Is it possible to easy get normal (deobfuscated) access to all files in a sandbox written using FileSystem API?

问题

I used Filesystems API to write to a new file in a sandboxed storage of Chrome:

preparing the FS:

window.requestFileSystem  = window.requestFileSystem || window.webkitRequestFileSystem;

function errorHandler(e) {
  var msg = '';

  switch (e.code) {
    case FileError.QUOTA_EXCEEDED_ERR:
      msg = 'QUOTA_EXCEEDED_ERR';
      break;
    case FileError.NOT_FOUND_ERR:
      msg = 'NOT_FOUND_ERR';
      break;
    case FileError.SECURITY_ERR:
      msg = 'SECURITY_ERR';
      break;
    case FileError.INVALID_MODIFICATION_ERR:
      msg = 'INVALID_MODIFICATION_ERR';
      break;
    case FileError.INVALID_STATE_ERR:
      msg = 'INVALID_STATE_ERR';
      break;
    default:
      msg = 'Unknown Error';
      break;
  };

  console.log('Error: ' + msg);
}

var fileSystem;

function onInitFs(fs) {
  console.log('Opened file system: ' + fs.name);
  fileSystem = fs;
}

navigator.webkitPersistentStorage.requestQuota(1024*1024, 
  function(gB){
    window.requestFileSystem(PERSISTENT, gB, onInitFs, errorHandler);
  }, function(e){
    console.log('Error', e);
})

writing the file:

fileSystem.root.getFile('log.txt', {create: true}, function(fileEntry) {

    // Create a FileWriter object for our FileEntry (log.txt).
    fileEntry.createWriter(function(fileWriter) {

      fileWriter.onwriteend = function(e) {
        console.log('Write completed.');
      };

      fileWriter.onerror = function(e) {
        console.log('Write failed: ' + e.toString());
      };

      // Create a new Blob and write it to log.txt.
      var blob = new Blob(['Lorem Ipsum'], {type: 'text/plain'});

      fileWriter.write(blob);

    }, errorHandler);

}, errorHandler);

So afterwards I found a new File in the ./ChromeFolder/FileSystem/003/p/00/00000000 with the Lorem Ipsum content (reading it with a hex-editor).

I thought that I could access the sandboxed FS as a normal mounted FS, so that I have normal File and Directories names. Instead I see some obfuscated file names (00000000 instead of expected log.txt) and not the structure I expected.

Like this:

Is it possible to access this Sandboxed FS as a normal FS, so I could manage all files as I create them in the Chrome using the FileSystems API (I mean structure and file names) or is it impossible and it stays obfuscated for the outside of the Chrome?

Are there any tricks, any flag changes in Chrome to get what I expected?

回答1:

As with many "How come I can't ______?" sort of questions, the answer is "Security." And the short answer to your question is "no." The File System API was specifically designed solely as a method for web clients (e.g., browsers) to give developers a file system-like storage structure only through the API not from the outside.

Section "4.3 Security Considerations" of the API specification addresses exactly the behavior you're attempting. If the client were to store the raw files with their actual file names (e.g., "FinanceReport.doc") then it would make things much easier for malicious software on a compromised machine to locate and exploit sensitive data stored through the File System API. Also, if the actual file name were used, then it could make those files executable, such as storing "EvilActions.exe" on the local file system with that name. (Note: Some clients, such as Chrome, won't even let you store executables.) These are some of the reasons you see the obfuscation of file and storage. In fact, the API does not explicitly specify how the client should store the data, only that local storage raises security concerns that clients should address.

I recently completed a full scale security assessment of HTML5, including the File System API. I assure you that as the API and client implementation if the API both mature, you will almost certainly see further measures to block or at least obfuscate locally stored data vis-a-vis access from outside the client. To further bolster local storage security, clients may even shift toward storing the whole thing as one big file, similar to how MS Access uses a .mdb file for storage. Again, it's entirely up to the client. Because you're attempting a sort of backdoor access to the data/files apart from the API, and doing so in an manner specifically called out as a "security concern" in the API, it's likely that any "solution" you employ today may fail tomorrow as client security matures. If you can do it for legitimate purposes, malware authors can do is for malicious purposes, and client manufacturers will do what they can to prevent that.

来源：https://stackoverflow.com/questions/19208984/is-it-possible-to-easy-get-normal-deobfuscated-access-to-all-files-in-a-sandbo