Description

Improper enforcement of message integrity in PostgreSQL GSSAPI support allows a user to negotiate GSSAPI contrary to pg_hba.conf rules, via initial direct TLS connection. Despite a pg_hba.conf that appears to require GSSAPI, the connection may exchange data over TLS encryption alone. If the TLS settings are more permissive than the GSS settings, the connection may continue with lesser protection. Within major versions 17-18, minor versions before PostgreSQL 18.5 and 17.11 are affected. Versions before PostgreSQL 17 are unaffected.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "4.2",
  "pubDate": "2026-08-13T13:17:45.523Z",
  "pubdate": "2026-08-13T13:17:45.523Z",
  "executiveSummary": "An improper enforcement of message integrity vulnerability exists within the PostgreSQL GSSAPI support mechanism. This flaw allows a user to successfully negotiate GSSAPI authentication contrary to the configured access control rules defined in pg_hba.conf by leveraging an initial direct TLS connection.\nThe primary impact of this vulnerability is a security degradation where a connection intended to require GSSAPI authentication instead exchanges data solely over TLS encryption. If the underlying TLS settings are configured with more permissive security parameters than the GSS settings, the connection proceeds with a lesser degree of protection than mandated by the database administrator.\nAffected systems include PostgreSQL major versions 17 through 18, specifically minor versions prior to PostgreSQL 18.5 and 17.11. Versions prior to PostgreSQL 17 are not affected.\nThe risk implications involve a breach of defense-in-depth principles and potential non-compliance with organizational security policies that mandate strict GSSAPI enforcement. An attacker capable of establishing network connections to the database can bypass intended authentication and encryption constraints, exploiting discrepancies between TLS and GSS security configurations.",
  "technicalDetails": "The root cause of the vulnerability stems from improper message integrity enforcement during the initial connection handshake within the PostgreSQL GSSAPI support component. Specifically, the protocol fails to correctly validate and enforce message integrity constraints when an initial direct TLS connection is established.\nThe vulnerable component is the authentication negotiation subsystem handling GSSAPI and TLS integration within PostgreSQL major versions 17 through 18 (specifically minor versions before PostgreSQL 18.5 and 17.11).\nThe step-by-step attack flow begins when a client initiates a direct TLS connection to the PostgreSQL server. During the connection establishment and authentication phase, the client manipulates the negotiation process to bypass the strict requirements specified in the server-side pg_hba.conf file. Although the pg_hba.conf configuration dictates that GSSAPI is required, the flawed integrity checks allow the connection to downgrade or bypass the GSSAPI negotiation phase.\nConsequently, the session defaults to exchanging data over TLS encryption alone. If the server's TLS cipher suites or protocol configurations are more permissive than the intended GSS security parameters, the session continues under weaker cryptographic or authentication protections, violating the security boundaries established by the database administrator.\nAuthentication and privilege requirements for exploitation depend on network exposure to the PostgreSQL server, but generally require the ability to initiate a direct TLS connection to the database port. The attack vector is network-based, exposing any improperly enforced GSSAPI configurations to unauthorized security degradation."
}