Malwrologist Profile picture
Jul 29, 2018 13 tweets 6 min read Twitter logo Read on Twitter
#linux #bash #obfuscation using octal vals
$'\143\141\164' really…
"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."

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

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

(deleted the previous one, as the picture had a problem)
To give you a headache

$'\143'''$'\141'$t''"$t"''`$t`''$'\164' really
#bash #linux
Using null character as filler for command #obfuscation

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

\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

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

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More from @DissectMalware

Aug 4, 2018
#linux #bash #path #obfuscation
goal: obfuscate /etc/passwd
1) #directorytraversal: /mnt/././../etc/././passwd
2) escape characher: /etc/.\/\/\/\/\/passwd /etc/.\/.\/.\/.\/passwd /\e\t\c/passwd
3) null character /et$'c/pa\u0000/notexist/path'sswd
#path #obfuscation using #symbolic #link
ln -s / anything
head -n 2 anything////../etc/passwd

ln -s ../../ wow
head -n 2 ././wow/../etc/passwd

* ../ after a sym link applies to the sym link not the current path
filename expansion #globbing

cat /et*/pa**wd

command expansion
$(echo /e)tc$(echo /pa*)wd

+ other string obfuscation techniques
Read 5 tweets
Jul 30, 2018
#linux #bash #obfuscation using shell parameter expansion:

${alphabet:2:1}at really
${alphabet: -24:-23}at really
${alphabet: -24:1}at really
${alphabet:2:-23}at really…
similar #bat #windows (…)
The goal is to obfuscate the following command:
cat really
in which, really is a file.
Using arithmetic (add, sub, mul, div, mod, and pow) for calculating index or lenght
${alphabet: 2:(-20-3)}at really
Read 6 tweets

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