SQL injection is one of the oldest and most well-understood vulnerability classes in software security. Most developers know the rules: use parameterized queries, avoid string concatenation, sanitize your inputs. And yet, SQL injection vulnerabilities continue to appear even in places where you least expect them.

This article documents a SQL injection vulnerability I discovered in sys.sp_dbmmonitorupdate, a Microsoft-signed system stored procedure. For other SQL Server security vulnerabilities I’ve discovered, see the full series here.

What makes this case particularly interesting is not just that the vulnerability exists in a trusted system object, but how it works: the injection bypasses a REPLACE-based sanitization attempt through a subtle Unicode character conversion that happens silently during a variable assignment.

The vulnerability was reported to Microsoft and they have since fixed it, but it’s still worth exposing and explaining given how intricate it is. So, that’s what I’ll do in this article.

For full details of the fix, check CVE-2025-53727.

What is sys.sp_dbmmonitorupdate?

Before we begin, it’s worth explaining what sys.sp_dbmmonitorupdate actually is. It’s a system stored procedure used to monitor the state of Database Mirroring, a high-availability feature in SQL Server. The procedure accepts a single parameter – @database_name – which specifies which mirrored database to update.

Since it’s a Microsoft system object, it’s treated as inherently trustworthy by both the SQL Server engine and many security tools. This trust is at the heart of why this vulnerability matters.

The vulnerability explained

Inside sys.sp_dbmmonitorupdate, after inserting a monitoring row, the procedure needs to call sys.sp_dbmmonitorresults to retrieve the latest data for alert evaluation.

The vulnerable code

It does this by building a dynamic SQL command string and executing it:

At first glance, it looks like the code is doing the right thing. The developer used REPLACE to double any single quotes in @database_name before embedding it in the SQL string – a classic and generally correct technique for preventing quote-based SQL injection.

So, what’s the problem?

The problem: an implicit type conversion

The problem is the type of the @command variable: CHAR(256).

CHAR is a non-Unicode type. @database_name, on the other hand, is declared as sysname – an alias for NVARCHAR(128), which is a Unicode type.

When the NVARCHAR expression on the right-hand side of the assignment is assigned to the CHAR(256) variable on the left, SQL Server must perform an implicit Unicode-to-non-Unicode conversion. This conversion is silent (no warning, no error), and is governed by the server’s collation and its best-fit character mapping rules.

This implicit conversion is the key to the attack.

The attack vector explained: Unicode lookalike characters

The Unicode standard contains many characters that are visually similar, or even nearly identical to common ASCII characters. One such character is:

These two characters look almost identical in most fonts but are completely different code points.

Now, consider what happens when an attacker crafts a database name containing ʼ (U+02BC) instead of ‘ (U+0027):

Step 1: The REPLACE call

REPLACE compares characters by their exact code point. It’s searching for U+0027, while the injected character is U+02BC – clearly, they don’t match. So, the REPLACE call finds nothing to replace, and passes the character through unchanged.

Step 2: The assignment to CHAR(256)

The right-hand side is a NVARCHAR expression. When assigned to @command (a CHAR(256) variable), SQL Server performs the implicit conversion. During this conversion, U+02BC (Modifier Letter Apostrophe) has no direct single-byte ASCII equivalent.

This isn’t a problem for SQL Server, with its collation best-fit mapping silently converting it to U+0027 – a real, standard apostrophe.

Step 3: The result

The @command variable now contains a real apostrophe that wasn’t there when REPLACE ran. This apostrophe breaks out of the string literal in the SQL command, creating an unmatched quote – the classic condition for SQL injection. Any SQL payload appended after the ʼ in the database name is now executable T-SQL.

A closer look at the conversion with a simplified proof of concept

The core vulnerability can be demonstrated with a much smaller proof of concept by isolating the vulnerable command construction logic.

The vulnerable procedure attempted to escape regular single quotes in @database_name, but then assigned the generated Unicode command to a non-Unicode char(256) variable:

The important character in the payload is this one:

ʼ

It is Unicode character U+02BC. It looks similar to a regular apostrophe, but it is not the same character as the SQL string delimiter '. As a result, the REPLACE() function does not escape it.

After the assignment to char(256), SQL Server performs an implicit Unicode-to-non-Unicode conversion. In the vulnerable scenario, the U+02BC character is converted into a regular apostrophe. The generated command becomes equivalent to:

At this point, the string passed to sys.sp_dbmmonitorresults is closed early, and the injected SELECT statement becomes a second command in the batch.

A slightly expanded local demonstration can show the difference between the vulnerable and fixed behavior:

In the vulnerable version, the generated command can contain a real apostrophe introduced by the implicit conversion to char.

In the fixed version, meanwhile, the command remains Unicode, so the U+02BC character stays U+02BC. It does not become a SQL string delimiter, and the injected text remains part of the database-name argument.

