security.rdoc

doc/security.rdoc
Last Update: 2014-04-22 10:09:17 -0700

Security Considerations with Sequel

When using Sequel, there are some security areas you should be aware of:

  • Code Execution

  • SQL Injection

  • Denial of Service

  • Mass Assignment

  • General Parameter Handling

Code Execution

The most serious security vulnerability you can have in any library is a code execution vulnerability. Sequel should not be vulnerable to this, as it never calls eval on a string that is derived from user input. However, some Sequel methods used for creating methods via metaprogramming could conceivably be abused to do so:

As long as you don't call those with user input, you should not be vulnerable to code execution.

SQL Injection

The primary security concern in SQL database libraries is SQL injection. Because Sequel promotes using ruby objects for SQL concepts instead of raw SQL, it is less likely to be vulnerable to SQL injection. However, because Sequel still makes it easy to use raw SQL, misuse of the library can result in SQL injection in your application.

There are basically two kinds of possible SQL injections in Sequel:

  • SQL code injections

  • SQL identifier injections

SQL Code Injections

Full SQL Strings

Some Sequel methods are designed to execute raw SQL, including:

Here are some examples of use:

DB.run 'SQL'
DB << 'SQL'
DB.execute 'SQL'
DB['SQL'].all
DB.fetch('SQL').all
DB.dataset.with_sql('SQL').all

If you pass a string to these methods that is derived from user input, you open yourself up to SQL injection. The Sequel::Database#run, Sequel::Database#<<, and Sequel::Database#execute methods are not designed to work at all with user input. If you must use them with user input, you should escape the user input manually via Sequel::Database#literal. Example:

DB.run "SOME SQL #{DB.literal(params[:user].to_s)}"

With Sequel::Database#[], Sequel::Database#fetch and Sequel::Dataset#with_sql, you should use placeholders, in which case Sequel automatically literalizes the input:

DB['SELECT * FROM foo WHERE bar = ?', params[:user].to_s]

Manually Created Literal Strings

Sequel generally treats ruby strings as SQL strings (escaping them correctly), and not as raw SQL. However, you can convert a ruby string to a literal string, and Sequel will then treat it as raw SQL. This is typically done through String#lit if the core_extensions are in use, or Sequel.lit if they are not in use.

'a'.lit
Sequel.lit('a')

Using String#lit or Sequel.lit to turn a ruby string into a literal string results in SQL injection if the string is derived from user input. With both of these methods, the strings can contain placeholders, which you can use to safely include user input inside a literal string:

'a = ?'.lit(params[:user_id].to_s)
Sequel.lit('a = ?', params[:user_id].to_s)

Even though they have similar names, note that Sequel::Database#literal operates very differently from String#lit or Sequel.lit. Sequel::Database#literal is for taking any supported object, and getting an SQL representation of that object, while String#lit or Sequel.lit are for treating a ruby string as raw SQL. For example:

DB.literal(Date.today) # "'2013-03-22'"
DB.literal('a') # "'a'"
DB.literal(Sequel.lit('a')) # "a"
DB.literal(:a => 'a') # "(\"a\" = 'a')"
DB.literal(:a => Sequel.lit('a')) # "(\"a\" = a)"

SQL Filter Fragments

The most common way to use raw SQL with Sequel is in filters:

DB[:table].where("name > 'M'")

If a filter method is passed a string as the first argument, it treats the rest of the arguments (if any) as placeholders to the string. So you should never do:

DB[:table].where("name > #{params[:id].to_s}") # SQL Injection!

Instead, you should use a placeholder:

DB[:table].where("name > ?", params[:id].to_s) # Safe

Note that for that type of query, Sequel generally encourages the following form:

DB[:table].where{|o| o.name > params[:id].to_s} # Safe

Sequel's DSL supports a wide variety of SQL concepts, so it's possible to code most applications without ever using raw SQL.

A large number of dataset methods ultimately pass down their arguments to a filter method, even some you may not expect, so you should be careful. At least the following methods pass their arguments to a filter method:

The Model.find and Model.find_or_create class methods also call down to the filter methods.

SQL Fragment passed to Dataset#update

Similar to the filter methods, Sequel::Dataset#update also treats a string argument as raw SQL:

DB[:table].update("column = 1")

So you should not do:

DB[:table].update("column = #{params[:value].to_s}") # SQL Injection!

Instead, you should do:

DB[:table].update(:column => params[:value].to_s) # Safe

SQL Fragment passed to Dataset#lock_style

The Sequel::Dataset#lock_style method also treats an input string as SQL code. This method should not be called with user input.

SQL Type Names

In general, most places where Sequel needs to use an SQL type that should be specified by the user, it allows you to use a ruby string, and that string is used verbatim as the SQL type. You should not use user input for type strings.

SQL Function Names

In most cases, Sequel does not quote SQL function names. You should not use user input for function names.

SQL Identifier Injections

Usually, Sequel treats ruby symbols as SQL identifiers, and ruby strings as SQL strings. However, there are some parts of Sequel that treat ruby strings as SQL identifiers if an SQL string would not make sense in the same context.

