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Introduction:
Cybercriminals are increasingly weaponizing trusted cloud platforms to bypass traditional security controls. In a newly documented campaign, attackers abuse Google Cloud Storage buckets to deliver Remcos RAT through convincing Google Drive–themed phishing emails that pass SPF, DKIM, and DMARC checks. Because the malicious payloads are hosted on Google’s own infrastructure (storage.googleapis.com), legacy email gateways and URL filters fail to flag them, allowing fileless, multi‑stage execution chains to compromise endpoints.
Learning Objectives:
- Understand how attackers exploit Google Cloud Storage to evade email security filters and deliver Remcos RAT.
- Learn to detect phishing lures that abuse legitimate cloud storage bucket names like `com-bid` or
contract-bid-0. - Implement technical countermeasures including email header analysis, network blocking rules, and endpoint detection for fileless malware.
You Should Know:
- How Attackers Weaponize Google Cloud Storage for Phishing & RAT Delivery
The attack begins with a phishing email that appears to come from a legitimate Google Drive sharing notification. The email contains a link to a Google Cloud Storage bucket (e.g., https://storage.googleapis.com/com-bid/...` orhttps://storage.googleapis.com/contract-bid-0/…`). Because the domain `storage.googleapis.com` is trusted and the email passes SPF/DKIM/DMARC (the sending and hosting infrastructure belongs to Google), the message lands in the victim’s inbox.
When the victim clicks the link, they are presented with a fake Google Drive access request page – a locally hosted HTML file stored inside the same Google Cloud bucket. This HTML mimics the real Google Drive interface and tricks the user into enabling “collaboration” or “viewing permissions.” In reality, clicking the button triggers a JavaScript‑driven download of a malicious payload (e.g., a VBScript, PowerShell script, or ISO file) that fetches and executes Remcos RAT.
Step‑by‑step breakdown of the attack chain:
- Email delivery – Attacker sends email with link to
storage.googleapis.com/<bucket>/phishing.html. - Bypass filters – Trusted domain + valid email authentication → no alert.
- User interaction – Victim clicks link, sees a convincing Google Drive‑themed HTML.
- Malicious download – HTML contains script that downloads a file (e.g., `invoice.iso` or
document.zip) from the same bucket. - Execution – Victim opens the ISO/zip, which contains a LNK file or script that runs PowerShell to download and inject Remcos RAT.
Detection & mitigation commands (Linux / Windows):
Linux – Monitor for suspicious outbound connections to storage.googleapis.com
sudo tcpdump -i eth0 -n 'host storage.googleapis.com and (tcp port 443)'
Linux – Check for recently created files in /tmp that might be staged malware
find /tmp -type f -mmin -10 -ls
Windows PowerShell – Log processes that access Google Cloud Storage URLs
Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Sysmon/Operational'; ID=3} |
Where-Object {$_.Message -match 'storage.googleapis.com'} |
Format-List TimeCreated, Message
Windows CMD – Block storage.googleapis.com via hosts file (temporary)
echo 127.0.0.1 storage.googleapis.com >> C:\Windows\System32\drivers\etc\hosts
How to use these commands:
Run the `tcpdump` command on a Linux gateway or sensor to detect any internal host reaching out to Google Cloud Storage buckets. On Windows, use Sysmon (Event ID 3 for network connections) to log and alert on connections to storage.googleapis.com. The hosts file modification can be used as an emergency block, but note it may break legitimate Google services.
2. Analyzing Suspicious Emails That Abuse Google Trust
Traditional secure email gateways (SEGs) rely on reputation and URL blacklists. Because `storage.googleapis.com` is not blacklisted, these emails slide through. Security teams must instead inspect email headers and look for anomalies in the “From” display name, urgency cues, and the actual bucket name.
Step‑by‑step email analysis guide:
- Extract full email headers (in Outlook: File → Properties → Internet headers; in Gmail: Show original).
- Check authentication results – Look for
spf=pass,dkim=pass,dmarc=pass. If all pass, the email is technically legitimate – but that does not make it safe. - Examine the link – Hover over the URL (without clicking). If it points to `storage.googleapis.com` followed by a random bucket name like `com-bid` or
contract-bid-0, it is suspicious. - Look for display name spoofing – The “From” display may say “Google Drive” but the actual sender address is unrelated (e.g.,
[email protected]).
Useful commands to analyze email content on Linux:
Extract all URLs from an email file (.eml or .msg converted to text) grep -oP 'https?://[^ ]+' suspicious_email.eml Check DNS records for the sender domain dig +short TXT _dmarc.senderdomain.com dig +short spf.senderdomain.com Download a suspicious file from a sandbox (using curl with user-agent spoofing) curl -A "Mozilla/5.0" -O https://storage.googleapis.com/com-bid/invoice.iso
Windows equivalent – using PowerShell to analyze email links:
Extract URLs from a .eml file
Get-Content .\phishing.eml | Select-String -Pattern 'https?://' -AllMatches |
ForEach-Object {$_.Matches.Value} | Out-File extracted_urls.txt
- Hardening Email Gateways and Web Filters Against Cloud‑Abuse Attacks
To stop these attacks, organizations must move beyond reputation‑based filtering and implement behavioral and content‑based rules. Below are configuration examples for common security tools and network controls.
Microsoft 365 Defender / Exchange Online Protection (EOP):
- Create a transport rule that quarantines any email containing a link to `storage.googleapis.com` combined with keywords like “bid,” “contract,” “invoice,” or “document.”
- Enable Safe Links and set “Do not rewrite URLs” to off – this forces Defender to detonate the link at click time.
