6 Below is a list of known issues in recent releases of Tahoe-LAFS, and how to
7 manage them. The current version of this file can be found at
8 https://tahoe-lafs.org/source/tahoe-lafs/trunk/docs/known_issues.rst .
10 If you've been using Tahoe-LAFS since v1.1 (released 2008-06-11) or if you're
11 just curious about what sort of mistakes we've made in the past, then you might
12 want to read `the "historical known issues" document`_.
14 .. _the "historical known issues" document: historical/historical_known_issues.txt
17 Known Issues in Tahoe-LAFS v1.9.0, released 31-Oct-2011
18 =======================================================
20 * `Potential unauthorized access by JavaScript in unrelated files`_
21 * `Potential disclosure of file through embedded hyperlinks or JavaScript in that file`_
22 * `Command-line arguments are leaked to other local users`_
23 * `Capabilities may be leaked to web browser phishing filter / "safe browsing" servers`_
24 * `Known issues in the FTP and SFTP frontends`_
25 * `Traffic analysis based on sizes of files/directories, storage indices, and timing`_
29 Potential unauthorized access by JavaScript in unrelated files
30 --------------------------------------------------------------
32 If you view a file stored in Tahoe-LAFS through a web user interface,
33 JavaScript embedded in that file might be able to access other files or
34 directories stored in Tahoe-LAFS which you view through the same web
35 user interface. Such a script would be able to send the contents of
36 those other files or directories to the author of the script, and if you
37 have the ability to modify the contents of those files or directories,
38 then that script could modify or delete those files or directories.
42 For future versions of Tahoe-LAFS, we are considering ways to close off
43 this leakage of authority while preserving ease of use -- the discussion
44 of this issue is ticket `#615`_.
46 For the present, either do not view files stored in Tahoe-LAFS through a
47 web user interface, or turn off JavaScript in your web browser before
48 doing so, or limit your viewing to files which you know don't contain
51 .. _#615: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/615
56 Potential disclosure of file through embedded hyperlinks or JavaScript in that file
57 -----------------------------------------------------------------------------------
59 If there is a file stored on a Tahoe-LAFS storage grid, and that file
60 gets downloaded and displayed in a web browser, then JavaScript or
61 hyperlinks within that file can leak the capability to that file to a
62 third party, which means that third party gets access to the file.
64 If there is JavaScript in the file, then it could deliberately leak
65 the capability to the file out to some remote listener.
67 If there are hyperlinks in the file, and they get followed, then
68 whichever server they point to receives the capability to the
69 file. Note that IMG tags are typically followed automatically by web
70 browsers, so being careful which hyperlinks you click on is not
71 sufficient to prevent this from happening.
75 For future versions of Tahoe-LAFS, we are considering ways to close off
76 this leakage of authority while preserving ease of use -- the discussion
77 of this issue is ticket `#127`_.
79 For the present, a good work-around is that if you want to store and
80 view a file on Tahoe-LAFS and you want that file to remain private, then
81 remove from that file any hyperlinks pointing to other people's servers
82 and remove any JavaScript unless you are sure that the JavaScript is not
83 written to maliciously leak access.
85 .. _#127: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/127
90 Command-line arguments are leaked to other local users
91 ------------------------------------------------------
93 Remember that command-line arguments are visible to other users (through
94 the 'ps' command, or the windows Process Explorer tool), so if you are
95 using a Tahoe-LAFS node on a shared host, other users on that host will
96 be able to see (and copy) any caps that you pass as command-line
97 arguments. This includes directory caps that you set up with the "tahoe
102 As of Tahoe-LAFS v1.3.0 there is a "tahoe create-alias" command that does
103 the following technique for you.
105 Bypass add-alias and edit the NODEDIR/private/aliases file directly, by
106 adding a line like this:
108 fun: URI:DIR2:ovjy4yhylqlfoqg2vcze36dhde:4d4f47qko2xm5g7osgo2yyidi5m4muyo2vjjy53q4vjju2u55mfa
110 By entering the dircap through the editor, the command-line arguments
111 are bypassed, and other users will not be able to see them. Once you've
112 added the alias, if you use that alias instead of a cap itself on the
113 command-line, then no secrets are passed through the command line. Then
114 other processes on the system can still see your filenames and other
115 arguments you type there, but not the caps that Tahoe-LAFS uses to permit
116 access to your files and directories.
