CVE-2026-15340 in lwIP
Summary
by MITRE • 10/09/2026
lwIP SMTP client does not check the size of inputs, potentially allowing a buffer overflow.
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Analysis
by VulDB Data Team • 10/09/2026
The Lightweight IP stack, commonly known as lwIP, contains a critical security vulnerability within its Simple Mail Transfer Protocol (SMTP) client implementation that stems from an absence of input validation regarding message payload sizes. This flaw represents a classic instance of CWE-120, Buffer Copy without Checking Size of Input, where the software fails to verify whether incoming data exceeds the allocated memory buffer boundaries before processing it. In networked embedded systems and IoT devices utilizing lwIP for email transmission or notification services, this oversight can be exploited by an attacker who controls the content of SMTP responses or maliciously crafted server interactions during the handshake process. The vulnerability arises because the parser does not enforce strict length checks on fields such as reply strings or header values received from the mail server, allowing oversized inputs to overwrite adjacent memory structures in the stack's execution context.
From a technical perspective, this buffer overflow condition occurs when the SMTP client processes responses from an untrusted or compromised mail relay without validating their byte count against the static buffers allocated for storing these messages. If an attacker can manipulate the SMTP server behavior through DNS spoofing, man-in-the-middle attacks, or by operating a rogue mail server within reach of the target device, they can inject payloads containing excessively long strings into specific protocol fields. When lwIP attempts to copy this unbounded input into fixed-size buffers on the stack or heap, it results in memory corruption that may overwrite return addresses, function pointers, or critical state variables. This type of exploitation aligns with MITRE ATT&CK technique T1059, Command and Scripting Interpreter, as successful exploitation typically leads to arbitrary code execution, allowing the attacker to gain full control over the embedded device running the vulnerable stack.
The operational impact of this vulnerability is severe, particularly in environments where lwIP is deployed on resource-constrained devices such as industrial controllers, smart home appliances, or medical equipment that rely on automated email alerts for status reporting. A successful exploit can lead to complete system compromise, including denial of service through application crashes and data exfiltration via remote code execution. Since many embedded systems lack robust sandboxing mechanisms compared to general-purpose operating systems, the consequences extend beyond simple process termination to potential physical safety risks or persistent backdoor installation within critical infrastructure networks. The lack of bounds checking also complicates forensic analysis because memory corruption may manifest unpredictably depending on heap layout and stack canaries, making it difficult for defenders to detect exploitation attempts in real-time without specialized monitoring tools.
Mitigation strategies must prioritize immediate code remediation alongside defensive architectural changes. Developers should update the lwIP source tree to include strict input validation routines that verify all SMTP response lengths against predefined buffer limits before copying data into memory structures. Implementing safe string handling functions such as strncpy or snprintf with explicit size constraints is essential to prevent overflow conditions. Additionally, enabling compiler-level protections like stack canaries and Address Sanitizer during development builds can help identify similar vulnerabilities early in the lifecycle. For deployed systems that cannot be immediately patched, network segmentation should be enforced to restrict SMTP traffic to trusted mail relays only, reducing the attack surface exposed to untrusted servers. Regular security audits focusing on protocol parsers are recommended to ensure compliance with secure coding standards and maintain resilience against evolving threat vectors targeting embedded communication stacks.