JSON SQL Injection and the Lessons Learned

JSON SQL Injection
Copyright (C) 2014 DeNA Co.,Ltd. All Rights Reserved.
and
the Lessons Learned
DeNA Co., Ltd.
Kazuho Oku
1

Who am I?
 Field:
⁃ network and web-related architecture
 Works:
⁃ Palmscape / Xiino (web browser for Palm OS)
⁃ Author of various Perl modules
• Class::Accessor::Lite, HTTP::Parser::XS,
Parallel::Prefork, Server::Starter, Starlet,
Test::Mysqld, ...
⁃ Author of various MySQL extensions
• Q4M, mycached, ...
⁃ Also the author of:
• JSX, picojson, ...
2

Agenda
 What is an SQL Query Builder?
 JSON SQL Injection
 SQL::QueryMaker and strict mode of SQL::Maker
 The Lessons Learned
3

What is an SQL Query Builder?
4

What is an SQL Query Builder?
 is a library for building SQL queries
⁃ exists below or as a part of an object-relational
mapping library (ORM)
• ORM understands the semantics of the database
schema / query builder does not
5
Application
ORM Library
SQL Query Builder
Database API (DBI)
Database Driver (DBD)

Popular Implementations
 SQL::Abstract
 SQL::Interp
 SQL::Maker
⁃ used by Teng
⁃ is today's main focus
6
Application
ORM Library
SQL Query Builder
Database API (DBI)
Database Driver (DBD)

Query Builders are Great!
 Easy to build query conditions
⁃ by using hashrefs and arrayrefs
⁃ arrayref is considered as IN queries
• e.g. foo => [1,2,3] becomes `foo` IN (1,2,3)
⁃ hashref is considered as (operator, value) pair
• e.g. foo => { '<', 30 } becomes `foo`<30
⁃ the API is common to the aforementioned query
builders
7

Examples
use SQL::Maker;
...
$maker->select('user', '*', { name => 'tokuhirom' });
# => SELECT * FROM ùser` WHERE `name`='tokuhirom';
$maker->select('user', '*', { name => [ 'tokuhirom', 'yappo' ] });
# => SELECT * FROM ùser` WHERE `name` IN ('tokuhirom','yappo');
$maker->select('user', '*', { age => { '>' => 30 } });
# => SELECT * FROM ùser` WHERE àge`>30;
$maker->delete('user', { name => 'sugyan' });
# => DELETE FROM ùser` WHERE `name`='sugyan';
8

Examples (cont’d)
use SQL::Maker;
...
$maker->select('user', '*', { name => [ 'tokuhirom', 'yappo' ] });
# => SELECT * FROM ùser` WHERE `name` IN ('tokuhirom','yappo');
$maker->select('user', '*', { name => [] });
# => SELECT * FROM ùser` WHERE 1=0;
$maker->select('user', '*', {
age => 30,
sex => 0, # male
});
# => SELECT * FROM ùser` WHERE àge`=30 AND `sex`=0;
9

So What is JSON SQL Injection?
10

The Problem
 User-supplied input might not be the expected type
# this API returns the entries of a blog (specified by $json->{blog_id})
my $json = decode_json($input);
my ($sql, @binds) = $maker->select(
'blog_entries', '*', { id => $json->{blog_id} });
my $rows = $dbi->selectall_arrayref($sql, { Slice => {} }, @binds);
send_output_as_json([ map { +{
entry_id => $_->{id},
entry_title => $_->{title},
} } @$rows ]);
11
What if $json->{name} was not a scalar?

The Problem (cont’d)
 Will return a list of all blog entries if the supplied
JSON was: { "blog_id": { "!=": -1 } }
# this API returns the entries of a blog (specified by $json->{blog_id})
my $json = decode_json($input);
# generated query: SELECT * FROM `blog_entries` WHERE `id`!=-1
'blog_entries', '*', { id => $json->{blog_id} });
my $rows = $dbi->selectall_arrayref($sql, { Slice => {} }, @binds);
send_output_as_json([ map { +{
entry_id => $_->{id},
entry_title => $_->{title},
} } @$rows ]);
12

Can it be used as an attack vector?
13

Yes
14

Information Leakage in a Twitter-like App.
# this API returns the tweets of a user specified by $json->{user_id}, who is following the authenticating user
if (! is_following($session->{user_id}, $json->{user_id})) {
return "cannot return the tweets of an user who is not following you";
}
my ($sql, @binds) = $maker->select('tweet', '*', { user => $json->{user_id} });
My $rows = $dbi->selectall_arrayref($sql, { Slice => {} }, @binds);
send_output_as_json([ map { ... } @$rows ]);
sub is_following {
my ($user, $following) = @_;
# builds query: SELECT * FROM following WHERE user=$user AND following=$following
'following', '*', { user => $user, following => $following });
my $rows = $dbi->selectall_arrayref($sql, @binds);
return @$rows != 0;
}
15

