CodeQL documentation
CodeQL resources

Uncontrolled data used in path expression

ID: js/path-injection
Kind: path-problem
Security severity: 7.5
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:
   - javascript-code-scanning.qls
   - javascript-security-extended.qls
   - javascript-security-and-quality.qls

Click to see the query in the CodeQL repository

Accessing files using paths constructed from user-controlled data 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.

Recommendation

Validate user input before using it to construct a file path.

The validation method you should use depends on whether you want to allow the user to specify complex paths with multiple components that may span multiple folders, or only simple filenames without a path component.

In the former case, a common strategy is to make sure that the constructed file path is contained within a safe root folder. First, normalize the path using path.resolve or fs.realpathSync to remove any “..” segments. You should always normalize the file path since an unnormalized path that starts with the root folder can still be used to access files outside the root folder. Then, after you have normalized the path, check that the path starts with the root folder.

In the latter case, you can use a library like the sanitize-filename npm package to eliminate any special characters from the file path. Note that it is not sufficient to only remove “../” sequences: for example, applying this filter to “…/…//” would still result in the string “../”.

Finally, the simplest (but most restrictive) option is to use an allow list of safe patterns and make sure that the user input matches one of these patterns.

Example

In the first (bad) example, the code reads the file name from an HTTP request, then accesses that file within a root folder. A malicious user could enter a file name containing “../” segments to navigate outside the root folder and access sensitive files.

const fs = require('fs'),
      http = require('http'),
      url = require('url');

const ROOT = "/var/www/";

var server = http.createServer(function(req, res) {
  let filePath = url.parse(req.url, true).query.path;

  // BAD: This function uses unsanitized input that can read any file on the file system.
  res.write(fs.readFileSync(ROOT + filePath, 'utf8'));
});

The second (good) example shows how to avoid access to sensitive files by sanitizing the file path. First, the code resolves the file name relative to a root folder, normalizing the path and removing any “../” segments in the process. Then, the code calls fs.realpathSync to resolve any symbolic links in the path. Finally, the code checks that the normalized path starts with the path of the root folder, ensuring the file is contained within the root folder.

const fs = require('fs'),
      http = require('http'),
      path = require('path'),
      url = require('url');

const ROOT = "/var/www/";

var server = http.createServer(function(req, res) {
  let filePath = url.parse(req.url, true).query.path;

  // GOOD: Verify that the file path is under the root directory
  filePath = fs.realpathSync(path.resolve(ROOT, filePath));
  if (!filePath.startsWith(ROOT)) {
    res.statusCode = 403;
    res.end();
    return;
  }
  res.write(fs.readFileSync(filePath, 'utf8'));
});

References

  • © GitHub, Inc.
  • Terms
  • Privacy