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Date: Fri, 03 May 2024 00:39:12 +0200
From: Steffen Nurpmeso <>
To: Sam James <>
Cc: Solar Designer <>,
Subject: Re: escaping terminal control characters (was
 Re: backdoor in upstream xz/liblzma leading to ssh server compromise)

Sam James wrote in
 |Solar Designer <> 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:
 |>>>>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:
 |>>>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:
 |>> 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:
 |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)

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 : '?';
               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))
            /* And some zero-width messes */
            if(wc == 0x00AD || (wc >= 0x200B && wc <= 0x200D))
            /* Oh about the ISO C wide character interfaces, baby! */
            if(wc == 0xFEFF)

         if((n = wctomb(mbb, wc)) <= 0)

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 <>

|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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