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AVG Mobilation for Windows Phone

Thursday, September 8, 2011

A new app hit the Windows Phone marketplace today that claims to keep your device safe from malware. I immediately became interested in it because:

  1. I don’t know of any malware for the Windows Phone.
  2. Even if there was malware that misuses some kind of hole in the Windows Phone security model, this app wouldn’t be able to catch it because of phone’s application model (unless the app itself uses some kind of security hole).
  3. After installing it, it claimed it offers real time protection that would suggest it’s capable of running in the background.

I would consider it a joke app, if it didn’t come from a well-known antivirus company. (Spoiler: It actually is a joke app, but the joke is on the antivirus company.)

A look inside

To satisfy my curiosity, I downloaded the XAP file of the app with Marketplace Browser and Downloader for Windows Phone 7 and opened it with Reflector. Surprise, surprise, this app was ported from Android (or at least that’s what *Droid namespace names suggest). Funny how the game has changed and instead of porting antivirus software from a Microsoft operating system to Linux, people started doing it the other way around.

The scanning UI is concentrated in the DroidSecurityPOC.Scan class and gets invoked in the OnNavigatedTo method. The OnNavigatedTo method is actually the first nugget:

    protected override void OnNavigatedTo(NavigationEventArgs e)
    {
        // uninteresting code removed
 
        // do the actual scanning, synchronously (we are scared of threads...)
        (Application.Current as App).malwareCollection.ScanContainingMedia(this.library);
 
        // simulate work in the UI even though the scanning is already completed at this point
        // people will love this
        this.StartScan();
    }

The StartScan method looks at the number of files to scan, divides 5 seconds with that number and starts a timer to update the “currently scanned” file name in the UI. Scanning will always take 5+ seconds to complete (closer to 5 seconds if you have few files to scan) and most of the time will be spent waiting for the next timer event to fire. Because all the “scanning” already happened in the ScanContainingMedia method, long time before the UI was first updated.

The scanning algorithm

The DroidSecurityPOC.Data.MalwareCollection class is where the hilarity starts. The ScanContainingMedia method is where all the “scanning” happens. It’s split up in 2 parts: scanning your picture library and scanning your music library. The method doesn’t look at anything else (but that’s not much of a surprise given a marketplace application really cannot access anything else).

At this point, I was still giving the app a chance. Maybe it’s scanning for damaged files that can trigger known exploits in music players or picture viewers. All my hopes disappeared when I looked at the code:

private void ScanContainingMedia(PictureCollection mediaFileCollectiont)
{
    // uninteresting code removed
    // malwareGroup contains a list of known "malware"
 
    // for each picture in the library
    foreach (Picture picture in mediaFileCollectiont)
    {
        // for each known malware (because HashSet is overrated)
        foreach (string str in malwareGroup.MalwareGroupList)
        {
            // compare malware name with current file name (!!!!!!!)
            // NOTE: We call ToLower() on each string to allocate a new string
            // and never cache the result. This way the garbage collector will
            // be busy picking up redundant trash and we can have some fun time
            // with his daughter.
            // Also, String.Equals(s1, s2, StringComparison.OrdinalIgnoreCase)
            // is for pussies.
            if (str.ToLower() == picture.Name.ToLower())
            {
                // uninteresting code - add malware to a collection of "effected malware"
            }
        }
    }
}

Basically, this code couldn’t be less bothered about the file contents. It only looks at the file name and if it matches the predicate, boom, it’s jailed. No questions asked. Do not pass Go. Do not collect $200.
The list of “dangerous file names” is downloaded from a web service and Rafael Rivera can show you the current “definition file”.

The code also contains an unused method that hints at a future update that will actually look at the file contents, but the method makes me really scared:

public bool ScanEicar(Picture picture)
{
    Stream image = picture.GetImage();
    image.Position = 0L;
    while ((image.Position + 0x44L) <= image.Length)
    {
        // the garbage collector still doesn't seem to be busy enough, so
        // let's allocate an array in a tight loop
        byte[] buffer = new byte[70];
        image.Read(buffer, 0, 0x44);
 
        // BLAM! Potentially triple the amount of allocated memory by allocating
        // a string with the contents of the buffer. Note each character
        // in a string takes up 2 bytes.
        // Except Convert.ToString will actually return string "System.Byte[]"
        // for each and every call. What the author probably wanted
        // is Encoding.ASCII.GetString().
        if (Convert.ToString(buffer).Contains(@"X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*"))
        {
            image.Close();
            return true;
        }
 
        // Scanning fail: the call to image.Read() already moved the position
        // by 0x44 bytes. What the author probably wanted to do is
        // image.Position -= 0x43, but if he did that, the while loop would
        // run for each byte in the file, allocating about 210 MB from the heap
        // for a 1 MB file, so the algorithm is probably better off this way.
        image.Position += 1L;
    }
    image.Close();
    return false;
}

Everything (including the release date) hints at this being some kind of a summer intern project at AVG (if it’s not, it’s very disturbing). But AVG, c’mon. Interns do all kinds of wonky stuff. You really don’t need to ship all of it…

Topics: .NET, Security | Comments Off

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