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Date: Fri, 03 May 2024 00:39:12 +0200
From: Steffen Nurpmeso <steffen@...oden.eu>
To: Sam James <sam@...too.org>
Cc: Solar Designer <solar@...nwall.com>, oss-security@...ts.openwall.com
Subject: Re: escaping terminal control characters (was
Re: backdoor in upstream xz/liblzma leading to ssh server compromise)
Sam James wrote in
<87o79nlwxl.fsf@...too.org>:
|Solar Designer <solar@...nwall.com> writes:
|> On Wed, Apr 03, 2024 at 11:03:17AM +1100, Matthew Fernandez wrote:
|>> On 4/1/24 08:30, Solar Designer wrote:
|>>>On Sat, Mar 30, 2024 at 04:37:48PM -0000, Tavis Ormandy wrote:
|>>>>It was also pointed out they submitted an odd PR to libarchive:
|>>>>
|>>>>https://github.com/libarchive/libarchive/pull/1609
|>>>>
|>>>>In summary, they replaced calls to safe_fprintf() with fprintf() --
|>>>>meaning control characters are no longer filtered from errors. That
|>>>>seems pretty minor, but now that we know they were in the business of
|>>>>obfuscating the presence of backdoors -- seems a bit suspicious.
|>>>>
|>>>>Regardless, that change has now been reverted:
|>>>>
|>>>>https://github.com/libarchive/libarchive/pull/2101
|>>>
|>>>This does look minor indeed - not usable for large-scale attacks, and
|>>>libarchive is quite unique in that it even bothered to filter control
|>>>characters, whereas most command-line tools outputting filenames don't
|>>>bother. My guess is it could have been an early experiment to see
|>>>whether the project would accept PRs degrading security.
|>>>
|>>>That said, here's an excellent write-up by David Leadbeater on specific
|>>>ways that specific terminal emulators may be usefully attacked with
|>>>control sequences:
|>>>
|>>>https://dgl.cx/2023/09/ansi-terminal-security#vulnerabilities-using-know\
|>>>n-replies
|>>
|>> Is the currently accepted wisdom that any application printing to
|>> stdout/stderr should take steps to avoid control characters in the
|>> output?
|>
|> First, let's limit this to cases where the control characters come from
|> potentially untrusted input to the program. Obviously, many programs
|> generate terminal escapes on their own (usually via a library), for
|> their intended functionality (colorized listings, TUIs, etc.) Some
|> programs pass potential control characters from their trusted input.
|>
|> Second, I think no, there isn't currently an established opinion on
|> whether programs should perform such filtering of untrusted input.
|
|Lasse has put up an initial implementation for xz:
|https://github.com/tukaani-project/xz/pull/118.
|
|Comments are welcome. It was a TODO from a long time ago ;)
|
|We're not sure how much is overkill (or underkill) for this, especially
|given it gets harder when Unicode is involved.
|
|> [...]
For this purpose there exists the (very very expensive)
https://man.netbsd.org/vis.3
series of functions. Or you do something like this, where "isuni"
gives you "this is a UTF-8 nl_langinfo(CODESET)".
if(!iswprint(wc) && wc != '\n' /*&& wc != '\r' && wc != '\b'*/ &&
wc != '\t'){
if((wc & ~S(wchar_t,037)) == 0)
wc = isuni ? 0x2400 | wc : '?';
else if(wc == 0177)
wc = isuni ? 0x2421 : '?';
else
wc = isuni ? 0x2426 : '?';
}else if(isuni){ /* TODO ctext */
/* Need to filter out L-TO-R and R-TO-R marks TODO ctext */
if(wc == 0x200E || wc == 0x200F || (wc >= 0x202A && wc <= 0x202E))
continue;
/* And some zero-width messes */
if(wc == 0x00AD || (wc >= 0x200B && wc <= 0x200D))
continue;
/* Oh about the ISO C wide character interfaces, baby! */
if(wc == 0xFEFF)
continue;
}
if((n = wctomb(mbb, wc)) <= 0)
continue;
This can be made better (for example the above requires "wc" to be
an actual ISO 10646 codepoint, which ISO C etc), but the key point
is that the Unicode standard gives you everything needed to
properly mask these sequences, from its very beginning in 1993:
2400..2424 ; 1.1 # [37] SYMBOL FOR NULL..SYMBOL FOR NEWLINE
and i have yet to see a font which does not support those.
(Whether and how users can make sense of them totally aside.)
Of course you loose the copy&paste capability.
--End of <87o79nlwxl.fsf@...too.org>
--steffen
|
|Der Kragenbaer, The moon bear,
|der holt sich munter he cheerfully and one by one
|einen nach dem anderen runter wa.ks himself off
|(By Robert Gernhardt)
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