Executive Summary

CVE-2026-38426 is a critical stack buffer overflow vulnerability in the Tasmota firmware versions 15.3.0.3 and earlier. It occurs in the fetch_jpg() function within the xdrv_10_scripter.ino file, where a fixed 40-byte buffer is used to store an MJPEG boundary string from an HTTP Content-Type header. The vulnerability arises because the boundary string is copied into this buffer using strcpy() without checking its length.

An attacker can exploit this by sending a boundary string longer than 39 characters from a malicious HTTP server, causing the buffer to overflow and corrupt adjacent heap memory, including important pointers. This corruption can lead to remote code execution on the ESP32 device when certain virtual methods are called.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability allows a remote attacker to execute arbitrary code on affected devices without any authentication. This means an attacker can gain unauthorized control over the device, potentially leading to data theft, manipulation of device functions, or causing denial-of-service conditions.

The impact affects confidentiality, integrity, and availability of the device and its connected systems, making it a high-risk security issue.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability can be detected by monitoring network traffic for suspicious HTTP Content-Type headers containing unusually long MJPEG boundary strings exceeding 39 characters, which may trigger the buffer overflow.

Additionally, running the available proof-of-concept exploit script from Resource 1 can help verify if a device is vulnerable by demonstrating a crash or information leakage.

Since the vulnerability is in the fetch_jpg() function of the xdrv_10_scripter.ino file, checking the firmware version of Tasmota devices is crucial; versions 15.3.0.3 and earlier are vulnerable.

• Use network packet capture tools (e.g., Wireshark) to filter HTTP traffic and inspect Content-Type headers for boundary strings longer than 39 characters.
• Check the Tasmota firmware version on your devices with commands or interfaces provided by Tasmota to ensure they are updated beyond version 15.3.0.3.
• Run the proof-of-concept exploit script from https://github.com/sermikr0/CVE-2026-38426 to test device susceptibility.

Useful 0 Not Useful 0

Mitigation Strategies

The immediate mitigation step is to upgrade all affected Tasmota devices to version 15.3.0.4 or later, where the vulnerability has been patched.

If upgrading immediately is not possible, restrict network access to Tasmota devices to trusted sources only, minimizing exposure to potentially malicious HTTP servers.

Disable or limit the use of the script feature (xdrv_10_scripter) if it is not required, as this component is directly involved in the vulnerability.

• Update Tasmota firmware to version 15.3.0.4 or later.
• Implement network segmentation or firewall rules to limit access to Tasmota devices.
• Disable the script interpreter feature if not needed.

Useful 0 Not Useful 0

Compliance Impact

The vulnerability allows remote attackers to execute arbitrary code on affected devices without authentication, potentially compromising confidentiality, integrity, and availability of data and device operations.

Such unauthorized access and control could lead to data breaches or manipulation, which may violate requirements under common standards and regulations like GDPR and HIPAA that mandate protection of personal and sensitive information.

Therefore, organizations using vulnerable versions of Tasmota firmware could face compliance risks if this vulnerability is exploited, especially if the devices handle regulated data or are part of critical infrastructure.

Useful 0 Not Useful 0