Divide-by-Zero in lwext4 1.0.0 Library

Executive Summary

CVE-2025-70100 is a divide-by-zero vulnerability in the lwext4 1.0.0 library, specifically in the ext4_block_set_lb_size function located in src/ext4_blockdev.c. This vulnerability occurs when the function processes a malformed ext4 filesystem image that has a logical block size (lb_size) set to zero. Because the function does not validate this input, it performs a division by zero, leading to a Floating-Point Exception (FPE) or a runtime crash.

The flaw is triggered during the mounting or image processing of the ext4 filesystem, causing the program to crash due to the unhandled division-by-zero error.

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Impact Analysis

This vulnerability can cause a denial of service (DoS) by crashing applications or systems that use the vulnerable lwext4 library when they attempt to mount or process a specially crafted ext4 filesystem image with a zero logical block size.

An attacker can exploit this by providing a malformed ext4 filesystem image, which leads to a runtime crash or Floating-Point Exception, potentially disrupting normal operations or services relying on this library.

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Detection Guidance

This vulnerability can be detected by attempting to mount or process a specially crafted ext4 filesystem image that contains a zero logical block size, which triggers a division-by-zero error in the lwext4 library.

One practical detection method is to run the lwext4 fuzzer with the malicious image named sig8_2_lwext4_ext4_blockdev_c_127, which causes an immediate crash due to the division-by-zero error.

For example, on a Kali Linux system with Clang 19.1.7 and lwext4 version 1.0.0, you can reproduce the issue by running the fuzzer against the crafted image to observe the Floating-Point Exception or crash.

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Compliance Impact

The provided information does not specify any impact of the CVE-2025-70100 vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Mitigation Strategies

To mitigate the CVE-2025-70100 vulnerability, immediate steps include avoiding the mounting or processing of untrusted or malformed ext4 filesystem images that could trigger the divide-by-zero error.

Additionally, applying patches or updates to the lwext4 library that add proper validation for the logical block size (lb_size) to prevent zero values before arithmetic operations is recommended once available.

Until a fix is applied, running the library or applications using it in a controlled environment and avoiding exposure to potentially malicious ext4 images can reduce the risk of denial of service.

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