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Date: Mon, 22 Feb 2021 17:58:40 +0100
From: Oswald Buddenhagen <oswald.buddenhagen@....de>
To: isync-devel@...ts.sourceforge.net
Cc: oss-security@...ts.openwall.com
Subject: CVE-2021-20247: isync/mbsync data leak/destruction vulnerability
description:
mbsync didn't validate the mailbox names returned by IMAP LIST/LSUB,
which would allow a malicious/compromised server to use specially
crafted mailbox names containing '..' path components to access data
outside the designated mailbox on the opposite end of the
synchronization channel. gory details follow below.
the attack vector is rather narrow, but the effects can be disastrous.
the vulnerability has been there "forever", though it wasn't of much
concern prior to 1.3 used with a specific configuration.
mitigation:
upgrade to the freshly released v1.3.5 or v1.4.1 available from
https://sourceforge.net/projects/isync/files/isync/ , or apply one of
the attached patches (patches for earlier versions can be produced
easily, should anyone care).
credit:
the possible existence of the vulnerability was suggested by a user who
does not wish to be credited. :-D
vulnerability details:
- the victim must be using the Pattern channel option containing the '*'
wildcard. this is fairly likely (even though only those who actually
use hierarchical mailboxes (presumably a minority) actually need that
- others may use the '%' wildcard).
- if the opposite end is also an IMAP server (which is presumed to be
rare), the exact impact depends on the server. most servers will
expose only a very restricted amount of data to any particular user,
and will likely reject weird paths. also, most servers use '.' as the
hierarchy delimiter, which would make mbsync reject the crafted paths
as non-representable after delimiter translation. however,
uw-imap/panda-imap for example would be vulnerable, as it basically
just exposes the file system via IMAP.
- the much more common case is the opposite end being a local Maildir
store, which is somewhat similar to the uw-imap case:
- users who don't actually use hierarchical mailboxes locally are
usually not vulnerable, as they won't set the SubFolders option,
which will make mbsync reject any mailbox names containing
hierarchy delimiters. (not applicable before v1.3.)
- if SubFolders is Maildir++, no attack is possible, as periods are
hierarchy delimiters and are consequently rejected by the
translation. (not applicable before v1.3.)
- if SubFolders is Legacy, an attack is limited to hidden
directories, as a '.' is prepended to each subfolder when mapping
to file system paths.
- if SubFolders is Verbatim, exposure is unlimited. (not applicable
before v1.3.)
- the most likely target are Maildir folders that are synchronized to
other servers (e.g., work vs. private mail stores)
- if the victim is using '*' for the SyncState option (which most
users seem to do judging by support requests; the example config
file suggests it), all previously synchronized messages will be
deleted from that folder (that this happens is a seperate bug
that v1.4.1 also fixes), while new messages will be stolen.
- otherwise, all messages from that folder will be stolen. i
consider this the main danger of this vulnerability.
- non-Maildir paths can be attacked only if they end with one of
{cur,new,tmp}, as this is imposed by the expected Maildir structure.
- if no 'cur' is present, the victim must have the Create option set
for that end of the channel (this is likely).
- all files from 'tmp' will be deleted
- all files from 'cur' and 'new' will be stolen, and in the process
renamed to add some Maildir "decorations"
- subdirectories are not affected
- in principle the attacker can deposit dangerous files, but these
will also have "weird" names that cannot be influenced much, so
they are unlikely to pose an actual threat. (general content
attacks are neglected here, as they don't require this
vulnerability to be executed.)
- the attacked paths must be guessed or known in advance. if guessing
is used and the victim has Create enabled, every attempted target
path will be actually created, so the attacker will leave rather
obvious traces, and might be even noticed before landing a single
hit.
- users who run mbsync interactively in verbose mode will likely spot
an attack immediately (this is presumed to be rare; cron jobs are
more likely).
i got an NVSS score of 'high', but there are lots of caveats that would
qualify as mitigating factors if the criteria are not interpreted quite
as literally (the case of a malicious server does not seem to fit very
well).
View attachment "reject-funny-mailbox-names--1.3.patch" of type "text/x-diff" (2222 bytes)
View attachment "reject-funny-mailbox-names--1.4.patch" of type "text/x-diff" (2116 bytes)
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