CVE-2026-23891: Decidim core Cross-Site Scripting Vulnerability

CRITICAL
Advisory Identifier: GHSA-fc46-r95f-hq7g
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CVSS 8.7
CWE-79
Published
Advisory Summary

CVE-2026-23891 is a CRITICAL (CVSS 8.7) cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Decidim core libraries in the RubyGems ecosystem. The issue involves decidim has a cross-site scripting (xss) in user name. Remediation is available by upgrading affected packages to version 0.31.1, 0.30.5.

CVE-2026-23891 at a Glance

CVE Identifier CVE-2026-23891
GHSA Identifier GHSA-fc46-r95f-hq7g
Severity CRITICAL
CVSS v3.1 Base Score 8.7
CWE Classification CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Product Decidim core
Package Ecosystem RubyGems
Published Date
Last Updated

Security Impact

CVSS: 8.7

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

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

How to Fix CVE-2026-23891

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-2026-23891, 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-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where the product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. 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 A stored code execution vulnerability in the user name field allows a low-privileged attacker to execute arbitrary code in the context of any user who passively visits a comment page, resulting in high confidentiality and integrity impact across security boundaries. ### Patches N/A ### Workarounds Not available ### References OWASP ASVS v4.0.3-5.1.3 ### Credits This issue was discovered in a security audit organized by [octree](https://octree.ch/) and made by [Secu Labs](https://seculabs.ch/) against Decidim financed by the city of Lausanne (Switzerland).

CVE-2026-23891 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
< 0.30.5
0.30.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/decidim/decidim/security/advisories/GHSA-fc46-r95f-hq7g
NVD — CVE-2026-23891 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-23891
Decidim core Source Repository REPOSITORY
https://github.com/decidim/decidim
Decidim core Release RELEASE
https://github.com/decidim/decidim/releases/tag/v0.30.5
Decidim core Release RELEASE
https://github.com/decidim/decidim/releases/tag/v0.31.1
Decidim core Source Repository REPOSITORY
https://github.com/rubysec/ruby-advisory-db/blob/master/gems/decidim-core/CVE-2026-23891.yml

Frequently Asked Questions

What is CVE-2026-23891?
CVE-2026-23891 is a security vulnerability in Decidim core involving Decidim has a cross-site scripting (XSS) in user name under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
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-2026-23891?
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-2026-23891?
CVE-2026-23891 has a CVSS v3.1 base score of 8.7 (CRITICAL) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N.
What CWE is associated with CVE-2026-23891?
CVE-2026-23891 is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.).
How can I check RubyGems dependencies for CVE-2026-23891?
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-fc46-r95f-hq7g",
  "modified" : "2026-05-13T16:14:13Z",
  "published" : "2026-04-13T16:35:13Z",
  "aliases" : [ "CVE-2026-23891" ],
  "summary" : "Decidim has a cross-site scripting (XSS) in user name",
  "details" : "### Impact\nA stored code execution vulnerability in the user name field allows a low-privileged attacker to execute arbitrary code in the context of any user who passively visits a comment page, resulting in high confidentiality and integrity impact across security boundaries.\n\n### Patches\nN/A\n\n### Workarounds\nNot available\n\n### References\nOWASP ASVS v4.0.3-5.1.3\n\n### Credits\nThis issue was discovered in a security audit organized by [octree](https://octree.ch/) and made by [Secu Labs](https://seculabs.ch/) against Decidim financed by the city of Lausanne (Switzerland).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:H/VI:H/VA:L/SC:H/SI:H/SA:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "decidim-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.31.0.rc1"
      }, {
        "fixed" : "0.31.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 0.31.0"
    }
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "decidim-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.30.5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/decidim/decidim/security/advisories/GHSA-fc46-r95f-hq7g"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-23891"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/decidim/decidim"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/decidim/decidim/releases/tag/v0.30.5"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/decidim/decidim/releases/tag/v0.31.1"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/decidim-core/CVE-2026-23891.yml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-13T16:35:13Z",
    "nvd_published_at" : "2026-04-13T17:16:28Z"
  }
}