CVE-2026-4208: 
PHP vulnerability analysis and mitigation

CVE-2026-4208 is an MFA bypass vulnerability in the TYPO3 extension "E-Mail MFA Provider" (mfa_mail) by mrsilaz. The extension fails to properly reset the generated MFA code after a successful authentication event, allowing subsequent login attempts to bypass multi-factor authentication entirely by submitting an empty string as the MFA code. Affected versions include all releases before 1.0.7 and version 2.0.0. The vulnerability was published on March 17, 2026, with a patch advisory released on April 25, 2026. It carries a CVSS v3.1 base score of 8.8 (High) and a CVSS v4.0 base score of 7.7 (High) (TYPO3 Advisory).

The root cause is classified as CWE-639 (Authorization Bypass Through User-Controlled Key). After a user successfully authenticates using the MFA email code, the extension does not invalidate or reset the stored MFA code server-side. As a result, an attacker who has valid primary credentials (username and password) can submit an empty string in the MFA code field on subsequent login attempts, and the extension's MFA provider accepts this as valid, bypassing the second factor entirely. Exploitation requires the attacker to have already obtained valid primary credentials (low privileges required) and that a prior successful authentication has occurred on the target account (TYPO3 Advisory, GitLab Advisory).

Successful exploitation allows an attacker with valid primary credentials to completely bypass multi-factor authentication, gaining unauthorized access to TYPO3 backend accounts protected by the mfa_mail extension. This results in high confidentiality, integrity, and availability impact — an attacker could access sensitive CMS content, modify or delete website data, escalate privileges within the TYPO3 instance, or use the compromised account as a foothold for further lateral movement within the hosting environment (TYPO3 Advisory).

No public proof-of-concept exploit code or in-the-wild exploitation has been reported as of the available data. The EPSS score is approximately 0.054%, indicating a low current probability of exploitation in the wild. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires an attacker to already possess valid primary credentials for a target account, which somewhat limits the attack surface, though the bypass itself is trivially simple (submitting an empty string) once credentials are obtained (TYPO3 Advisory, GitLab Advisory).

1. Reconnaissance: Identify TYPO3 instances using the mfa_mail (E-Mail MFA Provider) extension in a vulnerable version (< 1.0.7 or == 2.0.0). This can be done by inspecting TYPO3 backend login pages or extension metadata.
2. Obtain Primary Credentials: Acquire valid username and password for a target TYPO3 backend account through phishing, credential stuffing, or other means.
3. Trigger Initial Authentication: Log in once successfully with valid credentials and a legitimate MFA code to establish the session state where the MFA code is not reset.
4. Bypass MFA on Subsequent Login: On a subsequent login attempt, provide the valid username and password, then submit an empty string ("") in the MFA code field when prompted by the mfa_mail provider.
5. Gain Unauthorized Access: The extension accepts the empty string as a valid MFA code due to the missing reset logic, granting full backend access without a legitimate second factor (TYPO3 Advisory).

• Logs: TYPO3 backend authentication logs showing successful logins where the MFA step was completed with an empty or null MFA code value; repeated login events for the same account in a short timeframe without corresponding MFA email delivery records.
• Network: Absence of outbound email traffic (MFA code delivery) from the mail server coinciding with successful TYPO3 backend authentication events for accounts using mfa_mail.
• Application: TYPO3 backend session records showing authenticated sessions for accounts where no MFA email was sent during the login flow; unexpected administrative actions (content changes, user modifications) from accounts that should require MFA.

TYPO3 has released a patched version of the mfa_mail extension. Administrators should update to version 1.0.7 or later (for the 1.x branch) to remediate the vulnerability; version 2.0.0 is also affected and users should check for an updated 2.x release. No configuration-based workaround is available — upgrading the extension is the only effective remediation. As an interim measure, administrators may consider disabling the mfa_mail extension and switching to an alternative MFA provider until the patch is applied (TYPO3 Advisory).

The vulnerability was noted in Cyfirma's Weekly Intelligence Report for March 27, 2026, and picked up by several vulnerability tracking and threat intelligence platforms shortly after disclosure (Cyfirma Report). Community discussion was observed on Mastodon/infosec.exchange and security aggregator sites. No major vendor statements beyond the official TYPO3 security advisory have been identified.