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CISO Briefing: The “Slack Data Breach” Explained. (How 1 Employee’s Stolen Cookie Bypassed MFA & Leaked 17,000 Records) — by CyberDudeBivash

By CyberDudeBivash · 01 Nov 2025 · cyberdudebivash.com · Intel on cyberbivash.blogspot.com

LinkedIn: ThreatWire cryptobivash.code.blog

SESSION HIJACKING • EDR BYPASS • MFA BYPASS • INFOSTEALER

Situation: The “Slack Data Breach” (17,000+ records) was not a “hack” of Slack’s servers. It was a CISO-level failure of endpoint and identity security. Attackers used a fileless infostealer (like Gootloader) to breach *one* employee, *steal their active session cookie*, and *bypass MFA*.

This is a decision-grade CISO brief. This is the “Cephalus” TTP. Your EDR is blind (it *trusts* the fileless malware). Your Zero-Trust policy is *blind* (it *trusts* the valid session). This is the new playbook for ransomware and corporate espionage, and your SOC is *not* hunting for it.

TL;DR — Attackers stole *one* employee’s Slack *cookie* to bypass MFA.

The TTP: “Living off the Land” (LotL). A `.JS` file runs a *fileless* script *inside* your “trusted” `wscript.exe` process.
The “EDR Bypass”:** Your EDR is *whitelisted* to *trust* `wscript.exe`. It *cannot* see the infostealer running in-memory.
The “MFA Bypass”:** The infostealer *steals the active session cookie* (the “key”) *after* the user has already logged in with MFA.
The “Zero-Trust Fail”: The attacker “replays” this cookie from a “clean” IP. Slack (and your ZTNA) sees a “valid session” and *allows* the attacker to *log in as your employee*.
THE ACTION: 1) HARDEN: *De-weaponize `.JS` files* (change handler to `notepad.exe`). 2) DETECT: Deploy SessionShield to catch the *hijacked session*. 3) HUNT: Get a 24/7 MDR team to hunt for the initial `wscript.exe -> powershell.exe` TTP.

TTP Factbox: “Slack” Session Hijack (The “Cephalus” TTP)

TTP	Component	Severity	Exploitability	Mitigation
Infostealer (T1555.003)	Endpoint (Browser)	Critical	EDR Bypass (Fileless)	MDR / Kaspersky EDR
Session Hijacking (T1539)	Slack/SaaS Cookies	Critical	Bypasses MFA	SessionShield / FIDO2 Keys

Critical Data BreachMFA Bypass TTPEDR Bypass TTPContents

Phase 1: The “MFA Bypass” (How They Steal the “Key”)

As a CISO, you *mandated* MFA. Your Zero-Trust policy relies on it.

This TTP *bypasses* it. It doesn’t *break* MFA; it *steals the key* after MFA is already complete.

This is a Session Hijacking attack. An attacker doesn’t *need* your password. They need your *session cookie*.
Here is the kill chain that your EDR is *blind* to:

The Lure (Gootloader TTP): Your employee gets a phishing email with `invoice.zip`. Your SEG *allows* it.
The “Bypass”:** The user clicks `invoice.pdf.js`. Your EDR *allows* this “trusted” `wscript.exe` process to run.
The “Fileless” Payload: The `.JS` script runs `powershell.exe -e …` to download an Infostealer (like Vidar) *in-memory*.
The “Heist”: The infostealer *steals all session cookies* from the browser’s database (`chrome.cookies`). This includes the *active, post-MFA* cookie for `app.slack.com`.

The CISO Mandate: Your “Phish Training” is *obsolete*. You *must* assume the phish will work. You need AI to fight AI. Our PhishRadar AI app is *behavioral*. It doesn’t look for “bad links”; it detects the *psychological intent* (e.g., “Urgent Invoice”) of the phish and *blocks it*.
Explore PhishRadar AI by CyberDudeBivash →

Phase 2: The “Zero-Trust Fail” (How They Become Your Employee)

This is a CISO PostMortem because the kill chain is *invisible* to traditional tools.

Stage 1: The “Session Replay”

The attacker now has your employee’s *active Slack session cookie*. They *don’t* log in. They *import* this cookie into their *own* browser and hit `app.slack.com`.

Stage 2: The “Zero-Trust” Failure

Your Zero-Trust policy (and Slack’s servers) sees a *valid, authenticated session*. It *allows* the connection.
The attacker is now *logged in as your trusted employee*. They have *bypassed* your MFA. They are *inside* your “secure” corporate chat.

Stage 3: Data Exfiltration (The “17,000 Records”)

The attacker is now an *invisible insider*. They *silently* run “low-and-slow” searches:

`search: “password”`
`search: “AWS_SECRET_KEY”`
`search: “customer_list.csv”`

They *scrape* 17,000 customer records (PII). They *steal* your “crown jewel” source code. Your DLP is blind because it’s a “trusted user” accessing “trusted data.”

Stage 4: The “Trusted” Pivot (Ransomware)

*After* exfiltrating the data, the attacker *uses the trusted Slack session* to *phish your C-suite from the inside*.
(From `[Your_IT_Admin]`): “Hey [CEO], we’re testing a new security patch. Please run this ‘patch.exe’.”
The CEO *trusts* this. They run the file. You are now being hit with ransomware.

