Summary
yawkat LZ4 Java has a possible information leak in Java safe decompressor
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 usessafeInstance()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()andLZ4Factory.fastestJavaInstance()are affected. LZ4Factory.fastestInstance()uses thenativeInstanceorfastestJavaInstancedepending on platform.LZ4Factory.fastestInstance().fastDecompressor()is always affected, whileLZ4Factory.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.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2025-66566? CVE-2025-66566 is a high-severity security vulnerability in at.yawk.lz4:lz4-java (maven), affecting versions <= 1.10.0. It is fixed in 1.10.1.
- Which packages are affected by CVE-2025-66566?
at.yawk.lz4:lz4-java(maven) (versions <= 1.10.0)org.lz4:lz4-java(maven) (versions <= 1.8.1)org.lz4:lz4-pure-java(maven) (versions <= 1.8.1)net.jpountz.lz4:lz4(maven) (versions <= 1.8.1)
- Is there a fix for CVE-2025-66566? Yes. CVE-2025-66566 is fixed in 1.10.1. Upgrade to this version or later.
- Is CVE-2025-66566 exploitable, and should I be worried? Whether CVE-2025-66566 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-66566 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-66566? Upgrade
at.yawk.lz4:lz4-javato 1.10.1 or later.