CVE-2026-77129: TYPO3 Information Disclosure Vulnerability

CVE-2026-77129 Overview

CVE-2026-77129 is a server-side template injection vulnerability affecting a TYPO3 event registration extension. The extension passes an editor-configurable email subject string directly into a Fluid template source without restriction. An authenticated backend user with edit access to the event plugin or Backend Module can inject Fluid ViewHelper syntax. Successful exploitation discloses sensitive data or executes TypoScript content objects on the server. The flaw is classified under [CWE-1336: Improper Neutralization of Special Elements Used in a Template Engine].

Critical Impact
Authenticated backend users can leverage Fluid ViewHelper injection to disclose sensitive data or execute arbitrary TypoScript content objects within the TYPO3 application context.

Affected Products

TYPO3 event registration extension (see TYPO3 Security Advisory)
Deployments exposing the event plugin to editors
Deployments exposing the Backend Module to editors

Discovery Timeline

2026-08-25 - CVE-2026-77129 published to NVD
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-77129

Vulnerability Analysis

The vulnerability exists because the extension accepts an editor-supplied email subject and uses it as the source of a Fluid template. Fluid is TYPO3's templating engine, and its source is expected to be developer-controlled. When user input becomes the template source, ViewHelper tags in that input are parsed and executed by the rendering pipeline. An attacker submits Fluid ViewHelper syntax through the subject field to read protected records, dump configuration, or trigger TypoScript content objects. Because TypoScript objects can access filesystem and database resources, the impact extends beyond information disclosure.

Root Cause

The root cause is missing neutralization of template syntax in an input that is later compiled as a template. The design treats the subject string as trusted template content rather than as data to be interpolated. No allow-list or escape routine strips Fluid ViewHelper markup before compilation. This is a canonical server-side template injection pattern described by [CWE-1336].

Attack Vector

Exploitation requires an authenticated TYPO3 backend account with edit access to the event registration plugin or its Backend Module. The attacker edits the email subject field for an event and inserts Fluid ViewHelper syntax such as tags that render arbitrary variables or invoke content object rendering. When the extension sends or previews the notification email, the injected template is compiled and executed. The attack traverses the network over authenticated backend HTTP sessions.

No verified proof-of-concept code is published. Refer to the TYPO3 Security Advisory for vendor-provided technical details.

Detection Methods for CVE-2026-77129

Indicators of Compromise

Event records whose email subject fields contain Fluid syntax such as { and } delimiters or <f: ViewHelper prefixes.
Unexpected outbound emails or preview renderings that reference internal variables, file paths, or database content.
Backend audit log entries showing edits to event plugin or Backend Module records by non-administrative users immediately preceded by mail send events.

Detection Strategies

Scan the TYPO3 database for event registration records containing template metacharacters in subject fields.
Correlate backend user edits of event configuration with outbound SMTP traffic and template rendering errors in TYPO3 logs.
Alert on Fluid parser exceptions in typo3temp or PHP error logs, which often indicate malformed injection attempts.

Monitoring Recommendations

Enable TYPO3 backend audit logging and forward records to a centralized SIEM for retention and correlation.
Monitor for privilege changes granting edit access to the event extension and review those accounts quarterly.
Track file access patterns from the PHP process for reads of LocalConfiguration.php, .env, or credential files following event edits.

How to Mitigate CVE-2026-77129

Immediate Actions Required

Apply the fixed version of the affected TYPO3 extension as listed in the TYPO3 Security Advisory.
Audit backend user groups and remove edit permissions on the event registration plugin and Backend Module from accounts that do not require them.
Review existing event records for Fluid syntax in subject fields and sanitize any suspicious entries.

Patch Information

The extension maintainer has published fixed versions referenced in advisory typo3-ext-sa-2026-023. Administrators should upgrade the extension through Composer or the TYPO3 Extension Manager and clear caches after deployment. Consult the TYPO3 Security Advisory for the specific fixed release numbers.

Workarounds

Restrict backend edit permissions on event plugin content elements and Backend Module access to trusted administrators until patching completes.
Disable the event registration Backend Module in production if it is not actively used.
Enforce backend multi-factor authentication to reduce the risk of account compromise leading to exploitation.

bash

# Update the affected extension via Composer and flush caches
composer update vendor/event-extension --with-dependencies
vendor/bin/typo3 cache:flush
vendor/bin/typo3 extension:setup