GoAhead 3.4.1 Heap Overflow / Traversal

[KhiZaRix]

Affected software: GoAhead Web Server

Affected versions: 3.0.0 - 3.4.1 (3.x.x series before 3.4.2)

CVE ID: CVE-2014-9707

Description: The server incorrectly normalizes HTTP request URIs that

contain path segments that start with a "." but are not entirely equal

to "." or ".." (eg. ".x"). By sending a request with a URI that

contains these incorrectly handled segments, it is possible for remote

attackers to cause a heap overflow with attacker-controlled content or

perform a directory traversal attack.

Fixed version: 3.4.2

Bug entry: https://github.com/embedthis/goahead/issues/106

Fix: https://github.com/embedthis/goahead/commit/eed4a7d177bf94a54c7b06ccce88507fbd76fb77

Reported by: Matthew Daley

Detail:

The vulnerability lies in the websNormalizeUriPath function. This

function correctly handles the normalization of URIs consisting of

normal segments as well as "." and ".." segments, but fails to handle

other segments that start with a '.' character.

A quick runthrough of the important parts of this function:

The function starts by splitting up the URI into segments (at forward

slashes) into an array. At the same time, it calculates the total

length of these segments.

The function then iterates through the resulting array in order to

perform an in-place normalization (both the input and output pointers

point to the same array):

* If a given segment does not start with a '.', it is simply copied from the

current input pointer to the current output pointer. The for loop's

increment code will then advance both the input and output pointers.

* Otherwise, if the segment is "." or "..", the input and output pointers are

adjusted appropriately (taking into account the for loop's increment code)

but (correctly) no segment is copied.

* Otherwise the segment starts with a '.' but is not "." nor ".."; in this

case the function incorrectly does nothing and both the input and output

pointers are simply advanced by the for loop's increment code. This

effectively skips over a segment in the segment array without any

modification by the function.

After this iteration has completed, a string buffer for the final

output is allocated. The size used for this allocation comes from the

previously-calculated total segment length, with the addition of space

for forward slashes to join the segments back together again and a

null terminator. The segments in the array up to the final output

pointer are joined together in this buffer with forward slashes

separating them.

There are two ways to exploit this incorrect handling of certain segments:

1) Heap overflow

The heap overflow exploitation lies in the possibility to create a

disconnect between the lengths of the segments left in the segment

array after the iteration has completed and the previously-calculated

total segment length. The previously-calculated length should, in

theory, be the worst-case (longest) final output string buffer size

required (when all segments are left and none are removed by the

normalization iteration). However, since we can force the iteration to

skip over certain segments in the array, it is possible to effectively

duplicate segments in the resulting array; this is done by having the

segment copied from one location to another but then also having the

original copy skipped over, making it appear in the resulting array

twice. When this is done, the previously-calculated length is no

longer long enough for the final output's string buffer, and a heap

overflow occurs while joining together the final result.

As an example, take the following URI as input to the function:

"/./AAAAAAAA/.x".

The URI is first split into the segments "", ".", "AAAAAAAA" and ".",

with the total segment length calculated as 0 + 1 + 8 + 2 = 11 bytes.

The normalization iteration proceeds as follows:

* The "" segment is simply copied from input to output, and hence remains

unchanged. Both the input and output pointers are then advanced.

* The "." segment causes the output pointer to stay in place while the input

pointer advances forward.

* The "AAAAAAAA" segment is simply copied from input to output, and hence

overwrites the previous "." segment. Both the input and output pointers are

then advanced.

* Finally, the ".x" segment is incorrectly handled: no modification of

segments is performed but both the input and output pointers are still

advanced, moving the output pointer over the original "AAAAAAAA" segment.

Hence, the resulting segments in the array that are left up to the

final output pointer are "", "AAAAAAAA" and "AAAAAAAA". Note that the

"AAAAAAAA" segment has been duplicated. These segments, including

space for forward slashes to join them together with and a null

terminator, have a total length of 0 + 8 + 8 + 2 + 1 = 19 bytes.

A string buffer is then allocated for the final output, which uses the

previously-calculated total segment length of 11 bytes plus 3 bytes

for forward slashes and 1 byte for a null terminator, giving a total

size of 11 + 3 + 1 = 15 bytes.

The resulting segments are finally joined together into this final

output string buffer. In doing so in this case, however, the buffer is

overflowed by 19 - 15 = 4 bytes.

So, a remote attacker can make (ie.) a simple HTTP GET request for the

URI in question and cause a heap overflow. ASAN gives the following

output in this case, which shows the exact moment that the heap

overflow occurs:

=================================================================

==2613==ERROR: AddressSanitizer: heap-buffer-overflow on address

0x60200000d47f at pc 0x7ffff6f34020 bp 0x7fffffffd410 sp

0x7fffffffcbd0

WRITE of size 9 at 0x60200000d47f thread T0

#0 0x7ffff6f3401f in __interceptor_strcpy

(/usr/lib/x86_64-linux-gnu/libasan.so.1+0x2f01f)

