CVE-2025-66566

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.

JDK Mission Control is a powerful profiler for HotSpot JVMs and has an advanced set of tools that enables efficient and detailed analysis of the extensive data collected by JDK Flight Recorder. The tool chain enables developers and administrators to collect and analyze data from Java applications running locally or deployed in production environments.

Security Fix(es):

• lz4-java: lz4-java: Information Disclosure via Insufficient Output Buffer Clearing (CVE-2025-66566)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

JDK Mission Control is a powerful profiler for HotSpot JVMs and has an advanced set of tools that enables efficient and detailed analysis of the extensive data collected by JDK Flight Recorder. The tool chain enables developers and administrators to collect and analyze data from Java applications running locally or deployed in production environments.

Security Fix(es):

• lz4-java: lz4-java: Information Disclosure via Insufficient Output Buffer Clearing (CVE-2025-66566)

• org.eclipse.jetty/jetty-http: HTTP request smuggling via chunked extension quoted-string parsing (CVE-2026-2332)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

yawkat LZ4 Java provides LZ4 compression for Java. 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. This vulnerability is fixed in 1.10.1.

JDK Mission Control is a powerful profiler for HotSpot JVMs and has an advanced set of tools that enables efficient and detailed analysis of the extensive data collected by JDK Flight Recorder. The tool chain enables developers and administrators to collect and analyze data from Java applications running locally or deployed in production environments.

Security Fix(es):

• lz4-java: lz4-java: Information Disclosure via Insufficient Output Buffer Clearing (CVE-2025-66566)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

JDK Mission Control is a powerful profiler for HotSpot JVMs and has an advanced set of tools that enables efficient and detailed analysis of the extensive data collected by JDK Flight Recorder. The tool chain enables developers and administrators to collect and analyze data from Java applications running locally or deployed in production environments.

Security Fix(es):

• lz4-java: lz4-java: Information Disclosure via Insufficient Output Buffer Clearing (CVE-2025-66566)
• org.eclipse.jetty/jetty-http: HTTP request smuggling via chunked extension quoted-string parsing (CVE-2026-2332)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

TuxCare rebuilt 2 packages to address 1 CVE. Fixed in 1.8.1-tuxcare.1.

TuxCare rebuilt 2 org.lz4 packages to address 2 CVEs. Fixed in 1.8.0-tuxcare.1.

Root has patched CVE-2025-66566 in the io.root.org.lz4:lz4-java package for Root:Maven. Multiple fixed versions available.