tahoe-lafs/docs/known_issues.rst

Known Issues

• Overview
• Issues in Tahoe-LAFS v1.8.2, released 2011-01-30
  • Potential unauthorized access by JavaScript in unrelated files
  • Potential disclosure of file through embedded hyperlinks or JavaScript in that file
  • Command-line arguments are leaked to other local users
  • Capabilities may be leaked to web browser phishing filter / "safe browsing" servers
  • Known issues in the FTP and SFTP frontends

Overview

Below is a list of known issues in recent releases of Tahoe-LAFS, and how to manage them. The current version of this file can be found at http://tahoe-lafs.org/source/tahoe-lafs/trunk/docs/known_issues.rst.

If you've been using Tahoe-LAFS since v1.1 (released 2008-06-11) or if you're just curious about what sort of mistakes we've made in the past, then you might want to read the "historical known issues" document.

Issues in Tahoe-LAFS v1.8.2, released 2011-01-30

Potential unauthorized access by JavaScript in unrelated files

If you view a file stored in Tahoe-LAFS through a web user interface, JavaScript embedded in that file might be able to access other files or directories stored in Tahoe-LAFS which you view through the same web user interface. Such a script would be able to send the contents of those other files or directories to the author of the script, and if you have the ability to modify the contents of those files or directories, then that script could modify or delete those files or directories.

how to manage it

For future versions of Tahoe-LAFS, we are considering ways to close off this leakage of authority while preserving ease of use -- the discussion of this issue is ticket #615.

For the present, either do not view files stored in Tahoe-LAFS through a web user interface, or turn off JavaScript in your web browser before doing so, or limit your viewing to files which you know don't contain malicious JavaScript.

Potential disclosure of file through embedded hyperlinks or JavaScript in that file

If there is a file stored on a Tahoe-LAFS storage grid, and that file gets downloaded and displayed in a web browser, then JavaScript or hyperlinks within that file can leak the capability to that file to a third party, which means that third party gets access to the file.

If there is JavaScript in the file, then it could deliberately leak the capability to the file out to some remote listener.

If there are hyperlinks in the file, and they get followed, then whichever server they point to receives the capability to the file. Note that IMG tags are typically followed automatically by web browsers, so being careful which hyperlinks you click on is not sufficient to prevent this from happening.

how to manage it

For future versions of Tahoe-LAFS, we are considering ways to close off this leakage of authority while preserving ease of use -- the discussion of this issue is ticket #127.

For the present, a good work-around is that if you want to store and view a file on Tahoe-LAFS and you want that file to remain private, then remove from that file any hyperlinks pointing to other people's servers and remove any JavaScript unless you are sure that the JavaScript is not written to maliciously leak access.

Command-line arguments are leaked to other local users

Remember that command-line arguments are visible to other users (through the 'ps' command, or the windows Process Explorer tool), so if you are using a Tahoe-LAFS node on a shared host, other users on that host will be able to see (and copy) any caps that you pass as command-line arguments. This includes directory caps that you set up with the "tahoe add-alias" command.

how to manage it

As of Tahoe-LAFS v1.3.0 there is a "tahoe create-alias" command that does the following technique for you.

Bypass add-alias and edit the NODEDIR/private/aliases file directly, by adding a line like this:

fun: URI:DIR2:ovjy4yhylqlfoqg2vcze36dhde:4d4f47qko2xm5g7osgo2yyidi5m4muyo2vjjy53q4vjju2u55mfa

By entering the dircap through the editor, the command-line arguments are bypassed, and other users will not be able to see them. Once you've added the alias, if you use that alias instead of a cap itself on the command-line, then no secrets are passed through the command line. Then other processes on the system can still see your filenames and other arguments you type there, but not the caps that Tahoe-LAFS uses to permit access to your files and directories.

Capabilities may be leaked to web browser phishing filter / "safe browsing" servers

Firefox, Internet Explorer, and Chrome include a "phishing filter" or "safe browing" component, which is turned on by default, and which sends any URLs that it deems suspicious to a central server.

Microsoft gives a brief description of their filter's operation at http://blogs.msdn.com/ie/archive/2005/09/09/463204.aspx. Firefox and Chrome both use Google's "safe browsing API" which is documented at http://code.google.com/apis/safebrowsing/ and http://code.google.com/p/google-safe-browsing/wiki/Protocolv2Spec.

This of course has implications for the privacy of general web browsing (especially in the cases of Firefox and Chrome, which send your main personally identifying Google cookie along with these requests without your explicit consent, as described in Firefox bugzilla ticket #368255).

The reason for documenting this issue here, though, is that when using the Tahoe-LAFS web user interface, it could also affect confidentiality and integrity by leaking capabilities to the filter server.

Since IE's filter sends URLs by SSL/TLS, the exposure of caps is limited to the filter server operators (or anyone able to hack the filter server) rather than to network eavesdroppers. The "safe browsing API" protocol used by Firefox and Chrome, on the other hand, is not encrypted, although the URL components are normally hashed.

Opera also has a similar facility that is disabled by default. A previous version of this file stated that Firefox had abandoned their phishing filter; this was incorrect.

how to manage it

If you use any phishing filter or "safe browsing" feature, consider either disabling it, or not using the WUI via that browser. Phishing filters have very limited effectiveness (see http://lorrie.cranor.org/pubs/ndss-phish-tools-final.pdf), and phishing or malware attackers have learnt how to bypass them.

To disable the filter in IE7 or IE8:

• Click Internet Options from the Tools menu.
• Click the Advanced tab.
• If an "Enable SmartScreen Filter" option is present, uncheck it. If a "Use Phishing Filter" or "Phishing Filter" option is present, set it to Disable.
• Confirm (click OK or Yes) out of all dialogs.

If you have a version of IE that splits the settings between security zones, do this for all zones.

To disable the filter in Firefox:

• Click Options from the Tools menu.
• Click the Security tab.
• Uncheck both the "Block reported attack sites" and "Block reported web forgeries" options.
• Click OK.

To disable the filter in Chrome:

• Click Options from the Tools menu.
• Click the "Under the Hood" tab and find the "Privacy" section.
• Uncheck the "Enable phishing and malware protection" option.
• Click Close.

Known issues in the FTP and SFTP frontends

These are documented in docs/frontends/FTP-and-SFTP.rst and at http://tahoe-lafs.org/trac/tahoe-lafs/wiki/SftpFrontend.

Traffic analysis based on sizes of files/directories, storage indices, and timing

Files and directories stored by Tahoe-LAFS are encrypted, but the ciphertext reveals the exact size of the original file or directory representation. This information is available to passive eavesdroppers and to server operators.

For example, a large data set with known file sizes could probably be identified with a high degree of confidence.

Uploads and downloads of the same file or directory can be linked by server operators, even without making assumptions based on file size. Anyone who knows the introducer furl for a grid may be able to act as a server operator. This implies that if such an attacker knows which file/directory is being accessed in a particular request (by some other form of surveillance, say), then they can identify later or earlier accesses of the same file/directory.

Observing requests during a directory traversal (such as a deep-check operation) could reveal information about the directory structure, i.e. which files and subdirectories are linked from a given directory.

Attackers can combine the above information with inferences based on timing correlations. For instance, two files that are accessed close together in time are likely to be related even if they are not linked in the directory structure. Also, users that access the same files may be related to each other.