121 Capabilities may be leaked to web browser phishing filter / "safe browsing" servers
122 -----------------------------------------------------------------------------------
124 Firefox, Internet Explorer, and Chrome include a "phishing filter" or
125 "safe browing" component, which is turned on by default, and which sends
126 any URLs that it deems suspicious to a central server.
128 Microsoft gives `a brief description of their filter's operation`_. Firefox
129 and Chrome both use Google's `"safe browsing API"`_ (`specification`_).
131 This of course has implications for the privacy of general web browsing
132 (especially in the cases of Firefox and Chrome, which send your main
133 personally identifying Google cookie along with these requests without your
134 explicit consent, as described in `Firefox bugzilla ticket #368255`_.
136 The reason for documenting this issue here, though, is that when using the
137 Tahoe-LAFS web user interface, it could also affect confidentiality and integrity
138 by leaking capabilities to the filter server.
140 Since IE's filter sends URLs by SSL/TLS, the exposure of caps is limited to
141 the filter server operators (or anyone able to hack the filter server) rather
142 than to network eavesdroppers. The "safe browsing API" protocol used by
143 Firefox and Chrome, on the other hand, is *not* encrypted, although the
144 URL components are normally hashed.
146 Opera also has a similar facility that is disabled by default. A previous
147 version of this file stated that Firefox had abandoned their phishing
148 filter; this was incorrect.
150 .. _a brief description of their filter's operation: http://blogs.msdn.com/ie/archive/2005/09/09/463204.aspx
151 .. _"safe browsing API": http://code.google.com/apis/safebrowsing/
152 .. _specification: http://code.google.com/p/google-safe-browsing/wiki/Protocolv2Spec
153 .. _Firefox bugzilla ticket #368255: https://bugzilla.mozilla.org/show_bug.cgi?id=368255
158 If you use any phishing filter or "safe browsing" feature, consider either
159 disabling it, or not using the WUI via that browser. Phishing filters have
160 `very limited effectiveness`_ , and phishing or malware attackers have learnt
163 .. _very limited effectiveness: http://lorrie.cranor.org/pubs/ndss-phish-tools-final.pdf
165 To disable the filter in IE7 or IE8:
166 ++++++++++++++++++++++++++++++++++++
168 - Click Internet Options from the Tools menu.
170 - Click the Advanced tab.
172 - If an "Enable SmartScreen Filter" option is present, uncheck it.
173 If a "Use Phishing Filter" or "Phishing Filter" option is present,
176 - Confirm (click OK or Yes) out of all dialogs.
178 If you have a version of IE that splits the settings between security
179 zones, do this for all zones.
181 To disable the filter in Firefox:
182 +++++++++++++++++++++++++++++++++
184 - Click Options from the Tools menu.
186 - Click the Security tab.
188 - Uncheck both the "Block reported attack sites" and "Block reported
189 web forgeries" options.
193 To disable the filter in Chrome:
194 ++++++++++++++++++++++++++++++++
196 - Click Options from the Tools menu.
198 - Click the "Under the Hood" tab and find the "Privacy" section.
200 - Uncheck the "Enable phishing and malware protection" option.
207 Known issues in the FTP and SFTP frontends
208 ------------------------------------------
210 These are documented in `docs/frontends/FTP-and-SFTP.rst`_ and on `the SftpFrontend page`_ on the wiki.
212 .. _docs/frontends/FTP-and-SFTP.rst: frontends/FTP-and-SFTP.rst
213 .. _the SftpFrontend page: https://tahoe-lafs.org/trac/tahoe-lafs/wiki/SftpFrontend
218 Traffic analysis based on sizes of files/directories, storage indices, and timing
219 ---------------------------------------------------------------------------------
221 Files and directories stored by Tahoe-LAFS are encrypted, but the ciphertext
222 reveals the exact size of the original file or directory representation.
223 This information is available to passive eavesdroppers and to server operators.
225 For example, a large data set with known file sizes could probably be
226 identified with a high degree of confidence.