For example, Sequel::Database#from and Sequel::Dataset#from will treat a string as a table name:

DB.from('t') # SELECT * FROM "t"

Another place where Sequel treats ruby strings as identifiers are the Sequel::Dataset#insert and Sequel::Dataset#update methods:

DB[:t].update('b'=>1) # UPDATE "t" SET "b" = 1
DB[:t].insert('b'=>1) # INSERT INTO "t" ("b") VALUES (1)

Note how the identifier is still quoted in these cases. Sequel quotes identifiers by default on most databases. However, it does not quote identifiers by default on DB2 and Informix. On those databases using an identifier derived from user input can lead to SQL injection. Similarly, if you turn off identifier quoting manually on other databases, you open yourself up to SQL injection if you use identifiers derived from user input.

When Sequel quotes identifiers, using an identifier derived from user input does not lead to SQL injection, since the identifiers are also escaped when quoting. Exceptions to this are Oracle (can't escape ") and Microsoft Access (can't escape ]).

In general, even if doesn't lead to SQL Injection, you should avoid using identifiers derived from user input unless absolutely necessary.

Sequel also allows you to create identifiers using Sequel.identifier for plain identifiers, Sequel.qualify for qualified identifiers, and Sequel.as for aliased expressions. So if you pass any of those values derived from user input, you are dealing with the same scenario.

Note that the issues with SQL identifiers do not just apply to places where strings are used as identifiers, they also apply to all places where Sequel uses symbols as identifiers. However, if you are creating symbols from user input, you at least have a denial of service vulnerability, and possibly a more serious vulnerability.

Denial of Service

Sequel converts some strings to symbols. Because symbols in ruby are not garbage collected, if the strings that are converted to symbols are derived from user input, you have a denial of service vulnerability due to memory exhaustion.

The strings that Sequel converts to symbols are generally not derived from user input, so Sequel in general is not vulnerable to this. However, users should be aware of the cases in which Sequel creates symbols, so they do not introduce a vulnerability into their application.

Column Names/Aliases

Sequel returns SQL result sets as an array of hashes with symbol keys. The keys are derived from the name that the database server gives the column. These names are generally static. For example:

SELECT column FROM table

The database will generally use “column” as the name in the result set.

If you use an alias:

SELECT column AS alias FROM table

The database will generally use “alias” as the name in the result set. So if you allow the user to control the alias name:

DB[:table].select(:column.as(params[:alias]))

Then you have a denial of service vulnerability. In general, such a vulnerability is unlikely, because you are probably indexing into the returned hash(es) by name, and if an alias was used and you didn't expect it, your application wouldn't work.

Database Connection Options

All database connection options are converted to symbols. For a connection URL, the keys are generally fixed, but the scheme is turned into a symbol and the query option keys are used as connection option keys, so they are converted to symbols as well. For example:

postgres://host/database?option1=foo&option2=bar

Will result in :postgres, :option1, and :option2 symbols being created.

Certain option values are also converted to symbols. In the general case, the sql_log_level option value is, but some adapters treat additional options similarly.

This is not generally a risk unless you are allowing the user to control the connection URLs or are connecting to arbitrary databases at runtime.

Mass Assignment

Mass assignment is the practice of passing a hash of columns and values to a single method, and having multiple column values for a given object set based on the content of the hash. The security issue here is that mass assignment may allow the user to set columns that you didn't intend to allow.

The Model#set and Model#update methods do mass assignment. The default configuration of Sequel::Model allows all model columns except for the primary key column(s) to be set via mass assignment.

Example:

album = Album.new
album.set(params[:album]) # Mass Assignment

Both Model.new and Model.create call Model#set internally, so they also allow mass assignment:

Album.new(params[:album]) # Mass Assignment
Album.create(params[:album]) # Mass Assignment

Instead of these methods, it is encouraged to either use the Model#set_only, Model#update_only, Model#set_fields, or Model#update_fields methods, which allow you to specify which fields to allow on a per-call basis. This pretty much eliminates the chance that the user will be able to set a column you did not intend to allow:

album.set_only(params[:album], [:name, :copies_sold])
album.set_fields(params[:album], [:name, :copies_sold])

You can override the columns to allow by default during mass assignment via the Model.set_allowed_columns class method. This is a good practice, though being explicit on a per-call basis is still recommended:

Album.set_allowed_columns(:name, :copies_sold)
Album.create(params[:album]) # Only name and copies_sold set

For more details on the mass assignment methods, see the Mass Assignment Guide.

General Parameter Handling

This issue isn't necessarily specific to Sequel, but it is a good general practice. If you are using values derived from user input, it is best to be explicit about their type. For example:

Album.where(:id=>params[:id])

is probably a bad idea. Assuming you are using a web framework, params could be a string, an array, a hash, or nil.

Assuming that id is an integer field, you probably want to do:

Album.where(:id=>params[:id].to_i)

If you are looking something up by name, you should try to enforce the value to be a string:

Album.where(:name=>params[:name].to_s)

If you are trying to use an IN clause with a list of id values based on input provided on a web form:

Album.where(:id=>params[:ids].to_a.map{|i| i.to_i})

Basically, be as explicit as possible. While there aren't any known security issues in Sequel when you do:

Album.where(:id=>params[:id])

It allows the attacker to choose to do any of the following queries:

id IS NULL # nil
id = '1' # '1'
id IN ('1', '2', '3') # ['1', '2', '3']
id = ('a' = 'b') # {'a'=>'b'}
id = ('a' IN ('a', 'b') AND 'c' = '') # {'a'=>['a', 'b'], 'c'=>''}

While none of those allow for SQL injection, it's possible that they might have an issue in your application. For example, a long array or deeply nested hash might cause the database to have to do a lot of work that could be avoided.

In general, it's best to let the attacker control as little as possible, and explicitly specifying types helps a great deal there.