CVE-2026-77146: Frontend User Authentication Bypass Flaw

CVE-2026-77146 Overview

CVE-2026-77146 is a missing authorization vulnerability [CWE-862] in a TYPO3 extension's invitation controller. The controller redirects on invalid input but fails to halt further processing. An unauthenticated attacker can exploit this flaw to set a new password for an arbitrary existing frontend user account and re-enable disabled or deleted accounts. The vulnerability affects only the 8.x version branch of the extension.

Critical Impact
Unauthenticated attackers can take over any existing frontend user account by resetting its password and re-enabling deactivated accounts through the invitation flow.

Affected Products

TYPO3 extension (8.x versions) — invitation controller component
Frontend user accounts managed by the affected extension
TYPO3 installations relying on the vulnerable invitation workflow

Discovery Timeline

2026-08-25 - CVE-2026-77146 published to NVD
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-77146

Vulnerability Analysis

The invitation controller processes password-set requests tied to an invitation hash that identifies the target frontend user. When the controller encounters invalid input, such as a missing hash, a non-existent user, or a disabled or deleted account, it issues a redirect response but continues executing the remaining controller logic. That downstream logic performs the password update and account re-enable operations without re-validating the request state.

The result is a missing authorization condition. The controller trusts that a redirect terminates the request, but PHP execution proceeds through the rest of the action method. An attacker who supplies a crafted request identifying any existing frontend username can force the controller to write a new password hash and clear the disabled or deleted flags on that account.

Root Cause

The root cause is control-flow error in the invitation action handler. The code emits a redirect signal on invalid input but omits an explicit return statement or equivalent early exit. Because the redirect is queued rather than immediately terminal, subsequent statements in the action, including the password persistence and account activation calls, execute against attacker-supplied parameters. This maps to [CWE-862] Missing Authorization.

Attack Vector

The attack requires only network access to the TYPO3 frontend hosting the invitation endpoint. The attacker submits a POST request to the invitation controller referencing an existing frontend username, an invalid or empty invitation hash, and a chosen new password. The controller redirects due to the invalid hash but then applies the password change and clears deletion or disabled flags. No authentication, session, or valid invitation token is required.

Exploitation grants full frontend account takeover, including the ability to resurrect previously deleted accounts. See the TYPO3 Security Advisory 2026-024 for vendor-supplied technical details.

Detection Methods for CVE-2026-77146

Indicators of Compromise

POST requests to the extension's invitation controller endpoint containing password parameters but empty, malformed, or unknown invitation hash values.
Unexpected password changes on frontend user accounts without a corresponding valid invitation record in the database.
Frontend user records where the disable or deleted flag transitions from 1 to 0 outside of administrative activity.

Detection Strategies

Correlate web server access logs for the invitation route against the extension's invitation table to flag password-set actions lacking a matching active invitation row.
Enable database audit logging on the fe_users table and alert on password, disable, and deleted column updates originating from the invitation controller.
Hunt for repeated invitation controller requests from a single source IP that iterate through likely usernames.

Monitoring Recommendations

Alert on frontend authentication events where the account was previously marked deleted or disabled and became active again without administrator action.
Monitor for password change events across frontend users that occur in rapid succession, which suggests scripted enumeration.
Forward TYPO3 application and web logs to a centralized analytics platform and retain them for post-incident review.

How to Mitigate CVE-2026-77146

Immediate Actions Required

Upgrade the affected TYPO3 extension to a fixed release outside the vulnerable 8.x branch as identified in TYPO3 Security Advisory 2026-024.
Force a password reset for all frontend user accounts and invalidate active frontend sessions until the patch is deployed.
Review recent changes to the fe_users table for unauthorized password updates or reactivation of deleted or disabled accounts.

Patch Information

The extension maintainer has published fixed versions referenced in the TYPO3 Security Advisory 2026-024. Apply the vendor-supplied update for the 8.x branch or migrate to a patched major release. Verify installed versions using the TYPO3 Extension Manager after deployment.

Workarounds

Restrict access to the invitation controller route at the web server or reverse proxy layer until the extension is patched.
Temporarily disable the vulnerable extension in the TYPO3 backend if invitation functionality is not required.
Add a web application firewall rule that blocks requests to the invitation endpoint when the hash parameter is empty or malformed.

bash

# Example nginx rule to block invitation requests with missing hash parameter
location ~ ^/(index\.php)?.*tx_[a-z_]+_invitation.* {
    if ($arg_hash = "") { return 403; }
    if ($request_method = POST) {
        # require non-empty hash argument for POST
        set $block "";
        if ($arg_hash !~ ^[A-Za-z0-9]{20,}$) { set $block "1"; }
        if ($block = "1") { return 403; }
    }
}