CVE-2026-44161: Fluentd Fluentd Is Vulnerable To Server-Side Vulnerability

HIGH
Advisory Identifier: GHSA-72f5-rr8c-r6gr
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CVSS 7.2
CWE-918
Published
Advisory Summary

CVE-2026-44161 is a HIGH (CVSS 7.2) fluentd is vulnerable to server-side vulnerability (CWE-918: Server-Side Request Forgery (SSRF)) affecting Fluentd libraries in the RubyGems ecosystem. The issue involves fluentd is vulnerable to server-side request forgery (ssrf) via placeholder expansion in `out_http`. Remediation is available by upgrading affected packages to version 1.19.3.

CVE-2026-44161 at a Glance

CVE Identifier CVE-2026-44161
GHSA Identifier GHSA-72f5-rr8c-r6gr
Severity HIGH
CVSS v3.1 Base Score 7.2
CWE Classification CWE-918: Server-Side Request Forgery (SSRF)
Affected Product Fluentd
Package Ecosystem RubyGems
Published Date
Last Updated

Security Impact

CVSS: 7.2

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:C/C:L/I:N/A:L).

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

How to Fix CVE-2026-44161

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-2026-44161, 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-918: Server-Side Request Forgery (SSRF), where the web server receives a url or similar request from an upstream component and retrieves the contents of this url, but it does not sufficiently ensure that the request is being sent to the expected destination. 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

The `out_http` output plugin allows the use of placeholders (such as `${tag}`) in the `endpoint` configuration parameter. It was discovered that if the placeholder value is derived from untrusted user input, an attacker can maliciously control the destination hostname of the outbound HTTP requests made by Fluentd. ### Impact This vulnerability allows for a **Server-Side Request Forgery (SSRF)** attack. An unauthenticated attacker can force the Fluentd node to send HTTP requests to arbitrary internal services. This can lead to unauthorized access to internal APIs, data exfiltration, or the compromise of cloud metadata endpoints (e.g., AWS IMDS `169.254.169.254`). ### Patches v1.19.3 ### Workarounds If an immediate upgrade is not possible, users are strongly advised to apply the following mitigations: 1. Avoid Dynamic Hostnames * Do not use the placeholder in the endpoint parameter as hostname. 2. Restrict Network Access * Use firewall rules (e.g., iptables, AWS Security Groups) to block the Fluentd node from accessing sensitive internal IP addresses, specifically the cloud provider's metadata service and other internal microservices that Fluentd does not explicitly need to access. 3. Restrict allowed hosts * Inject filter to accept allowed hosts in placeholders explicitly if possible.

CVE-2026-44161 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-72f5-rr8c-r6gr
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-2026-44161?
CVE-2026-44161 is a security vulnerability in Fluentd involving Fluentd is Vulnerable to Server-Side Request Forgery (SSRF) via Placeholder Expansion in `out_http` under CWE-918: Server-Side Request Forgery (SSRF).
Which Fluentd versions are affected?
Affected versions include releases prior to 1.19.3 across affected packages: fluentd.
What version fixes CVE-2026-44161?
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-2026-44161?
CVE-2026-44161 has a CVSS v3.1 base score of 7.2 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:L.
What CWE is associated with CVE-2026-44161?
CVE-2026-44161 is classified under CWE-918: Server-Side Request Forgery (SSRF) (The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.).
How can I check RubyGems dependencies for CVE-2026-44161?
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-72f5-rr8c-r6gr",
  "modified" : "2026-06-26T16:36:11Z",
  "published" : "2026-06-26T16:36:11Z",
  "aliases" : [ "CVE-2026-44161" ],
  "summary" : "Fluentd is Vulnerable to Server-Side Request Forgery (SSRF) via Placeholder Expansion in `out_http`",
  "details" : "The `out_http` output plugin allows the use of placeholders (such as `${tag}`) in the `endpoint` configuration parameter.\nIt was discovered that if the placeholder value is derived from untrusted user input, an attacker can maliciously control the destination hostname of the outbound HTTP requests made by Fluentd.\n\n### Impact\nThis vulnerability allows for a **Server-Side Request Forgery (SSRF)** attack.\nAn unauthenticated attacker can force the Fluentd node to send HTTP requests to arbitrary internal services. This can lead to unauthorized access to internal APIs, data exfiltration, or the compromise of cloud metadata endpoints (e.g., AWS IMDS `169.254.169.254`).\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. Avoid Dynamic Hostnames\n   * Do not use the placeholder in the endpoint parameter as hostname.\n\n2. Restrict Network Access \n   * Use firewall rules (e.g., iptables, AWS Security Groups) to block the Fluentd node from accessing sensitive internal IP addresses, specifically the cloud provider's metadata service and other internal microservices that Fluentd does not explicitly need to access.\n\n3. Restrict allowed hosts\n   * Inject filter to accept allowed hosts in placeholders explicitly if possible.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:L"
  } ],
  "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-72f5-rr8c-r6gr"
  }, {
    "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-918" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-26T16:36:11Z",
    "nvd_published_at" : null
  }
}