Summary

LZ4BlockInputStream grows its compressed-input buffer to the attacker-controlled compressedLen value from the legacy LZ4Block stream header before reading any payload bytes. A header-only input can therefore trigger a near-2 GiB allocation and exhaust the JVM heap.

Details

In net.jpountz.lz4.LZ4BlockInputStream, refill() validates that compressedLen is nonnegative but does not cap it before allocation:

java case COMPRESSIONMETHODLZ4: if (compressedBuffer.length < compressedLen) { compressedBuffer = new byte[Math.max(compressedLen, compressedBuffer.length 3 / 2)]; } readFully(compressedBuffer, compressedLen);

The paired LZ4BlockOutputStream never emits such a block. If compression is not smaller than the original block, it writes the block as RAW:

java if (compressedLength >= o) { compressMethod = COMPRESSIONMETHODRAW; compressedLength = o; } else { compressMethod = COMPRESSIONMETHODLZ4; }

Existing readers generally accept noncanonical LZ4-method blocks where compressedLen >= originalLen, but no canonical writer found produces them.

Impact

Applications that pass attacker-controlled legacy LZ4Block streams to LZ4BlockInputStream can suffer heap exhaustion from a header-only input. The impact is availability-only and requires no valid compressed payload.

Patch

As of lz4-java 1.11.2, lz4-java rejects lz4 blocks where the compressed length is larger than uncompressed. Readers would generally emit these as raw blocks instead. You can use the new acceptOversizedBlocks flag to restore the old behavior, but this reintroduces the DoS vector.

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