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Date: Sun, 23 Nov 2014 01:24:11 -0800
From: Michal Zalewski <lcamtuf@...edump.cx>
To: oss-security <oss-security@...ts.openwall.com>
Subject: so, can we do something about lesspipe? (+ a cpio bug to back up the argument)
There have been some low-key discussions about this in the past, but...
In short, many Linux distributions ship with the 'less' command
automagically interfaced to 'lesspipe'-type scripts, usually invoked
via LESSOPEN. This is certainly the case for CentOS and Ubuntu.
Unfortunately, many of these scripts appear to call a rather large
number of third-party tools that likely have not been designed with
malicious inputs in mind. On CentOS, lesspipe appears to include
things such as groff + troff + grotty, man, and cpio. On Ubuntu,
there's isoinfo (?!), ar from binutils, and so on. Ancient and obscure
compression utilities and doc converters crop up, too.
Even grabbing something as seemingly innocuous as cpio, a short spin
with afl-fuzz (or, probably, anything else) will immediately yield
this:
http://lcamtuf.coredump.cx/afl/vulns/lesspipe-cpio-bad-write.cpio
It's a file with declared block length of 0xffffffff. That gets us
here, with the value populated to c_filesize (copyin.c, list_file()):
link_name = (char *) xmalloc ((unsigned int) file_hdr->c_filesize + 1);
link_name[file_hdr->c_filesize] = '\0';
...where we end up allocating a zero-byte buffer and then promptly
writing out of bounds (just under the buffer on 32-bit systems or
somewhere above it on 64-bit).
While it's a single bug in cpio, I have no doubt that many of the
other lesspipe programs are equally problematic or worse. The saving
grace is that lesspipe scripts make most of their routing decisions
based on file extensions. Alas, many of these extensions will be
completely alien and meaningless to all but the most seasoned users
(.cpi, .raw, .r42, .ear, .zoo, .a). And there are some instances of
utilities being called on * (e.g., iconv, fileutils).
Ultimately, I think that there's an expectation that running less on a
downloaded file won't lead to RCE, and the lesspipe behavior in many
distros is almost certainly violating that. I'm also not sure if the
automation actually scratches any real itch - I doubt that people try
to run 'less' on CD images or ar archives when knowingly working with
files of that sort.
WDYT?
/mz
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