External risk intelligence

lwIP SMTP Client Buffer Overflow Vulnerability.

CVE advisorySeverity: CRITICAL (CVSS 9.3)

CVE-2026-15340

The vulnerability affects an SMTP client implementation within the lwIP (Lightweight IP) TCP/IP stack. lwIP is typically used in embedded systems and IoT devices rather than public-facing servers. While the protocol operates over a network, these components are generally deployed within internal or isolated device networks rather than as internet-exposed services.

Buffer Overflow

Halo Surface Signal: 2 out of 5 — less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability has been identified in the lwIP SMTP client where it does not properly check the size of certain inputs, which could lead to a buffer overflow. This issue affects the lwIP TCP/IP stack, commonly found in embedded systems and IoT devices. While the protocol allows for network interaction, the typical deployment of these systems within internal networks lessens the likelihood of direct external exploitation.

Input size checks are missing in lwIP's SMTP client.
It's important to confirm if this affects your embedded systems.
Focus on understanding relevance and potential exposure.

Attack Path

How an attacker could exploit the issue

An attacker could remotely send specially crafted data to the affected system's SMTP client. This input is not properly validated, leading to a buffer overflow in the lwIP component. Successful exploitation could result in significant impact to the confidentiality, integrity, and availability of the system.

No authentication or user interaction required.
Sending oversized input to SMTP client.
Remote code execution and system compromise.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, a buffer overflow in the lwIP SMTP client could affect the integrity and availability of services that rely on this component, particularly in embedded systems and IoT devices. This could occur when the client processes untrusted input without adequate size checks.

Embedded system services.
Network input processing.
Service availability and integrity.

Operational Fix

Recommended remediation, mitigation, and detection steps

The lwIP SMTP client's vulnerability to buffer overflows due to unchecked input size requires immediate attention from teams responsible for embedded systems and network-enabled devices. Ownership likely falls to platform or embedded development teams, with support from network and security teams for exposure assessment. The first practical step is to identify all instances of the affected lwIP component, confirm their network reachability and criticality, and then prioritize remediation, potentially involving vendor coordination for embedded systems.

Platform or embedded development teams own the issue.
Verify network reachability and system criticality first.
Plan remediation based on exposure and vendor advisories.

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.

Free EASM Trial See How Signal Works

Frequently asked questions

What is the lwIP software component?

lwIP, or Lightweight IP, is a compact open-source TCP/IP stack designed specifically for embedded systems and IoT devices. It provides essential networking capabilities—like sending emails via SMTP—to hardware with limited memory and processing power, such as industrial controllers or network-connected appliances.

What does CWE-120 mean for CVE-2026-15340?

CWE-120 refers to 'Buffer Copy without Checking Size of Input.' In this CVE, the lwIP SMTP client fails to verify the length of data it receives. Because it doesn't check if the incoming information fits in the allocated memory, an attacker could supply more data than the buffer can hold, potentially overwriting adjacent memory and disrupting or compromising the system.

How is this buffer overflow triggered?

The flaw is triggered when the lwIP SMTP client processes maliciously crafted, oversized input sent over the network. It is important to note that the vulnerability does not occur during standard, properly formatted SMTP communications; it specifically requires input that exceeds expected buffer limits to cause the memory corruption.

Is my device at risk for CVE-2026-15340?

According to Halo Surface Signal, risk is often lower because lwIP components are typically deployed in internal, isolated device networks rather than as public-facing internet services. However, you should evaluate if your specific embedded devices are reachable from untrusted network segments, as this increases the likelihood of remote access.

Do I need to patch my lwIP-based systems?

Yes, but start by creating an inventory of devices running the affected lwIP component. Once identified, assess their network reachability and business criticality. Since these are often embedded devices, you should coordinate with your hardware vendors to obtain and apply the necessary firmware updates or software patches they provide.

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.