Summary

When no system-installed liblz4-java is found, net.jpountz.util.Native.load() extracts the bundled native library to java.io.tmpdir and loads it with System.load. The path of the extracted library can be predicted before it is created, and the library file is opened without exclusive creation. Another local user who can write to the same temporary directory can therefore create the file first and control the code that is loaded.

Details

java tempLibLock = File.createTempFile("liblz4-java-", "." + os().libExtension + ".lck"); tempLib = new File(tempLibLock.getAbsolutePath().replaceFirst(".lck$", "")); // copy to tempLib try (FileOutputStream out = new FileOutputStream(tempLib)) { ... } ... System.load(tempLib.getAbsolutePath());

Only the .lck file is created safely, with a random name and exclusive creation. The library path is the .lck name without the .lck suffix, and nothing reserves it. new FileOutputStream(tempLib) opens it with OCREAT|OTRUNC and follows symlinks.

In a shared, world-writable temporary directory, another user can watch for the .lck file and create the corresponding library file before it is opened. If they win that race, they own the file. The victim then writes into the attacker-owned file, and the attacker can overwrite it again before System.load.

Whether this can be exploited depends on the host:

- With fs.protectedregular = 0, the attacker can pre-create a world-writable file, and their code is then loaded into the victim's JVM. - With fs.protectedregular >= 1 (the default on many systemd-based distributions), the victim's open fails. The library then fails to load and the library falls back to the Java implementations, so the attacker can only cause a denial of service. - A symlink variant is blocked by fs.protectedsymlinks = 1. - Temporary directories without the sticky bit, or shared across users in containers, are exploitable whatever these settings are.

This behaviour was introduced upstream in commit c3ddae5 (lz4-java 1.7.0), which swapped which of the two files is created with createTempFile.

Impact

A local user who can write to the same temporary directory as a victim process that uses the bundled JNI library may be able to execute code as the victim. Exploitation requires a shared temporary directory, host settings that permit it, and winning a race.

The issue does not apply if liblz4-java is found on java.library.path, if java.io.tmpdir is private to the user (for example systemd PrivateTmp, or a per-user temporary directory), or if only the Java implementations are used.

Patch

Fixed in lz4-java 1.11.4. The native library is now extracted directly into a file created by File.createTempFile, which has a random name and is created exclusively, so a file or symlink planted in advance cannot be reused. The .lck lock files are no longer used. Instead, the startup cleanup removes stale liblz4-java- temporary files (including .lck files left by older versions) only once they are more than one hour old.

For older versions, the workaround is to set java.io.tmpdir to a directory writable only by the application user, or install liblz4-java on java.library.path.

https://github.com/yawkat/lz4-java/security/advisories/GHSA-cmp6-m4wj-q63q discloses: Java-based decompressor implementations can leak information from uninitialized output buffer

yawkat published GHSA-cmp6-m4wj-q63q Dec 5, 2025

Package Affected versions Patched versions ------- ----------------- ---------------- at.yawk.lz4:lz4-java (Maven) <= 1.10.0 1.10.1 net.jpountz.lz4:lz4 (Maven) <= 1.8.1 None org.lz4:lz4-java (Maven) <= 1.8.1 None org.lz4:lz4-pure-java (Maven) <= 1.8.1 None

Description ===========

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.

Severity: High 8.2 / 10 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N CVE ID: CVE-2025-66566 Weaknesses: Weakness CWE-201 Credits: @simonresch (Reporter) -- -Alan Coopersmith- alan.coopersmith () oracle com Oracle Solaris Engineering - https://blogs.oracle.com/solaris