Microsoft fixed the issue by changing the command buffer to nvarchar(4000), which prevents the Unicode-to-non-Unicode conversion that turned the U+02BC character into a real apostrophe.

Why REPLACE can’t save you here

This is the critical insight of this vulnerability: the sanitization runs before the conversion.

So, by the time the dangerous character transformation has occurred during the variable assignment, REPLACE has already finished its work and returned. There’s no second chance to catch the newly created apostrophe.

This is a general principle worth remembering: any REPLACE-based quote sanitization applied to a NVARCHAR value before it’s assigned to a CHAR/VARCHAR variable, is potentially bypassable via Unicode lookalike characters.

What’s the impact of this security vulnerability?

One important limitation of this vulnerability is that sys.sp_dbmmonitorupdate can only be executed by members of the sysadmin fixed server role.

The procedure explicitly performs the following security check before reaching the vulnerable dynamic SQL code path:

This significantly limits the practical exploitability in standard configurations because, if you are already a sysadmin, you can execute arbitrary SQL anyway. However, the vulnerability still matters, as I’ll explain next.

Why the vulnerability matters (3 key reasons explained)

Here are the 3 key reasons this SQL Server vulnerability matters:

It enables stealthy audit evasion

sys.sp_dbmmonitorupdate is a Microsoft-signed system object. Many Extended Events sessions, SQL Server Audit configurations, and SIEM integrations whitelist known system procedures or reduce the verbosity of their logging.

An attacker who is a sysadmin (perhaps through a compromised service account) can use this injection vector to execute arbitrary SQL in a way that looks, from the outside, like a routine monitoring procedure call concealing the true command from logging systems and administrators.

Cloud platform trust can be abused

Cloud providers including Azure, AWS RDS, and GCP Cloud SQL often grant elevated permissions to specific system procedures to support platform automation features. These include: enabling CDC, replication, and database mirroring.

In some configurations, these procedures may be executable by users who are not full sysadmins. If a cloud platform grants elevated execution rights to sys.sp_dbmmonitorupdate as part of a mirroring automation workflow, the privilege requirement changes entirely.

Automation context can be abused

If the procedure is invoked by an elevated service account on a schedule — common in monitoring setups — an attacker who can influence the name of a mirrored database could inject SQL that executes in that service account’s context. This might be possible, for example, through a compromised application with permission to create databases.

Therefore, the sysadmin requirement should reduce the assessed severity – but should not cause the bug to be ignored entirely. The vulnerability demonstrates a real SQL injection flaw in trusted SQL Server code, caused by unsafe dynamic SQL construction and a Unicode-to-non-Unicode conversion after quote escaping.

How Microsoft fixed the vulnerability

Microsoft’s fix is practical and easy. A single variable type change in the declaration block eliminates the vulnerability entirely.

Vulnerable declaration

Fixed declaration

4 key technical takeaways from this vulnerability

Here’s what you should know about this SQL Server vulnerability (the 4 key technical takeaways):

1. Type consistency in dynamic SQL is a security property

When building dynamic SQL strings, every variable in the chain must be the same type or you must explicitly account for what happens during conversion. Mixing NVARCHAR inputs with CHAR/VARCHAR command buffers is a latent security defect waiting to be triggered.

2. REPLACE-based sanitization has a blind spot

REPLACE operates on the string as-is at the moment it runs. If a subsequent operation (like a type conversion) transforms the string in a way that introduces new SQL metacharacters, REPLACE can’t help. The correct defense is to ensure that no such transformation can occur which means keeping everything NVARCHAR.

3. The attack requires a specific precondition

For this attack to work, the database name containing U+02BC must actually exist in SQL Server. While SQL Server does allow Unicode characters in database names (when using quoted identifiers), it also requires the attacker to first create a database with the crafted name. To do that, the CREATE DATABASE permission or sysadmin is required.

4. Signed system procedures are not immune

The presence of this vulnerability in a Microsoft-signed system procedure is a reminder that, while code signing establishes authenticity and integrity, it doesn’t establish security correctness. A signed procedure can still contain logic errors, including injection vulnerabilities.

Conclusion

The SQL injection vulnerability in sys.sp_dbmmonitorupdate is subtle, but instructive. The developer clearly thought about injection – the REPLACE call is evidence of that. However, the implicit conversion from NVARCHAR to CHAR created a blind spot that REPLACE couldn’t overcome. Consequently, a Unicode lookalike character was able to slip through the sanitization and emerge on the other side as a real SQL metacharacter.

For developers writing T-SQL that constructs dynamic SQL, the lesson is clear: keep your types consistent, use NVARCHAR throughout, and prefer sp_executesql with parameterization over string concatenation and REPLACE. No amount of REPLACE-based sanitization can protect you if a type conversion undoes your work after the fact.