Most Robust way of reading a file or stream using Java (to prevent DoS attacks)

Question

Currently I have the below code for reading an InputStream. I am storing the whole file into a StringBuilder variable and processing this string afterwards.

public static String getContentFromInputStream(InputStream inputStream)
// public static String getContentFromInputStream(InputStream inputStream,
// int maxLineSize, int maxFileSize)
{

    StringBuilder stringBuilder = new StringBuilder();
    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
    String lineSeparator = System.getProperty("line.separator");
    String fileLine;

    boolean firstLine = true;
    try {
        // Expect some function which checks for line size limit.
        // eg: reading character by character to an char array and checking for
        // linesize in a loop until line feed is encountered.
        // if max line size limit is passed then throw an exception
        // if a line feed is encountered append the char array to a StringBuilder
        // after appending check the size of the StringBuilder
        // if file size exceeds the max file limit then throw an exception

        fileLine = bufferedReader.readLine();

        while (fileLine != null) {
            if (!firstLine) stringBuilder.append(lineSeparator);
            stringBuilder.append(fileLine);
            fileLine = bufferedReader.readLine();
            firstLine = false;
        }
    } catch (IOException e) {
        //TODO : throw or handle the exception
    }
    //TODO : close the stream

    return stringBuilder.toString();

}

The code went for a review with the Security team and the following comments were received:

1. BufferedReader.readLine is susceptible to DOS (Denial of Service) attacks (line of infinite length, huge file containing no line feed/carriage return)

2. Resource exhaustion for the StringBuilder variable (cases when a file containing data greater than the available memory)

Below are the solutions I could think of:

1. Create an alternate implementation of readLine method (readLine(int limit)), which checks for the no. of bytes read and if it exceeds the specified limit, throw a custom exception.

2. Process the file line by line without loading the file in entirety. (pure non-Java solution :) )

Please suggest if there are any existing libraries which implement the above solutions. Also suggest any alternate solutions which offer more robustness or are more convenient to implement than the proposed ones. Though performance is also a major requirement, security comes first.

Answer 1

Updated Answer

You want to avoid all sorts of DOS attacks (on lines, on size of the file, etc). But in the end of the function, you're trying to convert the entire file into one single String!!! Assume that you limit the line to 8 KB, but what happens if somebody sends you a file with two 8 KB lines? The line reading part will pass, but when finally you combine everything into a single string, the String will choke all available memory.

So since finally you're converting everything into one single String, limiting line size doesn't matter, nor is safe. You have to limit the entire size of the file.

Secondly, what you're basically trying to do is, you're trying to read data in chunks. So you're using BufferedReader and reading it line-by-line. But what you're trying to do, and what you really want at the end - is some way of reading the file piece by piece. Instead of reading one line at a time, why not instead read 2 KB at a time?

BufferedReader - by its name - has a buffer inside it. You can configure that buffer. Let's say you create a BufferedReader with buffer size of 2 KB:

BufferedReader reader = new BufferedReader(..., 2048);

Now if the InputStream that you pass to BufferedReader has 100 KB of data, BufferedReader will automatically read it 2 KB at at time. So it will read the stream 50 times, 2 KB each (50x2KB = 100 KB). Similarly, if you create BufferedReader with a 10 KB buffer size, it will read the input 10 times (10x10KB = 100 KB).

BufferedReader already does the work of reading your file chunk-by-chunk. So you don't want to add an extra layer of line-by-line above it. Just focus on the end result - if your file at the end is too big (> available RAM) - how are you going to convert it into a String at the end?

One better way is to just pass things around as a CharSequence. That's what Android does. Throughout the Android APIs, you will see that they return CharSequence everywhere. Since StringBuilder is also a subclass of CharSequence, Android will internally use either a String, or a StringBuilder or some other optimized string class based on the size/nature of input. So you could rather directly return the StringBuilder object itself once you've read everything, rather than converting it to a String. This would be safer against large data. StringBuilder also maintains the same concept of buffers inside it, and it will internally allocate multiple buffers for large strings, rather than one long string.

So overall:

• Limit the overall file size since you're going to deal with the entire content at some point. Forget about limiting or splitting lines
• Read in chunks

Using Apache Commons IO, here is how you would read data from a BoundedInputStream into a StringBuilder, splitting by 2 KB blocks instead of lines:

// import org.apache.commons.io.output.StringBuilderWriter;
// import org.apache.commons.io.input.BoundedInputStream;
// import org.apache.commons.io.IOUtils;

BoundedInputStream boundedInput = new BoundedInputStream(originalInput, <max-file-size>);
BufferedReader reader = new BufferedReader(new InputStreamReader(boundedInput), 2048);

StringBuilder output = new StringBuilder();
StringBuilderWriter writer = new StringBuilderWriter(output);

IOUtils.copy(reader, writer); // copies data from "reader" => "writer"
return output;

Original Answer

Use BoundedInputStream from Apache Commons IO library. Your work becomes much more easier.

The following code will do what you want:

public static String getContentFromInputStream(InputStream inputStream) {
  inputStream = new BoundedInputStream(inputStream, <number-of-bytes>);
  // Rest code are all same

You just simply wrap your InputStream with a BoundedInputStream and you specify a maximum size. BoundedInputStream will take care of limiting reads up to that maximum size.

Or you can do this when you're creating the reader:

BufferedReader bufferedReader = new BufferedReader(
  new InputStreamReader(
    new BoundedInputStream(inputStream, <no-of-bytes>)
  )
);

Basically what we're doing here is, we're limiting the read size at the InputStream layer itself, rather than doing that when reading lines. So you end up with a reusable component like BoundedInputStream which limits reading at the InputStream layer, and you can use that wherever you want.

Edit: Added footnote

Edit 2: Added updated answer based on comments