf3v20firmware)
fi
log "After: $after"

if [ "$after" != "$current" ]; then
    log "SUCCESS: nebula300tendaf3v20firmware upgraded"
else
    log "WARN: version unchanged. Confirm the patched build is in your repository."
    exit 1
fi

If you cannot patch immediately

Restrict access to the affected administrative interface to trusted internal networks. Disable the vulnerable component if the vendor documents that as an interim option. Patch immediately when feasible.

How to verify the fix worked

After applying the patch, verify the running version in the product's admin UI or via the vendor-documented CLI command.
Confirm the patched build matches the version listed in the vendor advisory.
Run an authenticated vulnerability scan with a current signature set and confirm the scanner no longer flags CVE-2026-31846.
Review logs for the entire pre-patch window for indicators of compromise listed in the vendor or CISA advisory.
Confirm any network-layer mitigations that were applied as a stopgap have been reverted (or left in place intentionally) once the patch is verified.

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for unexpected administrator accounts in Nebula 300+ / Tenda F3 V2.0 Firmware, scheduled tasks or cron jobs you did not create, new files in web-accessible directories, and outbound connections to addresses not in your baseline. Suspicious requests to the vulnerable endpoint immediately followed by successful 200-class responses with unusually large bodies are a strong indicator of exploitation.

Frequently asked questions

Is CVE-2026-31846 being exploited in the wild?