Linux 'less' can probably get you owned

2014.11.24
Risk: High
Local: Yes
Remote: No
CVE: N/A
CWE: N/A

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. Some other folks have expressed their disbelief in the past: https://twitter.com/solardiz/status/526008283690455040 But even ignoring the most suspect tools and 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. In a thread yesterday, people immediately started pointing out other issues, say: http://seclists.org/oss-sec/2014/q4/773 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. At this point, my best advice would be for users to unset LESSOPEN and LESSCLOSE if set by their distros. /mz ------------------------ $ mkdir dir/ $ genisoimage -o empty.iso dir/ 2> /dev/null $ time zzuf -qcs: isoinfo -i empty.iso zzuf[s=48,r=0.004]: signal 11 (SIGSEGV) real 0m0.210s user 0m0.084s sys 0m0.008s :-) ------------------------

References:

http://cxsecurity.com/issue/WLB-2014110157
http://seclists.org/oss-sec/2014/q4/773
http://seclists.org/fulldisclosure/2014/Nov/74
https://twitter.com/solardiz/status/526008283690455040
http://lcamtuf.coredump.cx/afl/vulns/lesspipe-cpio-bad-write.cpio


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