CodeQL documentation

Uncontrolled data used in path expression

ID: cs/path-injection
Kind: path-problem
Severity: error
Precision: high
Tags:
   - security
   - external/cwe/cwe-022
   - external/cwe/cwe-023
   - external/cwe/cwe-036
   - external/cwe/cwe-073
   - external/cwe/cwe-099
Query suites:
   - csharp-code-scanning.qls
   - csharp-security-extended.qls
   - csharp-security-and-quality.qls

Click to see the query in the CodeQL repository

Accessing paths controlled by users can allow an attacker to access unexpected resources. This can result in sensitive information being revealed or deleted, or an attacker being able to influence behavior by modifying unexpected files.

Paths that are naively constructed from data controlled by a user may contain unexpected special characters, such as “..”. Such a path may potentially point to any directory on the file system.

Recommendation

Validate user input before using it to construct a file path. Ideally, follow these rules:

  • Do not allow more than a single “.” character.
  • Do not allow directory separators such as “/” or “\” (depending on the file system).
  • Do not rely on simply replacing problematic sequences such as “../”. For example, after applying this filter to “…/…//” the resulting string would still be “../”.
  • Use a whitelist of known good patterns.
  • Sanitize potentially tainted paths using HttpRequest.MapPath.

Example

In the first example, a file name is read from a HttpRequest and then used to access a file. However, a malicious user could enter a file name which is an absolute path - for example, “/etc/passwd”. In the second example, it appears that the user is restricted to opening a file within the “user” home directory. However, a malicious user could enter a filename which contains special characters. For example, the string “../../etc/passwd” will result in the code reading the file located at “/home/[user]/../../etc/passwd”, which is the system’s password file. This file would then be sent back to the user, giving them access to all the system’s passwords.

using System;
using System.IO;
using System.Web;

public class TaintedPathHandler : IHttpHandler
{
    public void ProcessRequest(HttpContext ctx)
    {
        String path = ctx.Request.QueryString["path"];
        // BAD: This could read any file on the filesystem.
        ctx.Response.Write(File.ReadAllText(path));

        // BAD: This could still read any file on the filesystem.
        ctx.Response.Write(File.ReadAllText("/home/user/" + path));

        // GOOD: MapPath ensures the path is safe to read from.
        string safePath = ctx.Request.MapPath(path, ctx.Request.ApplicationPath, false);
        ctx.Response.Write(File.ReadAllText(safePath));
    }
}

References