i am running a php+mysql site on a lamp ubuntu cloud server (amazon ec2). i use php sessions. the apache php user is called www-data.

for every session (SID), i need to create a temporary folder which is a subdirectory of www and has the session Id as its name (unpredictable, therefore cannot be pre-created). for example: /var/www/fileStore/s35S10nT3Mp/.

i will later access files in these folders from a javascript (client side) function.

i am working under the assumptions that:

  1. anything executed (or called) by the php code will run under the user www-data
  2. client-side javascript cannot access a subdirectory OUTSIDE www (for example: /var/outsideFileStore/s35S10nT3Mp/, where webroot = public_html = /var/www)

i would love to be wrong on one or both of the above, for it would solve my problem with no further ado. in case i am not wrong on either one of my assumptions:

i have a security issue with giving user www-data write permissions (necessary for php to mkdir the new folder) in a subdirectory of www, because that leaves the server and data in these folders too exposed.

i would like to know if a php session can trigger (or call) a server script / process that runs under another user (who would receive write permission in /var/www/fileStore/), not www-data. this process would then create (and garbage collect) these temporary folders.

corollary: in case this cannot be done under apache, would tomcat help (if i ran php under java as opposed to http_server)?

corollary 2: what if i ran a socket connection (from inside php) to a socket server (which i have in c++) and had the socket server (another user, obviously) do the file creation? is this overkill?


You can use setuid() functionality to run scripts and other things as another user.

I use the below wrapper script to run another script as a different user than www-data for a specific purpose on an Apache web server.


#include <stdlib.h>

 * C wrapper to run script as other user from Apache.
 * Instructions:
 *   Run as root:
 *     gcc suid_wrapper.c -o make_stats.cgi
 *     chmod a+s make_stats.cgi

// User ID and Group ID for wanted user.
int uid = 503;
int gid = 506;
// Path to script to be executed as above user.
const char* scriptpath = "/home/user/public/stats/make_stats.sh";

int main()
    // setgid() must be before setuid(); otherwise one has forsaken the privilege to change group.
    return 0;

Just call make_stats.cgi from Apache and the script given in the above file should be run as the user specified in the above file.

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