== How to fix it in .NET

=== Code examples

:code_impact: delete

include::../../common/fix/code-rationale.adoc[]

==== Noncompliant code example

[source,csharp,diff-id=1,diff-type=noncompliant]
----
public class ExampleController : Controller
{
    private static string TargetDirectory = "/path/to/target/directory/";

    public void Example(string filename)
    {
        string path = Path.Combine(TargetDirectory, filename);
        System.IO.File.Delete(path);
    }
}
----

==== Compliant solution

[source,csharp,diff-id=1,diff-type=compliant]
----
public class ExampleController : Controller
{
    private static string TargetDirectory = "/path/to/target/directory/";

    public void Example(string filename)
    {
        string path = Path.Combine(TargetDirectory, filename);
        string canonicalDestinationPath = Path.GetFullPath(path);

        if (canonicalDestinationPath.StartsWith(TargetDirectory, StringComparison.Ordinal))
        {
            System.IO.File.Delete(canonicalDestinationPath);
        }
    }
}
----

=== How does this work?

:canonicalization_function: System.IO.Path.GetFullPath

include::../../common/fix/self-validation.adoc[]

=== Pitfalls

include::../../common/pitfalls/partial-path-traversal.adoc[]

For example, the following code is vulnerable to partial path injection. Note
that the string `TargetDirectory` does not end with a path separator:


[source, csharp]
----
private static string TargetDirectory = "/Users/John";

public void Example(string filename)
{
    string canonicalDestinationPath = Path.GetFullPath(filename);

    if (canonicalDestinationPath.StartsWith(TargetDirectory, StringComparison.Ordinal))
    {
        System.IO.File.Delete(canonicalDestinationPath);
    }
}
----

This check can be bypassed because `"/Users/Johnny/file".startsWith("/Users/John")`
returns `true`. Thus, for validation, `"/Users/John"` should actually be
`"/Users/John/"`.

**Warning**: Some functions remove the terminating path separator in their
return value. +
The validation code should be tested to ensure that it cannot be impacted by this
issue.

https://github.com/aws/aws-sdk-java/security/advisories/GHSA-c28r-hw5m-5gv3[Here is a real-life example of this vulnerability.]

:joining_docs: https://learn.microsoft.com/en-us/dotnet/api/system.io.path.combine
:joining_func: Path.Combine

include::../../common/pitfalls/path-joining.adoc[]
