TI-2026-21608 Aliases CVE-2026-27166

Discourse vulnerable to HTML injection via prohibited iframe URLs

LOW 24CWE-80Published 2026-03-19 · Updated 2026-03-21

Key facts

Attack vector: Network
Privileges required: Low
User interaction: Required
Attack complexity: Low
Impact type: Cross-site scripting
Confidentiality · integrity · availability: Low · Low · None
Fix status: Fixed

Attack profile from CVSS 3.1 · NVD. Fix status from CNA. Impact type from CWE-80.

Summary

Discourse is an open source discussion platform. Prior to versions 2026.3.0-latest.1, 2026.2.1 and 2026.1.2, insufficient cleanup in the default Codepen allowed iframes value allows an attacker to trick a user into changing the URL of the main page. This issue has been fixed in versions 2026.3.0-latest.1, 2026.2.1 and 2026.1.2. To workaround this issue, remove Codepen from the list of allowed iframes.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00198 9th percentileScored 2026-11-07

Fix Fixed discourse (discourse): 2026.3.0-latest.1, 2026.2.1, 2026.1.2

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: No
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
discourse	discourse	before 2026.3.0-latest.1; 2026.2.0-latest to before 2026.2.1; 2026.1.0-latest to before 2026.1.2	2026.3.0-latest.1, 2026.2.1, 2026.1.2	Patch available	CNA

discourse Vendordiscourse Affected before 2026.3.0-latest.1; 2026.2.0-latest to before 2026.2.1; 2026.1.0-latest to before 2026.1.2 Fixed in 2026.3.0-latest.1, 2026.2.1, 2026.1.2 Patch available CNA

CPE configurations NVD · 2

CPE	Versions	Role	Vendor
cpe:2.3:a:discourse:discourse::::::::	before 2026.3.0; 2026.1.0 to before 2026.1.2; 2026.2.0 to before 2026.2.1	Vulnerable	discourse
cpe:2.3:a:discourse:discourse:2026.3.0::::latest:::	2026.3.0	Vulnerable	discourse

cpe:2.3:a:discourse:discourse:*:*:*:*:*:*:*:* Versions before 2026.3.0; 2026.1.0 to before 2026.1.2; 2026.2.0 to before 2026.2.1 Vulnerable ·discourse
cpe:2.3:a:discourse:discourse:2026.3.0:*:*:*:latest:*:*:* Versions 2026.3.0 Vulnerable ·discourse

Remediation

Fix · CNA discourse (discourse) Fixed in 2026.3.0-latest.1, 2026.2.1, 2026.1.2

Timeline

2026-10-02 Threatist record created from CVE-2026-27166

References

Patch 1

https://github.com/discourse/discourse/commit/c4762a4fac2f61f52f325396b11d3aeb955ea925

Advisory 1

https://github.com/discourse/discourse/security/advisories/GHSA-h653-cq78-vjj2

TI-2021-19656 Denial of Service in discourse CVSS 3.1MEDIUM 6.8
TI-2024-36150 Potential Backup file leaked via Nginx in Discourse CVSS 3.1HIGH 7.5
TI-2021-18002 RCE via malicious SNS subscription payload CVSS 3.1CRITICAL 10.0
TI-2026-24651 Discourse: Open redirect via `sso_destination_url` cookie in `enter` CVSS 4.0MEDIUM 5.1
TI-2026-22278 Discourse leaks private topic title and post excerpt via user action API endpoint CVSS 4.0HIGH 8.7
TI-2023-24152 Unauthenticated access to new private chat messages in Discourse CVSS 3.1HIGH 7.5

See all 262 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences