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Qualifying Event Style Sample Challenge - KPRCA_00001

Original Versions

Known Vulnerabilities

  • CWE-121 - Stack-based Buffer Overflow

Author Information

"Andrew Wesie"

DARPA performer group

Kaprica Security (KPRCA)


This service implements a simple protocol that lets the user call a couple functions which implement the root64 and parcour schemes.

The protocol requires the user to authenticate with a token in order to prevent trivial DDOS attacks. The server will generate a random sesson token which the user must pass back to the server in order to authenticate and access the primary functionality.

Feature List

This is a server that was roughly inspired by the Gopher protocol. It has a few protocol commands that are used to authenticate and set up the environment, and then the user can request pages which will perform computation and print the output. There are three pages: a summary screen showing which pages have been called, a root64 encoder and decoder, and a parcour obfuscator.


The vulnerability is triggered in the root64 encoding function (main.c:284). The check to verify the length of the input is wrong and can result in writing 260 bytes to a 256 byte buffer. This 4-byte overwrite gives the attacker limited control of the ebx register which is later used as a pointer when incrementing the call count for the function (main.c:245). Gaining control of eip from here is difficult, but causing a crash is trivial.

Generic class of vulnerability

Stack-based Buffer Overflow

CWE classification

CWE-121: Stack-based Buffer Overflow


This service uses many primitives that are found in real-world programs:

  • string processing (e.g. protocol messages, root64)
  • arrays of function pointers
  • dynamic memory allocation
  • unpredictable data (e.g. authorization token)

In order to reach vulnerable code, the automated binary analysis tool must be able to ingest data produced by random() and send it back to the service. The rest of the logic is very simple in C, but much more difficult since it requires understanding of the malloc implementation and following function pointers.

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