Microsoft has published SQL Server 2025 Cumulative Update 6 (CU6), build 17.0.4055.5. According to the release notes, this update includes 18 verified fixes across the Database Engine, SQL connectivity, Integration Services, and Linux-related areas.

The release also documents two known issues: one involving SESSION_CONTEXT in parallel plans, and another affecting linked server queries that use MSDASQL with a provider string.

Known issues

  • SESSION_CONTEXT in parallel plans: Queries that use the built-in SESSION_CONTEXT function can return incorrect results or generate access violation dump files when they run in parallel query plans. Microsoft notes that this behavior is related to how SESSION_CONTEXT interacts with parallel execution threads, especially when a session is reset for reuse.
  • Linked server queries using MSDASQL: Linked server queries that use the MSDASQL provider and specify a provider string can fail with Msg 7416: Access to the remote server is denied because no login-mapping exists. Microsoft says a stricter connection validation check in the Database Engine can reject some linked server configurations that worked in earlier builds.

Improvements and fixes

CU6 includes fixes and updates in several practical areas:

  • Query processing and SQL OS: Microsoft fixed an issue during parallel vector index builds that could lead to resource exhaustion and performance degradation, and it also addressed several uncommon cases where tempdb space usage accounting could be inaccurate.
  • Programmability and JSON/DDL behavior: The release fixes a memory access violation that can happen when altering a disabled DDL trigger, and it also addresses a possible failure when a DML operation inserts a NULL JSON document into a JSON index column.
  • High availability and disaster recovery: For availability groups, CU6 fixes a failed assertion related to thread trace elements on the primary replica. On Windows, it adds more detailed error information to the Failover Cluster log when the availability group resource DLL cannot retrieve diagnostic column information. On Linux, the Pacemaker resource agent is updated to treat a forwarder replica as the local primary for the local availability group orchestrator.
  • Linux engine fixes: Microsoft fixed several Linux-specific issues, including blocked mssql-conf behavior during collation changes on high-CPU systems, file creation failures on file systems without hard link support, sp_executesql error 17750 in a Linux container using Korean_Wansung_CI_AS, backup failures with 16 or more stripes when Transparent Huge Pages is enabled, a thread-suspension scheduling issue during index rebuilds, and date conversion errors during DDL operations on CDC tables.
  • Connectivity and TLS: The bcp utility now supports bulk import and export of vector(16) and vector(32) data types, and it adds -H (HostNameInCertificate) and -J (ServerCertificate) options for strict TLS certificate validation. CU6 also fixes handling of zero-length TLS records in the SNI SSL provider.
  • Integration Services: SSIS password-based encryption for packages targeting SQL Server 2025 now uses PBKDF2 with SHA-256 and 100,000 iterations. Microsoft also fixed an SSIS deployment and execution issue for connections that use Encrypt=Strict.
  • Search: On Azure SQL Managed Instance, the default value of FULLTEXT_INDEX_VERSION is changed to 2.

Microsoft’s release notes link each fix to a bug reference, including items such as 5094785, 5251310, 5255250, and 5267380.

Release details

  • Update: SQL Server 2025 CU6
  • KB: KB5093421
  • Build: 17.0.4055.5
  • Product version: SQL Server 2025
  • Official release date: 2026-06-17

Official source