Heap Buffer Overflow in Snap7 v1.4.3

Executive Summary

This vulnerability is a heap buffer overflow in the TS7Worker::PerformFunctionWrite() function of the snap7 software version 1.4.3. It occurs when the function processes a specially crafted packet, which can cause the program to behave unexpectedly or crash.

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Detection Guidance

This vulnerability can be detected by monitoring for crashes or Denial of Service (DoS) conditions on Snap7 servers running version 1.4.3 when they receive malformed S7 WriteVar requests.

A minimal server setup binding to 127.0.0.1 on a chosen port with a registered data block (DB1) can be used to reproduce the crash, indicating the presence of the vulnerability.

Using debugging tools such as AddressSanitizer on a debug build of the Snap7 server can help detect heap buffer overflows triggered by malformed packets.

While no specific detection commands are provided, you can monitor network traffic for suspicious or malformed S7 WriteVar requests sent to the Snap7 server's TCP port.

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Impact Analysis

An attacker exploiting this vulnerability can cause a Denial of Service (DoS) condition, meaning the affected snap7 service could crash or become unavailable, disrupting normal operations.

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Compliance Impact

The vulnerability in Snap7 v1.4.3 is a heap buffer overflow that allows an attacker to cause a Denial of Service (DoS) via a crafted packet. It does not impact confidentiality or integrity of data, as indicated by the CVSS vector (C:N/I:N/A:H).

Since the vulnerability only affects availability and does not lead to unauthorized access or data leakage, it has limited direct impact on compliance with data protection regulations such as GDPR or HIPAA, which primarily focus on confidentiality and integrity of personal or sensitive data.

However, availability is also a component of many security frameworks and standards, so organizations relying on Snap7 might need to consider the risk of service disruption in their compliance and risk management processes.

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Mitigation Strategies

Immediate mitigation steps include restricting network access to the Snap7 server to trusted hosts only, especially limiting access to the TCP port where Snap7 listens.

Avoid exposing the Snap7 server to untrusted networks to prevent attackers from sending crafted WriteVar requests.

Monitor the Snap7 server for crashes or abnormal behavior and restart the service if necessary.

Consider compiling and running a debug build with AddressSanitizer to detect exploitation attempts during testing.

Check for updates or patches from the Snap7 project repository or maintainers that address this vulnerability.

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