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CVE-2025-66566: yawkat LZ4 Java has a possible information leak in Java safe decompressor

Summary

Insufficient clearing of the output buffer in Java-based decompressor implementations in lz4-java 1.10.0 and earlier allows remote attackers to read previous buffer contents via crafted compressed input. In applications where the output buffer is reused without being cleared, this may lead to disclosure of sensitive data.

JNI-based implementations are not affected.

Details

During the decompression process, the lz4 algorithm may have to repeat data that was previously decompressed in the same input frame. In the Java implementation, this is implemented by copy operations within the output buffer.

With a crafted input, an attacker may induce the Java implementation to copy from a region in the output buffer that does not contain decompressed data yet. If that region contains sensitive information because the output buffer was not cleared prior to decompression, that data will then be copied to the decompressed output.

LZ4Factory.nativeInstance().safeDecompressor() is not affected.
LZ4Factory.nativeInstance().fastDecompressor() is affected because it actually uses safeInstance() since 1.8.1. In 1.8.0 and earlier versions, this implementation is instead vulnerable to the more severe CVE‐2025‐12183, so downgrading is not a solution.
Both decompressors of LZ4Factory.safeInstance(), LZ4Factory.unsafeInstance() and LZ4Factory.fastestJavaInstance() are affected.
LZ4Factory.fastestInstance() uses the nativeInstance or fastestJavaInstance depending on platform. LZ4Factory.fastestInstance().fastDecompressor() is always affected, while LZ4Factory.fastestInstance().safeDecompressor() is affected only when JNI cannot be used (e.g. on unsupported platforms).

Independent of this vulnerability, it is recommended that users migrate from fastDecompressor to safeDecompressor, as the latter is more performant (despite the name).

The impact of this vulnerability depends on how user code interacts with the decompression API. Users that allocate a new destination buffer each time, or use only zeroed buffers, are not impacted. When the buffer is reused, however, the confidentiality impact can be severe. This vulnerability is marked as VC:H out of caution.

Mitigation

lz4-java 1.10.1 fixes this issue without requiring changes in user code.

If you cannot upgrade to 1.10.1, you can mitigate this vulnerability by zeroing the output buffer before passing it to the decompression function.

Relation to CVE‐2025‐12183

This CVE is a different attack than CVE‐2025‐12183, affecting different implementations with different impact. This new vulnerability was discovered by CodeIntelligence during research that followed up on CVE‐2025‐12183. Users are recommended to upgrade to 1.10.1 to fix both vulnerabilities.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2025-66566

CVE-2025-66566:

N/A

CVSS Score

-

CVSS Score

-

Basic Information

Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
at.yawk.lz4:lz4-java	maven	<= 1.10.0	1.10.1
org.lz4:lz4-java	maven	<= 1.8.1
org.lz4:lz4-pure-java	maven	<= 1.8.1
net.jpountz.lz4:lz4	maven	<= 1.8.1

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in the core decompression logic of the pure Java LZ4 decompressors provided by the yawkat/lz4-java library. The issue stems from improper handling of a specific case in the LZ4 compression format where a 'match' operation has a distance of zero. In the vulnerable versions, this would cause the decompressor to copy bytes from a part of the output buffer that hadn't been written to yet during the current decompression operation.

If an application reuses its decompression buffers without clearing them between uses, this flaw could be exploited. An attacker could craft a special compressed data stream that, when decompressed, would cause the library to copy leftover data from the previous use of the buffer into the new decompressed output. This leftover data could be sensitive information, leading to an information leak.

The patch addresses this by modifying the shared decompression logic, which is defined in the src/build/source_templates/decompress.template file. This template is used to generate the decompress methods for the LZ4JavaSafeDecompressor and LZ4JavaUnsafeDecompressor classes. The fix introduces a check for matchDec == 0. When this condition is met, instead of performing a copy, it calls a new LZ4Utils.zero utility function to fill the relevant portion of the output buffer with zeros, effectively preventing the information leak. The analysis of the patch commit confirms that these are the key changes and the functions involved.

Vulnerable functions

net.jpountz.lz4.LZ4JavaSafeDecompressor.decompress

src/build/source_templates/decompress.template

The `decompress` method, generated from `decompress.template`, is vulnerable. When processing a crafted compressed input with a match distance of 0 (`matchDec == 0`), the decompressor would attempt to copy data from a location in the output buffer that has not yet been written to. If the buffer was reused without being cleared, this would result in copying stale, potentially sensitive, data into the decompressed output, leading to an information leak. The patch mitigates this by detecting this specific condition and zeroing out the relevant portion of the buffer instead of performing a copy.

net.jpountz.lz4.LZ4JavaUnsafeDecompressor.decompress

src/build/source_templates/decompress.template

The `decompress` method, generated from `decompress.template`, is vulnerable. When processing a crafted compressed input with a match distance of 0 (`matchDec == 0`), the decompressor would attempt to copy data from a location in the output buffer that has not yet been written to. If the buffer was reused without being cleared, this would result in copying stale, potentially sensitive, data into the decompressed output, leading to an information leak. The patch mitigates this by detecting this specific condition and zeroing out the relevant portion of the buffer instead of performing a copy. This class is also affected as it uses the same vulnerable template.

net.jpountz.lz4.LZ4Utils.zero

src/java/net/jpountz/lz4/LZ4Utils.java

This function was added as part of the fix to mitigate the vulnerability. It is called by the patched `decompress` method to clear the buffer when a potentially malicious match offset is detected. While not vulnerable itself, its presence in a stack trace would be a strong indicator that the patched code path is being executed.

WAF Protection Rules

WAF Rule

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Reasoning

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N/A

-

Basic Information

Basic Information

Concerned about an active attack path?

CVE-2025-66566: yawkat LZ4 Java has a possible information leak in Java safe decompressor

Summary

Details

Mitigation

Relation to CVE‐2025‐12183

CVE-2025-66566:

N/A

-

Basic Information

Basic Information

Technical Details

Technical Details

Vulnerability IntelligenceMiggo AI

Root Cause Analysis

Vulnerable functions

WAF Protection Rules

WAF Rule

Reasoning

Vulnerability IntelligenceMiggo AI

N/A

N/A

Vulnerability IntelligenceMiggo AI

Root Cause Analysis

Vulnerable functions

WAF Protection Rules

WAF Rule

Reasoning

Basic Information

Basic Information

Technical Details

CVE-2025-66566: yawkat LZ4 Java has a possible information leak in Java safe decompressor

Summary

Details

Mitigation

Relation to CVE‐2025‐12183

Vulnerability Intelligence
Miggo AI

Vulnerability Intelligence
Miggo AI

Vulnerability Intelligence
Miggo AI