KB5122770: Security update for SQL Server 2025, GDR branch, September 2026. Critical database flaws addressed; linked-server queries may fail

• The managed backup stored procedures receive a security fix.¹
  Addresses a vulnerability affecting managed_backup.sp_get_encryption_option and managed_backup.sp_do_backup. The source advises customers who use managed backup to apply the update through the standard SQL Server servicing process.
• Malformed or undersized backup metadata is rejected safely.²
  Improves internal validation of backup metadata processed by the SQL Server Volume Shadow Copy Service (VSS) Writer.

• SQL Server Agent applies stronger security checks when loading task-agent components.³
  Addresses a security validation issue when SQL Server Agent loads task-agent components.
• SQL Server Agent gets reliability and security fixes for processing specific input.⁴
  Addresses reliability and security issues in SQL Server Agent if it processes specific input.
• SQL Server Agent gets a security and reliability fix for specific operating conditions.¹⁴
  Addresses an issue affecting the security and reliability of SQL Server Agent under specific operating conditions.

For: SQL Server on Windows

• The SQL WMI Provider handles malformed input correctly while retaining its behavior for valid input.⁵
  Addresses incorrect processing of malformed input by the SQL WMI Provider.

• Microsoft Entra authentication login handling receives a security fix.⁶
  Addresses a security vulnerability in Microsoft Entra authentication login handling in SQL Server.
• An error condition now produces a correctly formatted error message.²⁶
  Addresses an error condition that causes an incorrectly formatted error message.
• Audit records consistently include sensitivity information when applicable.²⁹
  Fixes inconsistent population of data_sensitivity_information in audit records generated by SCHEMA_OBJECT_ACCESS_GROUP.

• Replication stored procedures receive stronger internal validation and security checks.⁷
  Improves internal validation and security checks to ensure replication operations use the expected security safeguards.
• Replication stored procedures handle supplied and stored identifiers more reliably.⁸
  Addresses identifier handling and input validation in replication stored procedures.
• Replication stored procedures handle identifiers with longer or special characters correctly.⁹
  Addresses replication stored procedures that incorrectly process identifiers containing longer or special characters.
• Replication stored procedures handle identifiers with special characters correctly.¹⁰
  Addresses replication stored procedures that incorrectly process identifiers containing special characters.
• A merge replication metadata stored procedure validates supplied values more reliably.¹¹
  Addresses input validation for a merge replication metadata stored procedure.
• A replication stored procedure handles a caller-supplied path value correctly.¹²
  Addresses a replication stored procedure that handles a caller-supplied path value.
• Replication stored procedures validate their parameter values more reliably.¹³
  Addresses input validation for parameters used by replication stored procedures.
• Buffer length checks in enclave code are strengthened.²⁸
  Strengthens buffer length validation checks within enclave code.
• Object Store Format deserialization is hardened against malformed or malicious stream data.³⁰
  Strengthens Object Store Format (OSF) deserialization against malformed or malicious stream data.
• Embedded attestation fields receive bounds checks.³¹
  Adds bounds validation for embedded attestation fields.
• A class ID lookup no longer reads arbitrary memory.³²
  Addresses an issue in which the class ID lookup call reads arbitrary memory.
• Protection of internal replication data during processing is addressed.³⁶
  Addresses an issue affecting the protection of internal replication data during processing operations.
• Merge Replication handles text data more reliably during processing.³⁷
  Addresses an issue in Merge Replication affecting the handling of text data during processing operations.
• Validation and handling in replication operations receive security and reliability changes.³⁸
  Addresses an issue in replication operations affecting validation and handling, enhancing security and reliability.

