A proof of concept technique for writing injection-proof APIs in Haskell. Posted by Thomas Sutton on September 16, 2012

This post contains a short proof-of-concept for a technique which allows library authors to implement injection-proof interfaces. It combines two techniques used by Bryan O’Sullivan in the mysql-simple and text packages:

• Wrap injection-vulnerable string parameters in a new type with a restricted API.

• Use GHC’s rewriting facilities to modify the code generated for literal values of string types.

This technique (which probably isn’t new, but is new to me) works like this:

1. Create a new type which wraps your problematic strings.

2. Define an instance of IsString for this new type.

3. Implement a new “unsafe” version for the fromString method.

4. Use a rule to specialise fromString to your fromStringUnsafe function.

5. Define a “safe” version of the fromString method.

6. Use a rule to replace “safe” calls to fromStringUnsafe with your fromStringSafe function.

The first two steps result in an API in which writing unsafe code requires explicit effort from the programmer. Rather than writing:

result <- query_ $"SELECT * FROM table WHERE id = " ++ theId a programmer will need to write: result <- query_$ fromString \$ "SELECT * FROM table WHERE id = " ++ theId

This is certainly an improvement, but still allows unsafe operations to be performed.

Steps three and four convince GHC to replace the fromString calls which convert string values (be they literal as in the first example or dynamic as in the second) into our custom type with calls to our own specialised function. Functionally, this should have no impact on the program.

In steps five and six we use a technique from text, where we use knowledge of GHC’s internals to rewrite the code which handles our string literals and replace this code with something a little more specialised. In the text package, this results in code which goes straight from GHC.CString.Addr# to Text values rather than going via an intermediate String.

So far this seems like a little bit of security theatre with a little bit of optimisation, but it becomes more powerful when we make our implementations of fromString and fromStringUnsafe call error and only fromStringSafe actually create a Query (or whatever your type is).

This post was published on September 16, 2012 and last modified on October 4, 2021. It is tagged with: haskell, ghc, security, static.