CVE-2026-42587

Netty: HttpContentDecompressor maxAllocation bypass when Content-Encoding set to br/zstd/snappy leads to decompression bomb DoS

• Denial of Service: An attacker can cause out-of-memory conditions on any Netty server that relies on maxAllocation for decompression bomb protection, by simply using a non-gzip content encoding.
• False sense of security: Developers who explicitly configure maxAllocation to protect against decompression bombs are not actually protected for brotli, zstd, or snappy encodings. The API documentation implies all encodings are covered.
• Trivial bypass: The attacker only needs to change one HTTP header (Content-Encoding: br instead of Content-Encoding: gzip) to circumvent the protection entirely.
• Both HTTP/1.1 and HTTP/2: The vulnerability exists in both HttpContentDecompressor (HTTP/1.1) and DelegatingDecompressorFrameListener (HTTP/2).

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-42587 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.2.13.Final, 4.1.133.Final); upgrading removes the vulnerable code path.

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Pass maxAllocation to all decoder constructors. For BrotliDecoder, which currently has no maxAllocation support, add the parameter:

HttpContentDecompressor.java, pass maxAllocation to all decoders:

// Line 120: BrotliDecoder, add maxAllocation support
.handlers(new BrotliDecoder(maxAllocation))

// Line 129: SnappyFrameDecoder, add maxAllocation support
.handlers(new SnappyFrameDecoder(maxAllocation))

// Line 138: ZstdDecoder, forward the configured maxAllocation
.handlers(new ZstdDecoder(maxAllocation))

DelegatingDecompressorFrameListener.java, same fix at lines 188-210.

BrotliDecoder, add maxAllocation parameter with the same semantics as ZlibDecoder.prepareDecompressBuffer(): set buffer maxCapacity and throw DecompressionException when the total decompressed output exceeds the limit.

SnappyFrameDecoder, add maxAllocation parameter with equivalent enforcement.

ZstdDecoder, ensure that when maxAllocation is set, total output across all buffers is bounded (not just per-buffer allocation size).

1. What is CVE-2026-42587? CVE-2026-42587 is a high-severity uncontrolled resource consumption vulnerability in io.netty:netty-codec-http (maven), affecting versions >= 4.2.0.Alpha1, <= 4.2.12.Final. It is fixed in 4.2.13.Final, 4.1.133.Final. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-42587? CVE-2026-42587 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-42587?
   • io.netty:netty-codec-http (maven) (versions >= 4.2.0.Alpha1, <= 4.2.12.Final)
   • io.netty:netty-codec-http2 (maven) (versions >= 4.2.0.Alpha1, <= 4.2.12.Final)
4. Is there a fix for CVE-2026-42587? Yes. CVE-2026-42587 is fixed in 4.2.13.Final, 4.1.133.Final. Upgrade to this version or later.
5. Is CVE-2026-42587 exploitable, and should I be worried? Whether CVE-2026-42587 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
6. What actually determines whether CVE-2026-42587 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.
7. How do I fix CVE-2026-42587?
   • Upgrade io.netty:netty-codec-http to 4.2.13.Final or later
   • Upgrade io.netty:netty-codec-http2 to 4.2.13.Final or later
   • Upgrade io.netty:netty-codec-http to 4.1.133.Final or later
   • Upgrade io.netty:netty-codec-http2 to 4.1.133.Final or later

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