CVE-2026-24031: 
Dovecot vulnerability analysis and mitigation

CVE-2026-24031 is a SQL injection-based authentication bypass vulnerability in Dovecot's SQL-based authentication mechanism. When an administrator clears the auth_username_chars configuration option, attackers can bypass authentication for any user and perform user enumeration. The vulnerability affects Dovecot versions before 2.4.3 and OX Dovecot Pro versions before 3.1.4, both maintained by Open-Xchange. It was published on March 27, 2026, and carries a CVSS v3.1 base score of 8.2 (High) (Feedly, OX Advisory).

The root cause is improper neutralization of special elements used in an SQL command (CWE-89), triggered when the auth_username_chars setting is explicitly cleared by an administrator. Under normal configuration, auth_username_chars restricts which characters are permitted in usernames, effectively preventing SQL metacharacters from reaching the authentication query. When this restriction is removed, an attacker can inject SQL syntax into the username field, manipulating the authentication query to bypass credential validation entirely. The vulnerability is network-accessible, requires no privileges or user interaction, and no authentication is needed to exploit it (OX Advisory, Feedly).

Successful exploitation allows an unauthenticated remote attacker to log in as any user on the affected mail server, resulting in high integrity impact and low confidentiality impact (e.g., access to email accounts, potential data exfiltration). Additionally, the vulnerability enables user enumeration, which can facilitate targeted follow-on attacks. Availability is not directly impacted, but unauthorized access to mail accounts could expose sensitive communications and enable lateral movement within an organization (OX Advisory, Feedly).

1. Reconnaissance: Identify Dovecot mail servers running versions prior to 2.4.3 (community) or 3.1.4 (OX Dovecot Pro) using banner grabbing or service fingerprinting tools (e.g., nmap -sV -p 143,993,110,995).
2. Verify misconfiguration: Confirm that the target server has auth_username_chars cleared in its Dovecot configuration — this is a prerequisite for exploitation. This may be inferred by attempting logins with special characters and observing whether they are rejected at the input validation stage.
3. Craft SQL injection payload: Construct a username containing SQL metacharacters (e.g., ' OR '1'='1) designed to manipulate the SQL authentication query so that it returns a valid result regardless of the password supplied.
4. Authenticate as arbitrary user: Submit the crafted username along with any password via IMAP, POP3, or another supported protocol. If the server is vulnerable, the injected SQL logic causes the authentication query to succeed, granting access to the targeted or any user account.
5. User enumeration: Leverage variations of the SQL injection payload to enumerate valid usernames by observing differences in server responses to crafted queries (OX Advisory, Feedly).

• Network: Unusual IMAP/POP3 login attempts containing SQL metacharacters (e.g., single quotes, OR, =, --) in the username field; repeated authentication attempts from a single IP targeting multiple usernames.
• Logs: Dovecot authentication logs (/var/log/dovecot.log or /var/log/mail.log) showing successful logins with usernames containing special characters or SQL syntax; unexpected successful logins from unfamiliar IP addresses or at unusual times.
• Configuration: Presence of auth_username_chars = (empty value) in Dovecot configuration files (/etc/dovecot/dovecot.conf or included files), indicating the vulnerable misconfiguration is active.
• Process/Session: Unexpected IMAP sessions or mail access for accounts that do not normally receive remote logins; anomalous mail access patterns such as bulk message reads or exports shortly after authentication.

The primary workaround is to not clear auth_username_chars — restoring this setting to its default value immediately eliminates the attack surface without requiring a software update (OX Advisory). For permanent remediation, upgrade to Dovecot 2.4.3 or later (community edition) or OX Dovecot Pro 3.1.4 or later. Distribution-specific patches have been released for Ubuntu (USN-8136-1, USN-8136-2), Debian (DSA-6197-1), and openSUSE (Ubuntu Advisory, openSUSE). Administrators should audit Dovecot configurations to ensure auth_username_chars is set to a safe, non-empty value.

The vulnerability was disclosed via the oss-security mailing list and Seclists Full Disclosure, generating moderate community attention (oss-sec, Full Disclosure). Security news outlets including The Hacker Wire and Infinit Security published coverage shortly after disclosure (Infinit Security). Major Linux distributions responded promptly with security advisories and patched packages for Ubuntu, Debian, and openSUSE. No notable threat actor attribution or significant social media controversy has been observed.