# CVE-2026-47321: Apache MINA: Unbounded Decompression Amplification DoS in Zlib.inflate

> Attackers can cause memory exhaustion and denial of service by sending highly compressed data to applications using Apache MINA's CompressionFilter.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 Sep 2026
- **Updated:** 21 Sep 2026
- **Also known as:** DEBIAN-CVE-2026-47321, GHSA-3c8x-fp36-75xv, RHLW-2026-2cb9a31c02a7f38b, UBUNTU-CVE-2026-47321

## What is CVE-2026-47321?

CVE-2026-47321 is a high-severity denial of service vulnerability in Apache MINA, published on 21 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The CompressionFilter class uses ZLib to deflate and inflate data sent and received. When we inflate incoming data, the filter does not control the resulting size, and create a buffer no matter what.

Some compressed data may have a compression ration greater than 1 thousand, leading to an exhaustion of the application memory, as we don't control the deflated size.

The fix adds such a control by allowing the application developer to provide a fixed size limit, which when reached throws an exception. It also allows the user to provide a compression ratio that should not be exceeded, protected the application from small inflated files that inflate in gigantic files, but with a grace limit for the resulting size (1Mb) to avoid false positive (like a very small file inflating with a high ratio, but resulting with a acceptable size, like a few thousands bytes)

For application using this feature, it is highly recommended to create the CompressionFilter and to pass the maximum limit as a forth constructor parameter, maxDecompressedSize:

public CompressionFilter(final boolean compressInbound, final boolean compressOutbound, final int compressionLevel, final int maxDecompressedSize)Optionally one can also provide a maxDecompressRatio fifth parameter, and a decompressRatioMinSize sixth parameter to allow small inflated files with a high compression ratio to still be accepted.

Here are the additional constructor:

public CompressionFilter(final boolean compressInbound, final boolean compressOutbound,

            final int compressionLevel, final int maxDecompressedSize,

            final long maxDecompressRatio, final long decompressRatioMinSize)

Also note that a fluent API has been added to spare the users the pain to call a constructor with that many parameters:

 CompressionFilter compressionFilter = new CompressionFilter()

                                                .setCompressionLevel(Zlib.COMPRESSION_MAX)

                                                .setMaxDecompressedSize(1_000_000)

                                                .setMaxDecompressRatio(100).

                                                .setDecompressRatioMinSize(100_000); 

Applications using Apache MINA are advised to upgrade and configure their CompressionFilter instance.

Weakness: CWE-789 (Memory Allocation with Excessive Size Value), CWE-409 (Improper Handling of Highly Compressed Data (Data Amplification))

## Which versions are affected by CVE-2026-47321?


## How do I fix CVE-2026-47321?

Apply the vendor's patch.

## Is CVE-2026-47321 being exploited?

There is no known exploitation of CVE-2026-47321 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-47321 and fix it if it is affected.

What it is: CVE-2026-47321 is a high-severity denial of service vulnerability in Apache MINA, published on 21 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-47321 in the wild and no public exploit code.

Affected product: Apache MINA. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-47321/ (Markdown: https://insights.sbomify.com/CVE-2026-47321/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-47321
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-47321
- OSV: https://osv.dev/vulnerability/CVE-2026-47321

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-47321/
Summaries are generated and may contain mistakes; the official description above is the record.