XZ Utils is a widely-used, general-purpose data compression library and set of command-line tools. However, a newly discovered vulnerability in XZ Utils (version 5.3.3alpha to 5.8.) permits faulty input data to result in severe crashes, compromising the system's resources. This long-read post will delve into the technical details regarding this vulnerability identified as CVE-2025-31115, provide snippets of code, link to original references, and discuss potential exploit possibilities.

Vulnerability Details

The CVE-2025-31115 vulnerability is centered around the multithreaded .xz decoder in liblzma, a crucial component in the XZ Utils library. Specifically, the consequences of the vulnerability include heap use after freeing and writing to an address derived from the null pointer in addition to an offset. Applications and libraries using the lzma_stream_decoder_mt function are significantly affected by this vulnerability.

To mitigate this issue, the bug has been fixed in the latest version of XZ Utils, 5.8.1. Additionally, standalone patches are available for application to all affected releases. The fix has been committed to the v5.4, v5.6, v5.8, and master branches in the XZ Utils Git repository(https://git.tukaani.org/?p=xz.git).

Code Snippet

The vulnerability emerges from an issue concerning the lzma-stream_decoder_mt function, evident in the following code snippet:

lzma_ret
lzma_stream_decoder_mt(lzma_stream *strm, uint64_t memlimit,
		const lzma_mt *options)
{
	// Validate options and set default values
	lzma_mt mt_options;
	if (parse_options(options, &mt_options) != LZMA_OK)
		return LZMA_OPTIONS_ERROR;
...

This code demonstrates how the lzma_stream_decoder_mt function is employed in the XZ Utils library. Under normal circumstances, it enables applications to leverage multithreading for enhanced performance. However, the CVE-2025-31115 vulnerability compromises the function's integrity, leading to crashes when processing malicious input data.

Exploit Possibilities

The CVE-2025-31115 vulnerability may be exploited by a malicious attacker who sends a carefully-crafted, erroneous .xz file. The corrupted file is designed to trigger the documented vulnerability, causing the application or library using lzma_stream_decoder_mt function to crash. In certain cases, the attacker could exploit this vulnerability to execute further attack steps, including memory corruption or outright code injection. Consequently, this poses a significant security risk to affected systems.

Mitigation

It is highly recommended that users and developers using the XZ Utils library update to the latest version, 5.8.1, as it addresses and corrects the CVE-2025-31115 vulnerability. Additionally, users should apply the standalone patch available for affected past versions. These patches can be found at the XZ Utils Git repository (https://git.tukaani.org/?p=xz.git) in the v5.4, v5.6, v5.8, and master branches.

Conclusion

The CVE-2025-31115 vulnerability in the XZ Utils library poses a significant security risk due to its ability to exploit applications and libraries using the lzma_stream_decoder_mt function. Through upgrading to the latest version 5.8.1 or applying the critical patch, users can protect their systems and maintain the integrity of their applications from potential attacks. Stay informed, stay vigilant, and ensure your applications are up-to-date to safeguard against such vulnerabilities.

Timeline

Published on: 04/03/2025 17:15:30 UTC
Last modified on: 04/07/2025 14:18:34 UTC