format string %n vulnerability

[S3cur|ty4ng31]

Ive been reading some safe programming techniques and theres a section on fromat strings.
basically is says that keep format strings like this out



printf(user);
//where user was supplied by the user from the command line

it says this can be expoitable or crashable with %n

Ive never really used the %n can someone tell me what that is and how that could make a program exploitable

[OverdueSpy]

Google Search "%n programming vulnerability"

http://lists.suse.com/archives/suse-...-Sep/0030.html

[S3cur|ty4ng31]

well I actually did search google already and did not get what I wanted. The problem with google is that when I put %n it doesnt respect the the % I even tried "%n" or "\%n" but I get a ton of google pages with just \n

So thats kinda why I posted here

maybe my question should have been using google how do i make sure the string '%n' is searched for
also I looked at the man page for printf and theres really nothing about %n

Also Im not just looking for what the vulnerablity is but also about what %n does with printf

[OverdueSpy]

This is a PDF white paper touting FormatGuard, but it contains some good information on Printf vulnerabilities.

http://www.immunix.org/formatguard.pdf

[Maestr0]

This is a fairly complex exploit method, but alas I will attempt to get to the crux of the matter. The "%n" format string writes the number of bytes already printed to a variable, which in itself is not a bad thing but this is the meat of a pure format string vuln. Without the "%n" or combining a format vuln with another technique (standard overflow) you are limited to reading memory only, granted you can read just about ANY memory you want which is pretty disasterous already but the "%n" actually WRITES to an address which is placed on the stack as a parameter (yipee!) and allows a savy coder to perform some very ingenious trickery in order to write the desired data to an arbitrary location in memory (of course writting to unmapped memory wont turn out well.). Of course this isnt the only format parameter used, the others %d %s %x %u are all used to adjust the stack pointer according to the coders purpose by abusing how format strings and their parameters are stored on the stack.

-Maestr0

[cyclops07]

even though i have used printf often, i didn't realize there are vulnerabilities..
hmm.. i think i'll try to explore it.. thanks for da info..

[SirDice]

You should probably read this paper by team Teso.
Also check out this post by The_Jinx.