#1 0x7ffff63a7d6d in websNormalizeUriPath src/http.c:3320

#2 0x7ffff639b4de in parseFirstLine src/http.c:969

#3 0x7ffff639a905 in parseIncoming src/http.c:880

#4 0x7ffff639a4c9 in websPump src/http.c:829

#5 0x7ffff639a19c in readEvent src/http.c:802

#6 0x7ffff6399de7 in socketEvent src/http.c:740

#7 0x7ffff6399cbc in websAccept src/http.c:719

#8 0x7ffff63ac8ed in socketAccept src/socket.c:327

#9 0x7ffff63ade95 in socketDoEvent src/socket.c:638

#10 0x7ffff63add5f in socketProcess src/socket.c:622

#11 0x7ffff639daf8 in websServiceEvents src/http.c:1307

#12 0x401b5c in main src/goahead.c:153

#13 0x7ffff597ab44 in __libc_start_main

(/lib/x86_64-linux-gnu/libc.so.6+0x21b44)

#14 0x4011d8

(/home/matthew/goahead-3.4.1/build/linux-x64-debug/bin/goahead+0x4011d8)

0x60200000d47f is located 0 bytes to the right of 15-byte region

[0x60200000d470,0x60200000d47f)

allocated by thread T0 here:

#0 0x7ffff6f5973f in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x5473f)

#1 0x7ffff63a7d04 in websNormalizeUriPath src/http.c:3318

#2 0x7ffff639b4de in parseFirstLine src/http.c:969

#3 0x7ffff639a905 in parseIncoming src/http.c:880

#4 0x7ffff639a4c9 in websPump src/http.c:829

#5 0x7ffff639a19c in readEvent src/http.c:802

#6 0x7ffff6399de7 in socketEvent src/http.c:740

#7 0x7ffff6399cbc in websAccept src/http.c:719

#8 0x7ffff63ac8ed in socketAccept src/socket.c:327

#9 0x7ffff63ade95 in socketDoEvent src/socket.c:638

#10 0x7ffff63add5f in socketProcess src/socket.c:622

#11 0x7ffff639daf8 in websServiceEvents src/http.c:1307

#12 0x401b5c in main src/goahead.c:153

#13 0x7ffff597ab44 in __libc_start_main

(/lib/x86_64-linux-gnu/libc.so.6+0x21b44)

(... snip ...)

As with all heap overflows, it's likely that this can then go on to be

exploited in order to gain full remote code execution, especially in

embedded systems which are less likely to have heap allocators with

modern hardening techniques.

2) Directory traversal

The directory traversal exploitation lies in the fact that we can

force the normalization iteration to skip over certain segments in the

array; namely, we can force it to skip over a ".." segment. The ".."

segment will pass through unchanged into the final output string

buffer, where it is treated by the rest of the server as an actual

parent-directory relative segment.

As an example, take the following URI as input to the function:

"/../../../../../.x/.x/.x/.x/.x/.x/etc/passwd".

The URI is first split into the segments "", "..", "..", "..", "..",

"..", ".x", ".x", ".x", ".x", ".x", ".x", "etc", and "passwd". (The

total segment length that is calculated during this operation is

irrelevant for this mode of exploitation.)

When the normalization iteration reaches the ".x" segments, the

contents of the segment array are still untouched (as all the previous

segments are either empty or are "..") and the output pointer is still

pointing back at the "" segment. The incorrect handling of the ".x"

segments only causes the output (and input) pointers to be advanced

forward over the "" and ".." segments.

When the iteration reaches the "etc" segment, all the "" and ".."

segments have been skipped over; the output pointer is now pointing at

the first ".x" segment. The "etc" is copied over the first ".x"

segment, and the "passwd" segment is copied over the second ".x"

segment.

Hence, the resulting segments in the array that are left up to the

final output pointer are "", "..", "..", "..", "..", "..", "etc" and

"passwd"; note that the ".." segments are still present.

The final output string buffer is created and the resulting segments

are joined together to give a string of "/../../../../../etc/passwd".

The rest of the server is expecting that the result from the function

is normalized and that it contains no relative segments. Hence, the

".." segments go unnoticed when opening the content file while

handling the HTTP request. The end result is that the local filesystem

is traversed up from the administrator-configured web root until

reaching the filesystem's root directory and back down again into the

"/etc/passwd" file. Hence, the file "/etc/passwd" is given in response

to the HTTP request, regardless of the configured web root.

So, a remote attacker can make (ie.) a simple HTTP GET request for the

URI in question and get the contents of the "/etc/passwd" file:

$ echo -ne 'GET /../../../../../.x/.x/.x/.x/.x/.x/etc/passwd

HTTP/1.0\r\n\r\n' | nc localhost 4700

HTTP/1.0 200 OK

Server: GoAhead-http

Date: Sun Nov 16 17:21:01 2014

Content-Length: 1346

Connection: close

Last-Modified: Sat Oct 25 17:07:25 2014

root: x: 0: 0:root:/root:/bin/bash

daemon: x:1:1: daemon:/usr/sbin:/usr/sbin/nologin

bin: x : 2 : 2 : bin:/bin:/usr/sbin/nologin

sys: x : 3 : 3 :sys:/dev:/usr/sbin/nologin

sync: x:4:65534:sync:/bin:/bin/sync

games: x:5:60:games:/usr/games:/usr/sbin/nologin

man: x:6:12:man:/var/cache/man:/usr/sbin/nologin

lp: x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin

mail: x:8:8:mail:/var/mail:/usr/sbin/nologin

(... snip ...)

Of course, 5 ".." segments may not be enough to reach the filesystem's

root directory in all cases and so the crafted URI may have to be

extended with more ".." and ".x" segments.

- Matthew Daley

Source: http://dl.packetstormsecurity.net/1503-exploits/goahead341-overflowtraversal.txt