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Date: Tue, 2 May 2017 00:05:27 +0200
From: Robert Święcki <>
Subject: Re: terminal emulators' processing of escape sequences

2017-05-01 23:13 GMT+02:00 Michal Zalewski <>:
> > Besides (mis)features, there may also be implementation bugs.
> It is perhaps worth noting that guided fuzzing has been used in this
> space with good results, too. For example, AFL was credited on at
> least the following in rxvt, tmux, screen, and mosh:
> Especially if what's highlighted in this thread can be found with a
> simple script, I'm betting there's far more beneath the surface.
> Guided fuzzers have the advantage of being able to discover features
> that may be undocumented or hard to spot, so a more comprehensive dive
> into all the terminal emulators in use today would probably be quite
> fruitful

On a slightly different note; memory corruption/abort() problems might
end up as RCE with some effort, but what *is* RCE is ability to push
back characters into terminal's input buffer. There are some
well-known vectors, like setting title of the current terminal and
printing it back with ESC codes, and hopefully it's something that is
mitigated in all modern terminal emulator software packages for many
years now.

But, it's not something that can be discovered simply by waiting for
SEGV and similar signals. Hence, I'd like to encourage everyone
looking for bugs in terminal emulators to add some form of
instrumentation to their fuzz setups aimed at finding such problems

A harmless example from rxvt - pushing back the new-line character:

$ echo -ne "\eGQ;"
;$ 0
bash: 0: command not found

Robert Święcki

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