CVE-2026-44160: Fluentd Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-j9cw-hwqf-85w7
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CVSS 7.5
CWE-409
Published
Advisory Summary

CVE-2026-44160 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)) affecting Fluentd libraries in the RubyGems ecosystem. The issue involves fluentd is vulnerable to denial of service (dos) via gzip decompression bomb in `in_http` and `in_forward`. Remediation is available by upgrading affected packages to version 1.19.3.

CVE-2026-44160 at a Glance

CVE Identifier CVE-2026-44160
GHSA Identifier GHSA-j9cw-hwqf-85w7
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)
Affected Product Fluentd
Package Ecosystem RubyGems
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-44160

To remediate this vulnerability, upgrade all affected Fluentd dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

fluentd fixed: 1.19.3 
Upgrade fluentd to 1.19.3
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for fluentd

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-44160, inspect your project dependencies for Fluentd:

inspect dependencies for fluentd

Analyze the terminal output to locate any version of Fluentd in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.19.3), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification), where the product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. In Fluentd, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

Fluentd's `in_http` and `in_forward` plugins support receiving gzip-compressed data. While Fluentd correctly enforces size limits on the incoming compressed payloads (e.g., via `body_size_limit` or `chunk_size_limit`), it was discovered that there is no limit enforced on the size of the decompressed data. If a Fluentd instance is exposed to untrusted networks, an attacker can send a maliciously crafted, highly compressed payload. When Fluentd attempts to decompress this payload in memory, it will expand to an excessive size, completely bypassing the intended payload size limits. ### Impact This vulnerability allows for a **Denial of Service (DoS)** attack via memory exhaustion. The rapid memory consumption during decompression can easily lead to an Out-of-Memory kill of the Fluentd process by the operating system. This results in the disruption of all log collection and forwarding capabilities on the affected node. ### Patches v1.19.3 ### Workarounds If an immediate upgrade is not possible, users are strongly advised to apply the following mitigations: 1. Restrict Network Access * Ensure that Fluentd input ports (such as `9880` for `in_http` and `24224` for `in_forward`) are deployed within a closed, trusted network. Use firewall rules (e.g., iptables, AWS Security Groups) to block access from untrusted networks or instances. 2. Use a Reverse Proxy * If developers must expose HTTP ingestion to external sources, place a robust reverse proxy (such as Nginx) in front of Fluentd. Configure the proxy to handle the gzip decompression and enforce strict limits on both compressed and uncompressed body sizes before passing the traffic to Fluentd.

CVE-2026-44160 Affected Fluentd Versions

1 component(s)
Fixed Versions:
1.19.3
Product / Package Ecosystem Affected Versions Fixed Version
fluentd
gem/fluentd
RubyGems
< 1.19.3
1.19.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fluent/fluentd/security/advisories/GHSA-j9cw-hwqf-85w7
Fluentd Source Repository REPOSITORY
https://github.com/fluent/fluentd
Fluentd Release RELEASE
https://github.com/fluent/fluentd/releases/tag/v1.19.3

Frequently Asked Questions

What is CVE-2026-44160?
CVE-2026-44160 is a security vulnerability in Fluentd involving Fluentd is Vulnerable to Denial of Service (DoS) via Gzip Decompression Bomb in `in_http` and `in_forward` under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification).
Which Fluentd versions are affected?
Affected versions include releases prior to 1.19.3 across affected packages: fluentd.
What version fixes CVE-2026-44160?
Upgrading to 1.19.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is fluentd affected?
Yes, fluentd is affected in releases prior to 1.19.3. Upgrade your dependency to 1.19.3 or later.
What is the CVSS score of CVE-2026-44160?
CVE-2026-44160 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-44160?
CVE-2026-44160 is classified under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification) (The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.).
How can I check RubyGems dependencies for CVE-2026-44160?
Run the dependency tree command: 'inspect dependencies for fluentd'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-j9cw-hwqf-85w7",
  "modified" : "2026-06-26T16:35:38Z",
  "published" : "2026-06-26T16:35:38Z",
  "aliases" : [ "CVE-2026-44160" ],
  "summary" : "Fluentd is Vulnerable to Denial of Service (DoS) via Gzip Decompression Bomb in `in_http` and `in_forward`",
  "details" : "Fluentd's `in_http` and `in_forward` plugins support receiving gzip-compressed data.\nWhile Fluentd correctly enforces size limits on the incoming compressed payloads (e.g., via `body_size_limit` or `chunk_size_limit`), it was discovered that there is no limit enforced on the size of the decompressed data.\n\nIf a Fluentd instance is exposed to untrusted networks, an attacker can send a maliciously crafted, highly compressed payload. \nWhen Fluentd attempts to decompress this payload in memory, it will expand to an excessive size, completely bypassing the intended payload size limits.\n\n### Impact\nThis vulnerability allows for a **Denial of Service (DoS)** attack via memory exhaustion. \nThe rapid memory consumption during decompression can easily lead to an Out-of-Memory kill of the Fluentd process by the operating system.\nThis results in the disruption of all log collection and forwarding capabilities on the affected node.\n\n### Patches\nv1.19.3\n\n### Workarounds\nIf an immediate upgrade is not possible, users are strongly advised to apply the following mitigations:\n\n1. Restrict Network Access\n   * Ensure that Fluentd input ports (such as `9880` for `in_http` and `24224` for `in_forward`) are deployed within a closed, trusted network. Use firewall rules (e.g., iptables, AWS Security Groups) to block access from untrusted networks or instances.\n2. Use a Reverse Proxy\n   * If developers must expose HTTP ingestion to external sources, place a robust reverse proxy (such as Nginx) in front of Fluentd. Configure the proxy to handle the gzip decompression and enforce strict limits on both compressed and uncompressed body sizes before passing the traffic to Fluentd.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "fluentd"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.19.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.19.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fluent/fluentd/security/advisories/GHSA-j9cw-hwqf-85w7"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fluent/fluentd"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fluent/fluentd/releases/tag/v1.19.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-409" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-26T16:35:38Z",
    "nvd_published_at" : null
  }
}