CVE-2026-40869: Decidim core Decidim Amendments Can Be Accepted Vulnerability

HIGH
Advisory Identifier: GHSA-w5xj-99cg-rccm
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CVSS 7.5
CWE-266
Published
Advisory Summary

CVE-2026-40869 is a HIGH (CVSS 7.5) decidim amendments can be accepted vulnerability (CWE-266: Incorrect Privilege Assignment) affecting Decidim core libraries in the RubyGems ecosystem. The issue involves decidim amendments can be accepted or rejected by anyone. Remediation is available by upgrading affected packages to version 0.31.1, 0.30.5.

CVE-2026-40869 at a Glance

CVE Identifier CVE-2026-40869
GHSA Identifier GHSA-w5xj-99cg-rccm
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-266: Incorrect Privilege Assignment
Affected Product Decidim core
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:H/A:N).

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 High (I:H)
Availability None (A:N)

How to Fix CVE-2026-40869

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

decidim-core fixed: 0.31.1 
Upgrade decidim-core to 0.31.1
decidim-core fixed: 0.30.5 
Upgrade decidim-core to 0.30.5
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 decidim-core

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-40869, inspect your project dependencies for Decidim core:

inspect dependencies for decidim-core

Analyze the terminal output to locate any version of Decidim core in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.31.1), 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-266: Incorrect Privilege Assignment, where a product incorrectly assigns a privilege to a particular actor, creating an unintended sphere of control for that actor. In Decidim core, 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

### Impact The vulnerability allows any registered and authenticated user to accept or reject any amendments. The impact is on any users who have created proposals where the amendments feature is enabled. This also elevates the user accepting the amendment as the author of the original proposal as people amending proposals are provided coauthorship on the coauthorable resources. The only check done when accepting or rejecting amendments is whether the amendment reactions are enabled for the component: https://github.com/decidim/decidim/blob/9d6c3d2efe5a83bb02e095824ff5998d96a75eb7/decidim-core/app/permissions/decidim/permissions.rb#L107 The permission checks have been changed at 1b99136 which was introduced in released version 0.19.0. I have not investigated whether prior versions are also affected. ### Patches Not available ### Workarounds Disable amendment reactions for the amendable component (e.g. proposals).

CVE-2026-40869 Affected Decidim core Versions

2 component(s)
Fixed Versions:
0.31.10.30.5
Product / Package Ecosystem Affected Versions Fixed Version
decidim-core
gem/decidim-core
RubyGems
≥ 0.31.0.rc1
< 0.31.1
0.31.1
decidim-core
gem/decidim-core
RubyGems
≥ 0.19.0
< 0.30.5
0.30.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/decidim/decidim/security/advisories/GHSA-w5xj-99cg-rccm
NVD — CVE-2026-40869 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-40869
Security Fix Commit FIX COMMIT
https://github.com/decidim/decidim/commit/1b99136a1c7aa02616a0b54a6ab88d12907a57a9
Decidim core Source Repository REPOSITORY
https://github.com/decidim/decidim
Decidim core Source Repository REPOSITORY
https://github.com/rubysec/ruby-advisory-db/blob/master/gems/decidim-core/CVE-2026-40869.yml

Frequently Asked Questions

What is CVE-2026-40869?
CVE-2026-40869 is a security vulnerability in Decidim core involving Decidim amendments can be accepted or rejected by anyone under CWE-266: Incorrect Privilege Assignment.
Which Decidim core versions are affected?
Affected versions include releases prior to 0.31.1, 0.30.5 across affected packages: decidim-core.
What version fixes CVE-2026-40869?
Upgrading to 0.31.1, 0.30.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is decidim-core affected?
Yes, decidim-core is affected in releases prior to 0.31.1, 0.30.5. Upgrade your dependency to 0.31.1, 0.30.5 or later.
What is the CVSS score of CVE-2026-40869?
CVE-2026-40869 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:H/A:N.
What CWE is associated with CVE-2026-40869?
CVE-2026-40869 is classified under CWE-266: Incorrect Privilege Assignment (A product incorrectly assigns a privilege to a particular actor, creating an unintended sphere of control for that actor.).
How can I check RubyGems dependencies for CVE-2026-40869?
Run the dependency tree command: 'inspect dependencies for decidim-core'. 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-w5xj-99cg-rccm",
  "modified" : "2026-05-29T21:46:09Z",
  "published" : "2026-04-14T22:22:58Z",
  "aliases" : [ "CVE-2026-40869" ],
  "summary" : "Decidim amendments can be accepted or rejected by anyone",
  "details" : "### Impact\nThe vulnerability allows any registered and authenticated user to accept or reject any amendments. The impact is on any users who have created proposals where the amendments feature is enabled. This also elevates the user accepting the amendment as the author of the original proposal as people amending proposals are provided coauthorship on the coauthorable resources.\n\nThe only check done when accepting or rejecting amendments is whether the amendment reactions are enabled for the component:\nhttps://github.com/decidim/decidim/blob/9d6c3d2efe5a83bb02e095824ff5998d96a75eb7/decidim-core/app/permissions/decidim/permissions.rb#L107\n\nThe permission checks have been changed at 1b99136 which was introduced in released version 0.19.0. I have not investigated whether prior versions are also affected.\n\n### Patches\n\nNot available\n\n### Workarounds\nDisable amendment reactions for the amendable component (e.g. proposals).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "decidim-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.31.0.rc1"
      }, {
        "fixed" : "0.31.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "decidim-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.19.0"
      }, {
        "fixed" : "0.30.5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/decidim/decidim/security/advisories/GHSA-w5xj-99cg-rccm"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-40869"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/decidim/decidim/commit/1b99136a1c7aa02616a0b54a6ab88d12907a57a9"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/decidim/decidim"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/decidim-core/CVE-2026-40869.yml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-266" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-14T22:22:58Z",
    "nvd_published_at" : "2026-04-21T20:17:00Z"
  }
}