Date: Wed, 19 May 2010 03:42:29 +0400 From: Solar Designer <solar@...nwall.com> To: oss-security@...ts.openwall.com Subject: Re: [oCERT-2010-001] multiple http client unexpected download filename vulnerability On Tue, May 18, 2010 at 09:50:27AM +0200, Ludwig Nussel wrote: > wget doesn't overwrite existing files by default anyways. Instead it appends a > suffix .1, .2 etc to the newly downloaded file. Well, a server can sometimes override that default - please see below. > wget also prints the file name it used. This is of limited help - and for interactive uses only. I am mostly concerned about uses from cron jobs and the like. > So IMO it's perfectly fine and useful for wget to take the server > provided file name by default. I disagree. Uses from scripts and cron jobs are too common, and they often don't care to specify an output filename explicitly. Let's suppose there's a cron job like this: 1 * * * * wget http://www.openwall.com/pvt/wget/log &> /dev/null If the server is malicious or compromised, it can have: RedirectMatch log $1/pvt/wget/.wgetrc in .htaccess, and reject=; exec id output-document=.bash_profile in .wgetrc. When the cron job runs for the first time after the above changes made on the server, it does: 02:01:02 (2.64 MB/s) - `.wgetrc' saved [47/47] At this point, .wgetrc is on the client system. The second time the cron job runs, it does: 03:01:02 (2.99 MB/s) - `.bash_profile' saved [47/47] This has happily overwritten my .bash_profile file. (I replaced "/dev/null" in the cron job with another filename for obtaining these wget output lines.) When I am logging in to the affected account, I get the output of "id". Of course, the shell command could as well be nastier than that. Although I used a somewhat tricky approach in the above exploit, eventually making wget overwrite a file, it is also possible to mount attacks that do not rely on overwriting any files. Many programs support optional startup/config files of fixed/known/guessable names that a malicious or compromised server could provide. In fact, I've just demonstrated this attack against wget itself, but it could also work against another program. Is this more convincing now? Alexander
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