Stephen A. Fuqua (SAF) is a Bahá'í, software developer, and conservation and interfaith advocate in the DFW area of Texas.

Protecting Against SQL Injection in Dynamic SQL Statements

February 26, 2011

Microsoft's Books Online article on SQL Injection does a great job of reviewing the possible attacks against dynamic SQL statements (using EXEC or sp_executesql). I won't re-hash their discussion and suggestions. What I offer below is a sample remediation effort for this set of statements (the @Fields and @Values variables are actually stored procedure parameters):

SELECT @SQL = 'INSERT INTO MyTable (' + @Fields + ') VALUES (' + @Values + ')';

This represents the heart of what this stored procedure does. To protect against SQL Injection, I first added a section of code that checks to make sure that all of the fields in @Fields are real fields – this is positive input validation:

CREATE TABLE #temp (Field NVARCHAR(200)) 

INSERT INTO #temp(Field)
SELECT QUOTENAME(Field) FROM dbo.fnSplitString(@Fields,',')   

-- Validate that all fields are real fields in the table
                           AND QUOTENAME(c.COLUMN_NAME) = t.Field))
       DECLARE @v_strMessage AS NVARCHAR(500);

       SELECT @v_strMessage = 'Invalid  field(s) in the @Fields parameter detected: ';
       SELECT @v_strMessage = @v_strMessage  + QUOTENAME(t.Field) + ', '
            FROM #tempOrderField t WHERE NOT EXISTS
                           (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS c WHERE c.TABLE_NAME = 'MyTable'
                                  AND QUOTENAME(c.COLUMN_NAME) = QUOTENAME(t.Field));
       RAISERROR(@v_strMessage, 17, 0);

If there are any invalid fields, then they will all be listed in an error message and the stored procedure will stop executing. Note that all of the fields are note “quoted” by the QUOTENAME function. Next, the @VALUES needs to be protected. In this case, the application may send an apostrophe in the data, legitimately. So I use the Replace statement twice – once to guard against stray single quotes, and a second time to overcome incorrectly doubled apostrophes.

INSERT INTO #temp2 (Values)
SELECT REPLACE(REPLACE(Field, '''', ''''''), '''''', '''') FROM dbo.fnSplitString(@VALUES,',')

Finally, to avoid buffer overflows, I change the @SQL to NVARCHAR(MAX). In general it is better to use sp_executesql instead of EXEC, so I also change to that. In this case I can’t benefit from it (exercise for the reader to understand why), but I still use it out of good habit.

DECLARE @v_strSql NVARCHAR(MAX) -- statement needs 5028 characters, but to protect from buffer overflows, changing to MAX
SELECT @v_strSql = N'INSERT INTO dbo.MyTable ('

-- add  the fields
SELECT @v_strSql = @v_strSql + Field + ', ' FROM #temp;

-- remove extra comma and add VALUES()
SELECT @v_strSql = SUBSTRING(@v_strSql, 1, LEN(@v_strSql) - 1) + ') VALUES (';

-- add the values
SELECT @v_strSql = @v_strSql +  Value + ', ' FROM #temp2;

-- again remove extra comma and close VALUES()
SELECT @v_strSql = SUBSTRING(@v_strSql, 1, LEN(@v_strSql) - 1) + ');';

EXEC sp_executesql @v_strSql;

In summary, four changes were applied, as suggested by Microsoft:

  • Validated input data where I could (the field listing)
  • "Quoted" fields with QUOTENAME (can be done for fields in INSERT, SELECT, ORDER BY, etc.)
  • Doubled-up on the apostrophes
  • Tried to protect against buffer overflow

Before making these changes, I made sure that I had automated unit tests that were passing with the old version. After updates, and a few bug fixes, the unit tests are again passing. I also wrote unit tests that included bad data to make sure everything still worked fine.

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