Thread by @DissectMalware

#linux #bash #obfuscation using octal vals
$'\143\141\164' really

gnu.org/software/bash/…
"Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard."

https://twitter.com/DissectMalware/status/1020794867952365568

Variations
$'\143'$'\141'$'\164' really
$'\143'''$'\141'''''''''''''$'\164' really
$'\143'''$'\141'''""''''""''$'\164' really

Explanation:
'' -> empty string
""-> empty string
'a''b' -> 'ab' (concatenation)

(deleted the previous one, as the picture had a problem)

To give you a headache

t=
$'\143'''$'\141'$t''"$t"''`$t`''$'\164' really

#bash #linux
Using null character as filler for command #obfuscation

#empty strings
$'\x0'
$'\x00'
$'\x'
$'\000'
$'\00'
$'\0'
$'\u0000'
$'\u000'
$'\u00'
$'\u0'
$'\u'
$'\u0000000000000000'
$'\x000000000000'
$'\00000000000000000000'

bonus?
#empty string
$'\u0000 anything you want enter here it will be ignored'

Explanation
\u0000 is a #null character, in bash end of string is a null character like c; thus anything after null is ignored

$'\143\141\164' is equivalent to 'cat'

really is a file

so basically we want to obfuscate the following command

cat really

* Correction
$'\u' and $'\x' are not #empty strings.

the picture is a little bit misleading, see the bonus to find out why.

For completeness
$'\U00000063\U0000061\U00074' really

"\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits) "

Want to test yourself:

what this command do?
cat $'/e\u0`ec$(echo tset000 oh|rev)` > /dev/null/| rm /e't?/pa???d

(answer will be in the reply)

#bash #deobfuscation of the previous tweet
[1] $(echo tset000 oh|rev) # ho 000test
[2] `ec[1]` -> `echo 000test` # 000test
[3] $'/e\u0[2] > /dev/null/| rm /e' # '/e\u0000' after null will be truncated
[4] cat [3]t?/pa???d # cat /et?/pa???d
[5] cat /etc/passwd

Maybe this is clearer

@threadreaderapp unroll me #bashfuscation part 0

cc: @ItsReallyNick