CVE-2026-107350 in PgBouncer
Summary
by MITRE • 10/14/2026
Improper authentication in PgBouncer 1.17.0 through 1.26.0 allows a remote attacker who knows the password of a user listed in auth_file to log in as any other user that authenticates using SCRAM, including PgBouncer admin users and backend superusers. A second StartupMessage sent during SCRAM authentication against a non-existent database is not rejected and switches the user being logged in as, while the SCRAM exchange is still verified against the keys of the first user.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified in PgBouncer versions 1.17.0 through 1.26.0 represents a critical failure in authentication logic that allows for privilege escalation and unauthorized access to database resources. This flaw specifically targets systems utilizing SCRAM-SHA-256 as the authentication mechanism, which is commonly employed due its superior security properties over plain text or MD5 methods. The core of the issue lies in how PgBouncer handles session state during the multi-step SCRAM handshake process when a client initiates a connection to a database that does not exist within the configuration. Under normal operational parameters, a request for a non-existent database should be rejected immediately with an appropriate error message, preventing any further authentication attempts or resource allocation. However, due to improper handling of protocol messages, PgBouncer fails to enforce this boundary check at the critical juncture where user identity is being established.
The technical mechanism behind this vulnerability involves the manipulation of StartupMessage packets during the SCRAM exchange. When a remote attacker initiates a connection, they first authenticate as a legitimate user whose credentials are present in the auth_file configuration. This initial authentication establishes a valid session context and retrieves the necessary cryptographic keys associated with that specific user account. Instead of terminating the connection upon receiving an error for the non-existent database, the attacker sends a second StartupMessage packet within the same TCP stream. PgBouncer incorrectly processes this subsequent message by switching the target identity to a different user specified in the new startup parameters, while crucially retaining the authentication state and cryptographic verification results from the first user's SCRAM exchange. This decoupling of identity context from authentication proof allows the attacker to bypass access controls entirely.
The operational impact of this flaw is severe, as it enables an unauthenticated or low-privileged remote attacker to assume the identity of any other user on the system. Because PgBouncer acts as a connection pooler and proxy for PostgreSQL databases, gaining control over its authentication logic effectively grants direct access to the underlying database infrastructure. The vulnerability specifically allows attackers to log in as administrative users defined within PgBouncer itself, which typically possess elevated privileges such as creating new roles or modifying configuration settings. More critically, it permits login as backend superusers of the actual PostgreSQL databases behind the proxy. This means that even if strict password policies are enforced for application-level accounts, an attacker can leverage this flaw to escalate their privileges to database superuser status, thereby gaining unrestricted read and write access to all data stored within those databases.
This vulnerability aligns with CWE-287, which describes Improper Authentication, specifically illustrating a failure in verifying the association between authentication credentials and session identity. It also maps closely to MITRE ATT&CK technique T1078, Valid Accounts, as it allows an attacker to use legitimate but potentially low-level credentials to gain access to higher privileged accounts without needing those specific passwords. The flaw exploits the stateful nature of network protocols where intermediate messages can alter context if not strictly validated against prior authentication steps. This is particularly dangerous in environments where PgBouncer is used for load balancing or connection pooling across multiple database clusters, as a single compromised application credential could lead to widespread data exfiltration and system compromise.
Mitigation strategies must prioritize immediate software updates to address the underlying logic error in message handling. Organizations running affected versions of PgBouncer should upgrade to version 1.26.1 or later, where this specific race condition and state management flaw have been corrected by enforcing strict validation of database existence before allowing any identity switching during authentication sequences. In addition to patching, defensive configurations can help reduce the attack surface. Administrators should ensure that SCRAM-SHA-256 is enforced for all client connections rather than falling back to weaker methods like MD5 or plain text, although this vulnerability specifically targets SCRAM implementations so reliance on stronger hashing alone does not mitigate it. Network-level controls such as firewalls and intrusion detection systems can be configured to monitor for anomalous patterns of connection attempts involving multiple startup messages within a single session from external sources. Furthermore, implementing strict least-privilege principles for PgBouncer configuration ensures that even if an attacker successfully exploits this vulnerability, the scope of damage is limited by restricting which backend roles are accessible through specific database aliases in the configuration file. Regular auditing of authentication logs and monitoring for unusual login patterns can also aid in early detection of exploitation attempts targeting this flaw.