228 Uploads and downloads of the same file or directory can be linked by server
229 operators, even without making assumptions based on file size. Anyone who
230 knows the introducer furl for a grid may be able to act as a server operator.
231 This implies that if such an attacker knows which file/directory is being
232 accessed in a particular request (by some other form of surveillance, say),
233 then they can identify later or earlier accesses of the same file/directory.
235 Observing requests during a directory traversal (such as a deep-check
236 operation) could reveal information about the directory structure, i.e.
237 which files and subdirectories are linked from a given directory.
239 Attackers can combine the above information with inferences based on timing
240 correlations. For instance, two files that are accessed close together in
241 time are likely to be related even if they are not linked in the directory
242 structure. Also, users that access the same files may be related to each other.
247 Known Issues in Tahoe-LAFS v1.8.2, released 30-Jan-2011
248 =======================================================
251 Unauthorized deletion of an immutable file by its storage index
252 ---------------------------------------------------------------
254 Due to a flaw in the Tahoe-LAFS storage server software in v1.3.0 through
255 v1.8.2, a person who knows the "storage index" that identifies an immutable
256 file can cause the server to delete its shares of that file.
258 If an attacker can cause enough shares to be deleted from enough storage
259 servers, this deletes the file.
261 This vulnerability does not enable anyone to read file contents without
262 authorization (confidentiality), nor to change the contents of a file
265 A person could learn the storage index of a file in several ways:
267 1. By being granted the authority to read the immutable file—i.e. by being
268 granted a read capability to the file. They can determine the file's
269 storage index from its read capability.
271 2. By being granted a verify capability to the file. They can determine the
272 file's storage index from its verify capability. This case probably
273 doesn't happen often because users typically don't share verify caps.
275 3. By operating a storage server, and receiving a request from a client that
276 has a read cap or a verify cap. If the client attempts to upload,
277 download, or verify the file with their storage server, even if it doesn't
278 actually have the file, then they can learn the storage index of the file.
280 4. By gaining read access to an existing storage server's local filesystem,
281 and inspecting the directory structure that it stores its shares in. They
282 can thus learn the storage indexes of all files that the server is holding
283 at least one share of. Normally only the operator of an existing storage
284 server would be able to inspect its local filesystem, so this requires
285 either being such an operator of an existing storage server, or somehow
286 gaining the ability to inspect the local filesystem of an existing storage
291 Tahoe-LAFS version v1.8.3 or newer (except v1.9a1) no longer has this flaw;
292 if you upgrade a storage server to a fixed release then that server is no
293 longer vulnerable to this problem.
295 Note that the issue is local to each storage server independently of other
296 storage servers—when you upgrade a storage server then that particular
297 storage server can no longer be tricked into deleting its shares of the
300 If you can't immediately upgrade your storage server to a version of
301 Tahoe-LAFS that eliminates this vulnerability, then you could temporarily
302 shut down your storage server. This would of course negatively impact
303 availability—clients would not be able to upload or download shares to that
304 particular storage server while it was shut down—but it would protect the
305 shares already stored on that server from being deleted as long as the server
308 If the servers that store shares of your file are running a version of
309 Tahoe-LAFS with this vulnerability, then you should think about whether
310 someone can learn the storage indexes of your files by one of the methods
311 described above. A person can not exploit this vulnerability unless they have
312 received a read cap or verify cap, or they control a storage server that has
313 been queried about this file by a client that has a read cap or a verify cap.
315 Tahoe-LAFS does not currently have a mechanism to limit which storage servers
316 can connect to your grid, but it does have a way to see which storage servers
317 have been connected to the grid. The Introducer's front page in the Web User
318 Interface has a list of all storage servers that the Introducer has ever seen
319 and the first time and the most recent time that it saw them. Each Tahoe-LAFS
320 gateway maintains a similar list on its front page in its Web User Interface,
321 showing all of the storage servers that it learned about from the Introducer,
322 when it first connected to that storage server, and when it most recently
323 connected to that storage server. These lists are stored in memory and are
324 reset to empty when the process is restarted.
326 See ticket `#1528`_ for technical details.
328 .. _#1528: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1528