SQL Injection Defense in Python Edgar Román [email protected] October 4, 2011.

SQL Injection Defense in Python

Edgar Romá[email protected]

October 4, 2011

What is SQL Injection?Unauthorized database access by an external source using specially crafted code to piggyback on standard user input to bypass normal protections.

Why?• Gain access to restricted website areas• Query unauthorized data• Delete or corrupt data

import MySQLdb

def book_search_view(request): if 'bookname' not in request.GET:

raise Http404 conn = MySQLdb.connect (host = "localhost", user = "testuser",

passwd = "testpass", db = "test") cursor = conn.cursor () name = request.GET['bookname'] cursor.execute ("SELECT * FROM table_books WHERE book_name = ‘%s’" % name) row = cursor.fetchone ()

cursor.close ()conn.close () return render_to_response('booklist.html', row, context_instance=RequestContext(request))

• Normal SQL – name=“Moby Dick”

SELECT * FROM table_books WHERE book_name = ‘Moby Dick’

• SQL Injection – bad day– name=“1’; SELECT * from Users; --”

SELECT * FROM table_books WHERE book_name = ‘1’; SELECT * from Users; --’

• SQL Injection 2 – really bad day– name=“1’; DROP TABLE Users; --”

SELECT * FROM table_books WHERE book_name = ‘1’; DROP TABLE Users; --’

Security is about multiple layers

Multiple Layers

• Assume the worst and plan for it• Coding protection is only one layer

– Which we will focus on for this presentation

• Database lockdown– User partitioning– Password protection

• But there are other attacks too: Open Web Application Security Project (OWASP)– https://www.owasp.org/

General approaches to SQL Injection Defense

• Escape User Input• White Lists• Stored Procs• Parameterized Queries

Escape User Input

• Hard to do right• You’ll probably screw it up if you don’t

cover all the cases– So don’t write your own regex

• MySQLdb.escape_string– Pro: Handles almost all encoding

evasions– Con: Error prone because it depends on

humans to always use it

import MySQLdb

def book_search_view(request): if 'bookname' not in request.GET:

raise Http404 conn = MySQLdb.connect (host = "localhost", user = "testuser",

passwd = "testpass", db = "test") cursor = conn.cursor () name = MySQLdb.escape_string(request.GET['bookname'] )cursor.execute ("SELECT * FROM table_books WHERE book_name = ‘%s’" % name) row = cursor.fetchone ()

cursor.close ()conn.close () return render_to_response('booklist.html', row, context_instance=RequestContext(request))

What does the escaped version look like?

• SQL Injection – bad day– name=“1’; SELECT * from Users; --”

SELECT * FROM table_books WHERE book_name = ‘1\’; SELECT * from Users; --’

• SQL Injection 2 – really bad day– name=“1’; DROP TABLE Users; --”

SELECT * FROM table_books WHERE book_name = ‘1\’;DROP TABLE Users; --’

Evasion Techniques

http://www.f5.com/pdf/white-papers/sql-injection-detection-wp.pdf

Even more Evasion Techniques

• Multibyte atttacks– http://

shiflett.org/blog/2006/jan/addslashes-versus-mysql-real-escape-string

– http://ilia.ws/archives/103-mysql_real_escape_string-versus-Prepared-Statements.html

• Even the experts don’t get it right– MySQL patches bugs in their escaping

routines

White List

• Scrub data to a known set of inputs• Pros

– Works well for variables with limited range

– Fast• Cons

– Can only be used in customized locations

– Error prone• You might forgot• Or the intern might not understand

• Example: user id must only contain 6 numbers

Stored Procedures

• Use the inherent store procedure capabilities

• Pros– Forces parameterization of all user input

• Cons– Can still be bypassed if sql string is

generated in code and passed to stored procedure

– Not portable between databases

Parameterized Queries

• Use DB API (mysqldb.execute) properly• Use Django ORM• Use SQLAlchemy (pylons, flask)

– Really have to work hard to expose yourself

• Pros– Generally easier to model data

• Cons– ORMs sometimes limit advanced SQL

• Bottom line: use a framework!

MySQLdb.execute

Bad:cursor.execute ("SELECT * FROM table_books WHERE book_name = ‘%s’" % name)

Good:cursor.execute ("SELECT * FROM table_books WHERE book_name = ‘%s’" , name)

Seriously?

Yes

Django ORM

• Automatically escapes all input parameters

• Be aware of extra() method – this is raw!• More info

– http://www.djangobook.com/en/2.0/chapter20/

Conclusions

• Use a db framework• If possible, white list your inputs• Be careful if writing raw SQL

http://xkcd.com/327/