What’s Vulnerable › Vulnerabilities

CVE-2026-33393: Discourse fixes loose hostname matching in spam host allowlist

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-19 · updated 2026-06-17
• Weakness: CWE-284

Description

Discourse is an open-source discussion platform. Prior to versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2, the `allowed_spam_host_domains` check used `String#end_with?` without domain boundary validation, allowing domains like `attacker-example.com` to bypass spam protection when `example.com` was allowlisted. Versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2 require exact match or proper subdomain match (preceded by `.`) to prevent suffix-based bypass of `newuser_spam_host_threshold`. No known workarounds are available.

Affected products

• discourse: from 2026.1.0 before 2026.1.2; from 2026.2.0 before 2026.2.1; 2026.3.0 — fixed in 2026.1.2, 2026.2.1

How to fix

Upgrade to a fixed version: discourse 2026.1.2, 2026.2.1. Check the vendor advisory for mitigations.

References

• https://github.com/discourse/discourse/commit/80b19c15fe9c7bc890d1a54f454c8446312ac6d2
• https://github.com/discourse/discourse/commit/d8467b9fbb3d9ed6047b4e508d3fef88a37b8a02
• https://github.com/discourse/discourse/commit/f99099cfbc6b76fe39d6fa2daa48efd69497fb8e
• https://github.com/discourse/discourse/security/advisories/GHSA-95r5-p6qr-hgw6

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