External risk intelligence

TP-Link TL-WR840N Remote Code Execution Via IP Input.

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2021-41653

This vulnerability affects a home/small office router interface. Router administration consoles are frequently exposed to the network, and while best practices discourage exposing them to the public internet, they are commonly reachable edge services in many residential and small business deployment scenarios.

Code Injection

Tp Link Tl Wr840n Firmware

tl-wr840n\(eu\)_v5_171211 and earlier

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability exists in the PING function of certain TP-Link routers, allowing for remote code execution. This could enable an attacker to compromise the device and potentially gain control over the network it serves. The main concern is confirming relevance and exposure of these devices.

Unauthenticated attackers can run malicious code.
Affects network edge devices that may be exposed.
Assess exposure and confirm if devices are in scope.

Attack Path

How an attacker could exploit the issue

An attacker can reach this router from the internet and send it a specially crafted request to the PING function. By providing malicious input in an IP address field, they can execute arbitrary code on the device, potentially leading to full compromise.

Accessible from the internet.
Triggered via crafted IP address input.
Allows remote code execution.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could allow an unauthenticated attacker to execute arbitrary code on the router when a specially crafted payload is sent through the PING function's IP address input field. This could affect the router's service behavior and potentially any data it processes or transmits.

Router's service behavior.
Remote unauthenticated network access.
Compromised router functionality.

Operational Fix

Recommended remediation, mitigation, and detection steps

Addressing this critical vulnerability requires a coordinated effort, likely involving the network or security team responsible for edge devices, and potentially the vendor-management team if external support is needed. The initial focus should be on identifying all instances of the affected router, assessing their exposure (particularly to the internet), and determining their business criticality. This information will guide the prioritization of remediation or the implementation of compensating controls.

Identify affected router instances.
Verify external accessibility and criticality.
Plan coordinated remediation or mitigation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the TP-Link TL-WR840N?

The TL-WR840N is a wireless router commonly used in homes and small offices to provide internet connectivity. It handles network traffic and manages connected devices through an administrative interface. This specific vulnerability involves the device's firmware, which controls how the router processes internal commands like the PING diagnostic tool.

What does CVE-2021-41653 mean for the router?

This CVE represents a vulnerability classified as CWE-94, which refers to Improper Control of Generation of Code. In plain terms, the router's PING function fails to properly validate data entered into the IP address field. This allows an attacker to inject and execute their own unauthorized code directly on the device, effectively taking control of its operating system.

How is this vulnerability triggered?

An attacker triggers the vulnerability by sending a specially crafted request to the router's PING function. The attack relies on malicious content placed specifically within the IP address input field. Legitimate use of the PING function with valid, standard IP address formats does not trigger this flaw.

Is my router at risk from this vulnerability?

According to Halo Surface Signal, this vulnerability is considered likely for many users because these routers function as edge services. If your router’s administrative console is reachable from the public internet, it faces a higher risk. Devices restricted to internal, private network access only are generally safer, though they remain vulnerable to local attackers.

How do I respond to this threat?

Start by identifying all instances of the TP-Link TL-WR840N within your environment. Verify if these devices are exposed to the internet. If you find affected units, prioritize them based on their criticality and work to apply vendor-supplied firmware updates. If an update is unavailable, restrict management access so the device is no longer reachable from the internet.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.