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printbf

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Brainfuck interpreter inside printf

// repository documentation

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# printbf -- Brainfuck interpreter in printf

## Authors

* [Mathias Payer](http://nebelwelt.net)
* [Nicholas Carlini](http://nicholas.carlini.com)

## Background

Generic POSIX printf itself can be Turing complete as shown in [Control-Flow
Bending](http://nebelwelt.net/publications/#15SEC). Here we take
printf-oriented programming one step further and preset a brainfuck
interpreter inside a single printf statement.

An attacker can control a printf statement through a format string
vulnerability (where an attacker-controlled string is used as first parameter
to a printf-like statement) or if the attacker can control the first argument
to a printf statement through, e.g., a generic memory corruption. See the
disclaimer below for practical *in the wild* considerations.

Brainfuck is a Turing-complete language that has the following commands (and
their mapping to format strings):

* `>` == dataptr++  (`%1$.*1$d %2$hn`)
* `<` == dataptr--  (`%1$65535d%1$.*1$d%2$hn`)
* `+` == (\*dataptr)++  (`%3$.*3$d %4$hhn`)
* `-` == (\*dataptr)--  (`%3$255d%3$.*3$d%4$hhn` -- plus check for ovfl)
* `.` == putchar(\*dataptr)  (`%3$.*3$d%5$hn`)
* `,` == getchar(dataptr)  (`%13$.*13$d%4$hn`)
* `[` == if (\*dataptr == 0) goto `]`  (`%1$.*1$d%10$.*10$d%2$hn`)
* `]` == if (\*dataptr != 0) goto `[`  (`%1$.*1$d%10$.*10$d%2$hn`)

## Demo and sources

Have a look at the bf_pre.c sources to see what is needed to setup the
interpreter and also look at the tokenizer in toker.py.

Run make in ./src to generate a couple of sample programs (in ./src).

## Disclaimer

Keep in mind that this printbf interpreter is supposed to be a fun example of
Turing completeness that is available in current programs and not a new
generic attack vector. This demo is NOT intended to be a generic
FORTIFY_SOURCE bypass.

Current systems often either (i) disable %n (which is used to write to memory
and allowed according to the standard but rarely used in practice) or (ii)
through a set of of patches that test for attack-like conditions, e.g., if
the format string is in writable memory.

To use printbf in the wild an attacker will either have to disable
FORTIFY_SOURCE checking or get around the checks by placing lining up the
format strings and placing them in readonly memory. The FORTIFY_SOURCE
mitigations are glibc specific. The attacker model for printbf assumes that
the attacker can use memory corruption vulnerabilities to set-up the attack or
that the sources are compiled without enabled FORTIFY_SOURCE defenses.

printbf

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