CVE-2026-26077 in Discourse
Summary
by MITRE • 02/26/2026
Discourse is an open source discussion platform. Prior to versions 2025.12.2, 2026.1.1, and 2026.2.0, several webhook endpoints (SendGrid, Mailjet, Mandrill, Postmark, SparkPost) in the `WebhooksController` accepted requests without a valid authentication token when no token was configured. This allowed unauthenticated attackers to forge webhook payloads and artificially inflate user bounce scores, potentially causing legitimate user emails to be disabled. The Mailpace endpoint had no token validation at all. Starting in versions 2025.12.2, 2026.1.1, and 2026.2.0, all webhook endpoints reject requests with a 406 response when no authentication token is configured. As a workaround, ensure that webhook authentication tokens are configured for all email provider integrations in site settings (e.g., `sendgrid_verification_key`, `mailjet_webhook_token`, `postmark_webhook_token`, `sparkpost_webhook_token`). There's no current workaround for mailpace before getting this fix.
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Analysis
by VulDB Data Team • 03/03/2026
The vulnerability described in CVE-2026-26077 represents a critical authentication flaw within the Discourse open source discussion platform that affects multiple email provider webhook integrations. This issue stems from inadequate input validation and authentication mechanisms in the WebhooksController component, which handles incoming webhook requests from various email delivery services including SendGrid, Mailjet, Mandrill, Postmark, and SparkPost. The flaw specifically manifests when no authentication token is configured for these webhook endpoints, creating a scenario where any remote attacker can submit malicious payloads without proper authorization. This vulnerability falls under CWE-345 Insufficient Verification of Data Authenticity, as the system fails to properly authenticate incoming webhook requests, and aligns with ATT&CK technique T1566.002 for social engineering through email.
The technical implementation of this vulnerability allows attackers to exploit the absence of authentication checks in the webhook processing logic, particularly affecting the Mailpace endpoint which had no token validation whatsoever. When no authentication token is configured for the affected email providers, the system accepts all incoming requests without verification, enabling attackers to forge webhook payloads that simulate email delivery failures or bounces. This capability directly impacts the platform's email delivery infrastructure by artificially inflating user bounce scores, which can trigger automatic account disabling or delivery throttling mechanisms. The vulnerability specifically targets the authentication flow in the WebhooksController where the system should validate incoming requests against configured tokens before processing them, but instead accepts all requests when no tokens are present.
The operational impact of this vulnerability extends beyond simple authentication bypass to potentially disrupt legitimate user communications and account management. Attackers can manipulate bounce scores to cause legitimate user emails to be marked as undeliverable, leading to account disabling or delivery restrictions that affect normal platform operations. This creates a significant risk for platform administrators who rely on accurate email delivery metrics for user management and communication. The vulnerability particularly affects email integration workflows where bounce handling is critical for maintaining valid user contact information and preventing delivery issues. The severity increases when considering that the Mailpace endpoint had no token validation at all, making it completely vulnerable to manipulation without any authentication checks.
The remediation for this vulnerability involves implementing proper authentication validation across all webhook endpoints, with version releases 2025.12.2, 2026.1.1, and 2026.2.0 introducing mandatory token validation that rejects unauthorized requests with a 406 status code. This fix addresses the core issue by ensuring that all webhook endpoints require valid authentication tokens before processing any incoming requests, regardless of whether tokens are configured or not. The recommended workaround for administrators involves configuring proper webhook authentication tokens for all email provider integrations through site settings, including specific token parameters for each provider such as sendgrid_verification_key, mailjet_webhook_token, postmark_webhook_token, and sparkpost_webhook_token. However, the vulnerability presents a significant challenge for users of older versions who cannot implement the workaround for the Mailpace endpoint, as there is no current mitigation path available for this specific integration, leaving it permanently vulnerable until upgrading to patched versions.