TaintedPath.java

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
import java.nio.file.Path;
import java.nio.file.Paths;

public class TaintedPath {
    public void sendUserFile(Socket sock, String user) throws IOException {
        BufferedReader filenameReader =
                new BufferedReader(new InputStreamReader(sock.getInputStream(), "UTF-8")); // $ Source
        String filename = filenameReader.readLine();
        // BAD: read from a file without checking its path
        BufferedReader fileReader = new BufferedReader(new FileReader(filename)); // $ Alert
        String fileLine = fileReader.readLine();
        while (fileLine != null) {
            sock.getOutputStream().write(fileLine.getBytes());
            fileLine = fileReader.readLine();
        }
    }

    public void sendUserFileGood(Socket sock, String user) throws IOException {
        BufferedReader filenameReader =
                new BufferedReader(new InputStreamReader(sock.getInputStream(), "UTF-8"));
        String filename = filenameReader.readLine();
        // GOOD: ensure that the file is in a designated folder in the user's home directory
        if (!filename.contains("..") && filename.startsWith("/home/" + user + "/public/")) {
            BufferedReader fileReader = new BufferedReader(new FileReader(filename));
            String fileLine = fileReader.readLine();
            while (fileLine != null) {
                sock.getOutputStream().write(fileLine.getBytes());
                fileLine = fileReader.readLine();
            }
        }
    }

    public void sendUserFileGood2(Socket sock, String user) throws Exception {
        BufferedReader filenameReader =
                new BufferedReader(new InputStreamReader(sock.getInputStream(), "UTF-8"));
        String filename = filenameReader.readLine();

        Path publicFolder = Paths.get("/home/" + user + "/public").normalize().toAbsolutePath();
        Path filePath = publicFolder.resolve(filename).normalize().toAbsolutePath();

        // GOOD: ensure that the path stays within the public folder
        if (!filePath.startsWith(publicFolder + File.separator)) {
            throw new IllegalArgumentException("Invalid filename");
        }
        BufferedReader fileReader = new BufferedReader(new FileReader(filePath.toString()));
        String fileLine = fileReader.readLine();
        while (fileLine != null) {
            sock.getOutputStream().write(fileLine.getBytes());
            fileLine = fileReader.readLine();
        }
    }

    public void sendUserFileGood3(Socket sock, String user) throws Exception {
        BufferedReader filenameReader =
                new BufferedReader(new InputStreamReader(sock.getInputStream(), "UTF-8"));
        String filename = filenameReader.readLine();
        // GOOD: ensure that the filename has no path separators or parent directory references
        if (filename.contains("..") || filename.contains("/") || filename.contains("\\")) {
            throw new IllegalArgumentException("Invalid filename");
        }
        BufferedReader fileReader = new BufferedReader(new FileReader(filename));
        String fileLine = fileReader.readLine();
        while (fileLine != null) {
            sock.getOutputStream().write(fileLine.getBytes());
            fileLine = fileReader.readLine();
        }
    }

    public void sendUserFileGood5(Socket sock, String user) throws Exception {
        BufferedReader filenameReader =
                new BufferedReader(new InputStreamReader(sock.getInputStream(), "UTF-8"));
        String filename = filenameReader.readLine();

        Path publicFolder = Paths.get("/home/" + user + "/public").toRealPath();
        Path filePath = publicFolder.resolve(filename).toRealPath();

        // GOOD: toRealPath() normalizes the path (resolves ".." and symlinks),
        // equivalent to File.getCanonicalPath()
        if (!filePath.startsWith(publicFolder + File.separator)) {
            throw new IllegalArgumentException("Invalid filename");
        }
        BufferedReader fileReader = new BufferedReader(new FileReader(filePath.toString()));
        String fileLine = fileReader.readLine();
        while (fileLine != null) {
            sock.getOutputStream().write(fileLine.getBytes());
            fileLine = fileReader.readLine();
        }
    }

    public void sendUserFileGood4(Socket sock, String user) throws IOException {
        BufferedReader filenameReader =
                new BufferedReader(new InputStreamReader(sock.getInputStream(), "UTF-8"));
        String filename = filenameReader.readLine();
        File file = new File(filename);
        String baseName = file.getName();
        // GOOD: only use the final component of the user provided path
        BufferedReader fileReader = new BufferedReader(new FileReader(baseName));
        String fileLine = fileReader.readLine();
        while (fileLine != null) {
            sock.getOutputStream().write(fileLine.getBytes());
            fileLine = fileReader.readLine();
        }
    }
}