Proofpoint / Mimecast (custom filter):
Example regex for custom URL category ^https?://storage.googleapis.com/(com-bid|contract-bid-0|drive-[a-z0-9]+)/..(html|htm|iso|zip|vbs|ps1)$
Network‑level blocking (Fortinet / Palo Alto):
- Create a custom URL category for `storage.googleapis.com` with the condition
path contains "com-bid" OR "contract-bid-0". - Apply a security policy to block downloads from that category and log all access attempts.
Linux iptables blocking (for edge gateways):
Block all traffic to storage.googleapis.com (resolve IPs first)
host storage.googleapis.com | grep "has address" | awk '{print $4}' | while read ip; do
sudo iptables -A OUTPUT -d $ip -j DROP
done
Log attempts to access the bucket
sudo iptables -A INPUT -p tcp --dport 443 -m string --string "com-bid" --algo bm -j LOG --log-prefix "GS_BUCKET_HIT"
Windows Firewall via PowerShell:
Block outbound to specific IP ranges (use nslookup to get current IPs)
$ips = @("216.58.192.0/24", "172.217.0.0/16") Example Google IP ranges – adjust after lookup
foreach ($ip in $ips) {
New-NetFirewallRule -DisplayName "Block Google Storage Abuse" -Direction Outbound -RemoteAddress $ip -Action Block
}
4. Detecting Remcos RAT Execution and Fileless Payloads
Remcos RAT is often delivered via fileless techniques – PowerShell scripts that load directly into memory without writing a PE file to disk. The phishing HTML may use mshta.exe, regsvr32.exe, or `rundll32.exe` to execute script from a remote URL.
Indicators of compromise (IOCs) to hunt for:
- Process chains: `explorer.exe` → `mshta.exe` → `powershell.exe` → `regsvr32.exe`
- Command line arguments containing `-enc` (base64 encoded PowerShell) or `-windowstyle hidden`
- Network connections to C2 servers on ports 443, 8080, or 5555 (default Remcos ports)
Linux – Hunt for suspicious processes (if monitoring Windows hosts via Sysmon logs forwarded to Linux):
Using grep on Sysmon Event ID 1 (process creation) cat /var/log/sysmon/events.json | jq 'select(.EventID==1) | .CommandLine' | grep -E 'powershell.-enc|mshta.http|regsvr32./s' Monitor for unusual outbound connections on non‑standard ports sudo tcpdump -i eth0 'tcp port 5555 or tcp port 8080' -n
Windows – Real‑time detection using Sysmon and PowerShell:
Enable Sysmon with a config that logs process creation and network connections
Download sysmon-config from SwiftOnSecurity
.\sysmon64.exe -accepteula -i .\sysmonconfig.xml
Query for suspicious LOLBin usage in the last hour
Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Sysmon/Operational'; ID=1; StartTime=(Get-Date).AddHours(-1)} |
Where-Object {$<em>.Message -match 'mshta|powershell|regsvr32|rundll32'} |
Select-Object TimeCreated, @{n='CommandLine';e={$</em>.Properties[bash].Value}}
- Incident Response Steps When a User Clicks the Malicious Link
If a user has already clicked and executed the payload, follow this IR playbook:
- Isolate the endpoint – Disable network adapter or unplug Ethernet.
- Capture memory – Use `DumpIt` or `FTK Imager` on Windows for later analysis.
- Identify the execution chain – Check prefetch (
C:\Windows\Prefetch), Amcache, and recent PowerShell logs (%userprofile%\AppData\Roaming\Microsoft\Windows\PowerShell\PSReadLine\ConsoleHost_history.txt). - Block C2 domains – Extract domains from network logs or sandbox the sample.
- Remove persistence – Remcos RAT often adds a scheduled task or registry run key.
Windows commands for IR:
List scheduled tasks created in the last 24 hours schtasks /query /fo LIST /v | findstr "Task To Run" /A:5 Check Run/RunOnce registry keys reg query HKCU\Software\Microsoft\Windows\CurrentVersion\Run reg query HKLM\Software\Microsoft\Windows\CurrentVersion\Run Terminate suspicious processes by name taskkill /F /IM mshta.exe taskkill /F /IM powershell.exe Remove persistent WMI event subscriptions (used by some fileless strains) wmic /NAMESPACE:"\root\subscription" PATH __EventFilter DELETE
What Undercode Say:
- Trusted cloud infrastructure is the new phishing vector – Attackers don’t need to compromise domains; they rent legitimate storage and abuse it. Legacy filters are blind to this.
- User training must include “look at the URL path, not just the domain” – A Google link is safe only if it points to
drive.google.com, not `storage.googleapis.com` with suspicious bucket names. - Defense requires layered detection – Email authentication alone is insufficient. Combine header analysis, content inspection, network egress filtering, and endpoint behavioral monitoring to catch cloud‑abuse campaigns.
Prediction:
As Google Cloud Storage and similar platforms (AWS S3, Azure Blob) continue to be abused, we will see a rise in “brand‑trust” phishing where attackers mimic not only the email but also the hosting provider’s login pages. Security vendors will respond by introducing dynamic URL sandboxing that detonates links to cloud storage even if the domain is trusted. Meanwhile, threat actors will pivot to abusing cloud functions (e.g., AWS Lambda, Google Cloud Functions) to host short‑lived, rotating payload delivery endpoints, making static blocking ineffective. Organizations that adopt zero‑trust email security – treating every link as suspicious until verified – will have the upper hand.
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IT/Security Reporter URL:
Reported By: Mayura Kathiresh – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