For: SQL Server on Windows

• A malformed double-byte character input issue that could make the server read beyond a buffer is addressed.¹⁵
  The SUBSTRING function could read beyond the allocated buffer while processing malformed DBCS varchar input in a natively compiled In-Memory OLTP procedure.
• A conversion issue that could make the server stop responding is addressed.¹⁶
  Converting or truncating an NVARCHAR value containing an Ideographic Variation Sequence under a Variation Selector Sensitive (_VSS) collation could cause an access violation in a natively compiled In-Memory OLTP procedure.
• A stored procedure issue that could overwrite memory and stop the server responding is addressed.¹⁷
  Addresses a heap buffer overwrite when a natively compiled stored procedure streams nvarchar(max) data with SET TEXTSIZE truncation under a _VSS collation and uses small TDS packet sizes.

For: SQL Server on Windows and Linux

• PolyBase services no longer open raw TCP listeners for the DW Engine and DMS channels when started with modified command-line parameters.¹⁸
  Addresses a remote code execution vulnerability involving modified PolyBase service command-line parameters that activate a deprecated feature. The fix prevents raw TCP listeners for the DW Engine and DMS control and data channels in this scenario.

• XML data type handling for DTD attribute lists is corrected.¹⁹
  Addresses an XML data type handling bug in Document Type Definition (DTD) attribute lists.
• A very large query plan declaration no longer causes the described access violation and unexpected process termination.²⁰
  Addresses an access violation when SQL Server processes a query with an extremely large OPTION (USE PLAN) XML declaration.
• A specially crafted XML format file no longer causes the described integer overflow when processed.²⁵
  Addresses an integer overflow in the SQL Server XML reader when processing a specially crafted XML format file through OPENROWSET(BULK...) or BULK INSERT; the overflow could stop or unexpectedly terminate the process.

• The LEFT_SHIFT and RIGHT_SHIFT functions receive safeguards against inputs that could cause the server to terminate unexpectedly.²¹
  Addresses arithmetic overflow and out-of-bounds memory access when these functions process specially crafted shift values or binary inputs.
• Input validation is strengthened against an authenticated user's ability to disclose a limited amount of memory.²⁷
  Addresses improper input validation that allows an authenticated user to disclose a limited amount of memory.
• Input validation is strengthened against an authenticated user's ability to disclose in-memory information.³³
  Addresses improper input validation that allows an authenticated user to disclose in-memory information.
• Input validation is strengthened against a pointer that could cause dump file generation or out-of-memory conditions.³⁴
  Addresses an issue in which an authenticated user can provide a pointer that SQL Server dereferences and reads but does not execute.

• The handling and output of certain internal event data is updated to help protect information.²²
  Addresses a security issue by updating how SQL Server processes and outputs certain internal event data.
• Invalid field offsets in SqlDataRecord.SetBytes are addressed to prevent an out-of-bounds heap write.³⁵
  Addresses improper field offset validation for sql_variant values in SqlDataRecord.SetBytes. The issue can cause denial of service or elevation of privilege when CLR is enabled, including in SQL Server installations and Azure SQL Managed Instances.

For: SQL Server on Windows and Linux

• An authenticated remote procedure call buffer overflow that could terminate the SQL Server process is addressed.²³
  Addresses a buffer overflow during an authenticated Tabular Data Stream (TDS) remote procedure call (RPC).

For: SQL Server on Windows and Linux

• A memory-corruption risk in Extended Events is addressed.²⁴
  Addresses insufficient validation in Extended Events that can cause memory corruption.

For: SQL Server on Windows and Linux

• Responses from S3-compatible external storage are validated more carefully.³⁹
  Improves validation of responses from S3-compatible external storage endpoints during external data access.

For: SQL Server on Windows

• Showplan input handling is improved for security and reliability.⁴⁰
  Improves the handling of input to Showplan to enhance security and reliability.

For: SQL Server on Windows

• Queued Updating Subscriptions in Transactional Replication receive security and reliability improvements.⁴¹
  Includes security and reliability improvements for the Queued Updating Subscriptions in Transactional Replication.
• Queued Updating Subscriptions in Transactional Replication receive security and reliability improvements.⁴²
  Includes security and reliability improvements for the Queued Updating Subscriptions in Transactional Replication.