Overview
========

libgd [1] is an open-source image library.  It is perhaps primarily used
by the PHP project.  It has been bundled with the default installation
of PHP since version 4.3 [2].

A signedness vulnerability (CVE-2016-3074) exist in libgd 2.1.1 which
may result in a heap overflow when processing compressed gd2 data.


Details
=======

4 bytes representing the chunk index size is stored in a signed integer,
chunkIdx[i].size, by `gdGetInt()' during the parsing of GD2 headers:

libgd-2.1.1/src/gd_gd2.c:
,----
|  53 typedef struct {
|  54     int offset;
|  55     int size;
|  56 }
|  57 t_chunk_info;
`----


libgd-2.1.1/src/gd_gd2.c:
,----
|  65 static int
|  66 _gd2GetHeader (gdIOCtxPtr in, int *sx, int *sy,
|  67                int *cs, int *vers, int *fmt, int *ncx, int *ncy,
|  68                t_chunk_info ** chunkIdx)
|  69 {
| ...
|  73     t_chunk_info *cidx;
| ...
| 155     if (gd2_compressed (*fmt)) {
| ...
| 163         for (i = 0; i < nc; i++) {
| ...
| 167             if (gdGetInt (&cidx[i].size, in) != 1) {
| 168                 goto fail2;
| 169             };
| 170         };
| 171         *chunkIdx = cidx;
| 172     };
| ...
| 181 }
`----


`gdImageCreateFromGd2Ctx()' and `gdImageCreateFromGd2PartCtx()' then
allocates memory for the compressed data based on the value of the
largest chunk size:

libgd-2.1.1/src/gd_gd2.c:
,----
| 371|637     if (gd2_compressed (fmt)) {
| 372|638         /* Find the maximum compressed chunk size. */
| 373|639         compMax = 0;
| 374|640         for (i = 0; (i < nc); i++) {
| 375|641             if (chunkIdx[i].size > compMax) {
| 376|642                 compMax = chunkIdx[i].size;
| 377|643             };
| 378|644         };
| 379|645         compMax++;
| ...|...
| 387|656         compBuf = gdCalloc (compMax, 1);
| ...|...
| 393|661     };
`----


A size of <= 0 results in `compMax' retaining its initial value during
the loop, followed by it being incremented to 1.  Since `compMax' is
used as the nmemb for `gdCalloc()', this leads to a 1*1 byte allocation
for `compBuf'.

This is followed by compressed data being read to `compBuf' based on the
current (potentially negative) chunk size:

libgd-2.1.1/src/gd_gd2.c:
,----
| 339 BGD_DECLARE(gdImagePtr) gdImageCreateFromGd2Ctx (gdIOCtxPtr in)
| 340 {
| ...
| 413         if (gd2_compressed (fmt)) {
| 414
| 415             chunkLen = chunkMax;
| 416
| 417             if (!_gd2ReadChunk (chunkIdx[chunkNum].offset,
| 418                                 compBuf,
| 419                                 chunkIdx[chunkNum].size,
| 420                                 (char *) chunkBuf, &chunkLen, in)) {
| 421                 GD2_DBG (printf ("Error reading comproessed chunk\n"));
| 422                 goto fail;
| 423             };
| 424
| 425             chunkPos = 0;
| 426         };
| ...
| 501 }
`----


libgd-2.1.1/src/gd_gd2.c:
,----
| 585 BGD_DECLARE(gdImagePtr) gdImageCreateFromGd2PartCtx (gdIOCtx * in, int srcx, int srcy, int w, int h)
| 586 {
| ...
| 713         if (!gd2_compressed (fmt)) {
| ...
| 731         } else {
| 732             chunkNum = cx + cy * ncx;
| 733
| 734             chunkLen = chunkMax;
| 735             if (!_gd2ReadChunk (chunkIdx[chunkNum].offset,
| 736                                 compBuf,
| 737                                 chunkIdx[chunkNum].size,
| 738                                 (char *) chunkBuf, &chunkLen, in)) {
| 739                 printf ("Error reading comproessed chunk\n");
| 740                 goto fail2;
| 741             };
| ...
| 746         };
| ...
| 815 }
`----


The size is subsequently interpreted as a size_t by `fread()' or
`memcpy()', depending on how the image is read:

libgd-2.1.1/src/gd_gd2.c:
,----
| 221 static int
| 222 _gd2ReadChunk (int offset, char *compBuf, int compSize, char *chunkBuf,
| 223            uLongf * chunkLen, gdIOCtx * in)
| 224 {
| ...
| 236     if (gdGetBuf (compBuf, compSize, in) != compSize) {
| 237         return FALSE;
| 238     };
| ...
| 251 }
`----

libgd-2.1.1/src/gd_io.c:
,----
| 211 int gdGetBuf(void *buf, int size, gdIOCtx *ctx)
| 212 {
| 213     return (ctx->getBuf)(ctx, buf, size);
| 214 }
`----


For file contexts:

libgd-2.1.1/src/gd_io_file.c:
,----
|  52 BGD_DECLARE(gdIOCtx *) gdNewFileCtx(FILE *f)
|  53 {
| ...
|  67     ctx->ctx.getBuf = fileGetbuf;
| ...
|  76 }
| ...
|  92 static int fileGetbuf(gdIOCtx *ctx, void *buf, int size)
|  93 {
|  94     fileIOCtx *fctx;
|  95     fctx = (fileIOCtx *)ctx;
|  96
|  97     return (fread(buf, 1, size, fctx->f));
|  98 }
`----


And for dynamic contexts:

libgd-2.1.1/src/gd_io_dp.c:
,----
|  74 BGD_DECLARE(gdIOCtx *) gdNewDynamicCtxEx(int initialSize, void *data, int freeOKFlag)
|  75 {
| ...
|  95     ctx->ctx.getBuf = dynamicGetbuf;
| ...
| 104 }
| ...
| 256 static int dynamicGetbuf(gdIOCtxPtr ctx, void *buf, int len)
| 257 {
| ...
| 280     memcpy(buf, (void *) ((char *)dp->data + dp->pos), rlen);
| ...
| 284 }
`----


PoC
===

Against Ubuntu 15.10 amd64 running nginx with php5-fpm and php5-gd [3]:

,----
| $ python exploit.py --bind-port 5555 http://1.2.3.4/upload.php
| [*] this may take a while
| [*] offset 912 of 10000...
| [+] connected to 1.2.3.4:5555
| id
| uid=33(www-data) gid=33(www-data) groups=33(www-data)
|
| uname -a
| Linux wily64 4.2.0-35-generic #40-Ubuntu SMP Tue Mar 15 22:15:45 UTC
| 2016 x86_64 x86_64 x86_64 GNU/Linux
|
| dpkg -l|grep -E "php5-(fpm|gd)"
| ii  php5-fpm       5.6.11+dfsg-1ubuntu3.1 ...
| ii  php5-gd        5.6.11+dfsg-1ubuntu3.1 ...
|
| cat upload.php
| <?php
|     imagecreatefromgd2($_FILES["file"]["tmp_name"]);
| ?>
`----


Solution
========

This bug has been fixed in git HEAD [4].

Full Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/39736.zip

Footnotes
_________

[1] [http://libgd.org/]
[2] [https://en.wikipedia.org/wiki/Libgd]
[3] [https://github.com/dyntopia/exploits/tree/master/CVE-2016-3074]
[4] [https://github.com/libgd/libgd/commit/2bb97f407c1145c850416a3bfbcc8cf124e68a19]