Information Leakage in a Twitter-like App.
(cont'd)
# in case the JSON is: { user_id: { "!=": -1 } }
}
# generated query is SELECT * FROM `tweet` WHERE `user`!=-1, returns tweets of all users
sub is_following {
# the generated query becomes like bellow and the function likely returns TRUE:
# SELECT * FROM `following` WHERE `user`=$user AND `following`!=-1
return @$rows != 0;
}
16

Is the problem in the SQL query builders
using hash for injecting arbitrary operators?
17

No
18

Handling of Array is problematic as well
# in case the JSON is: { user_id: [ 12, 34 ] }, returns tweets of both users if either is following the authenticating
user
}
# generates: SELECT * FROM `tweet` WHERE `user` IN (12,34)
sub is_following {
# generates: SELECT * FROM `following` WHERE `user`=$user AND `following` IN (12,34)
return @$rows != 0;
}
19

Does the same problem exist in web
application frameworks written in
programming languages other than Perl?
20

Yes.
Many web application frameworks (e.g. Ruby
on Rails) automatically convert condition
specified by an array into an IN query.
21

Is the problem only related to the handling
of JSON?
22

Yes & No
23

Query decoders may return nested data
 in case of PHP and Ruby on Rails
query?id=1 => { id: 1 }
query?id[]=1&id[]=2 => { id: [1, 2] }
query?user[name]=yappo => { user: { name: "yappo" } }
⁃ also when using Data::NestedParams in Perl
 Catalyst switches from scalars to using arrayrefs
when the property is defined more than once
query?id=1 => { id => 1 }
query?id=1&id=2 => { id => [1, 2] }
 not the case for CGI.pm and Plack
24

SQL::QueryMaker
and
the strict mode of SQL::Maker
25

Overview of SQL::QueryMaker
 Provides a fail-safe API
⁃ provides functions to specify the SQL operators;
that return blessed refs to represent them
(instead of using arrayrefs / hashrefs)
sql_eq(name => 'yappo') # `name`='yappo'
sql_in(id => [ 123, 456 ]) # `id` IN (123,456)
sql_and([ # `sex`=1 AND `age`<=30
sex => 1, # female
age => sql_ge(30),
])
 Added strict mode to SQL::Maker
⁃ raises error when arrayref / hashref is given as a
condition
26

The snippet becomes safe in strict mode
# this API returns the tweets of a user specified by $json->{user_id}, who is following the authenticating user
my $maker = SQL::Maker->new(..., strict => 1);
}
# in strict mode, SQL::Maker raises an error if $json->{user_id} is not a scalar
sub is_following {
# ditto as above
return @$rows != 0;
}
27

Existing code may not work under strict mode
my $maker = SQL::Maker->new(..., strict => 1);
# equality comparison works as is
$maker->select(..., { foo => 123 });
# non-equality comparison needs to be rewritten
$maker->select(..., { foo => [ 123, 456 ]);
=> $maker->select(..., { foo => sql_in([ 123, 456 ]) });
$maker->select(..., { foo => { '<' => 30 } });
=> $maker->select(..., { foo => sql_lt(30) });
28

Supported in Teng >= 0.24
my $teng = My::DB->new({
...,
sql_builder_args => {
strict => 1,
}
,});
29

The Lessons Learned
30

Always validate the type of the input
 do not forget to validate the type of the input
⁃ even if you do not need to check the value of
the input
 checks can be done either within the Controller or
within the Model (in case of SQL::Maker)
⁃ validation in the Controller is preferable
31

Write fail-safe code
 do not change the behavior based on the type of
the input
⁃ instead, provide different method for each type
of the input
⁃ e.g. sql_eq vs. sql_in
32

Use blessed refs for dynamic behavior
 use blessed refs in case you need to change the
behavior based on the type of the input
⁃ this is a common idiom; many template engines
use blessed refs to determine whether if a string
is already HTML-escaped
# in case of Text::MicroTemplate
sub escape_html {
my $str = shift;
return ''
unless defined $str;
return $str->as_string
if ref $str eq 'Text::MicroTemplate::EncodedString';
$str =~ s/([&><"'])/$_escape_table{$1}/ge;
return $str;
}
33

Use blessed refs for dynamic behavior (cont'd)
 do not use serialization libraries with support for
blessed objects (e.g. YAML or Storable) for user
input
⁃ such use may lead to XSS or other code
injection vulnerability
⁃ always only accept scalars / arrays / hashes and
validate their type and value
34

Thanks to
 Toshiharu Sugiyama
⁃ original reporter of the issue
⁃ http://developers.mobage.jp/blog/2014/7/3/jsonsql-injection
 @tokuhirom, @cho45
⁃ for coordinating / working on the fix
 @miyagawa
⁃ for the behavior of Catalyst, Ruby, etc.
 @ockeghem
⁃ for looking into other impl. sharing the problem
⁃ http://blog.tokumaru.org/2014/07/json-sql-injectionphpjson.
html
35

JSON SQL Injection and the Lessons Learned

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