This is the “Session Hijacking” gap.
This is why we built SessionShield. Your ZTNA *stops* at the login. Our tool *starts*. SessionShield “fingerprints” your *real* employee’s session (Device, IP, Location, *Behavior*). The *instant* the attacker logs in with that *stolen cookie* from a new, anomalous location (e.g., a datacenter in Russia), SessionShield sees the “fingerprint” mismatch, flags it as a *hijacked session*, and kills it in real-time.
Explore SessionShield by CyberDudeBivash →

Exploit Chain (Engineering)

This is a “Trusted Process” Hijack (T1219/T1059). The “exploit” is a *logic* flaw in your EDR Whitelisting policy.

Trigger: User double-clicks `.js` file.
Precondition: EDR/AV is configured to *automatically trust* all `wscript.exe` / `cscript.exe` processes. Windows “Hides known file extensions” is ON.
Sink (The RCE): `explorer.exe` → `wscript.exe file.js` → `powershell.exe -e …` (Fileless C2)
Module/Build: `wscript.exe` (Trusted), `powershell.exe` (Trusted).
Patch Delta: There is no “patch.” The “fix” is GPO Hardening (changing the default `.js` handler) and MDR (Threat Hunting).

Reproduction & Lab Setup (Safe)

You *must* test your EDR’s visibility for this TTP.

Harness/Target: A sandboxed Windows 11 VM with your standard EDR agent installed.
Test: 1) Create a file named `test.js`. 2) Put this *one line* of code in it: `WScript.CreateObject(“WScript.Shell”).Run(“calc.exe”);`
Execution: Double-click the `test.js` file.
Result: Did `calc.exe` launch? Did your EDR fire a P1 (Critical) alert for `wscript.exe -> calc.exe`? If it was *silent*, your EDR is *blind* to this TTP.
Safety Note: If `calc.exe` can run, so can the “EndClient” RAT.

Detection & Hunting Playbook (The New SOC Mandate)

Your SOC *must* hunt for this. Your SIEM/EDR is blind to the exploit itself; it can *only* see the *result*. This is your playbook.

Hunt TTP 1 (The #1 IOC): “Anomalous Child Process.” This is your P1 alert. Your `wscript.exe` process should *NEVER* spawn a shell (`powershell.exe`, `cmd.exe`, `/bin/bash`).# EDR / SIEM Hunt Query (Pseudocode) SELECT * FROM process_events WHERE (parent_process_name = ‘wscript.exe’ OR parent_process_name = ‘cscript.exe’) AND (process_name = ‘powershell.exe’ OR process_name = ‘cmd.exe’)
Hunt TTP 2 (The C2): “Show me all *network connections* from `wscript.exe` or `cscript.exe` to a *newly-registered domain* or *anomalous IP*.”
Hunt TTP 3 (The *Result*): “Impossible Travel / Anomalous Session.” Hunt your *cloud* logs (Slack, M365) for a *session hijack*. This is what our SessionShield app automates.

Mitigation & Hardening (The CISO Mandate)

This is a Windows Configuration failure. This is the fix.

1. HARDEN (The *Real* Fix): This is your CISO mandate. De-weaponize JavaScript files.
You must *change the default file handler* for `.JS` files. An employee should *never* “execute” a `.JS` file. It should *open* in Notepad.
The Fix: Use GPO to change the default handler for `.js` files from `wscript.exe` (Execute) to `notepad.exe` (View). This *kills* the TTP.
2. HUNT (The “MDR” Fix): You *cannot* run a 9-to-5 SOC. You *must* have a 24/7 human-led MDR team (like ours) to hunt for the *behavioral* TTPs (like Hunt TTP 1) that your EDR will log but *not* alert on.
3. VERIFY (The “Red Team” Fix): You *must* run an Adversary Simulation (Red Team) to *prove* your EDR and your SOC team *can* detect this TTP.

Audit Validation (Blue-Team)

Run this *today*. This is not a “patch”; it’s an *audit*.

# 1. Audit your EDR (The "Lab" Test)
# Run the "Lab Setup" test (`test.js -> calc.exe`). 
# Did your EDR *see* it? If not, it is BLIND.

# 2. Audit your File Handlers
# (Run `ftype JScript.file`)
# Does it say "wscript.exe"? If yes, you are VULNERABLE.
# Run the GPO to change it to "notepad.exe".

# 3. Run the "Lab Test" again
# Did `calc.exe` launch? Or did `notepad.exe` open?
# If Notepad opened, you have *successfully* hardened your fleet.

Is Your EDR Blind to “Fileless” Attacks?
Your SOC is slow. Your EDR is whitelisted. CyberDudeBivash is the leader in Ransomware Defense. We are offering a Free 30-Minute Ransomware Readiness Assessment to show you the *exact* gaps in your “LotL” and “Session Hijacking” defenses.

Book Your FREE 30-Min Assessment Now →

Recommended by CyberDudeBivash (Partner Links)

You need a layered defense. Here’s our vetted stack for this specific threat.

Kaspersky EDR
This is your *sensor*. It’s the #1 tool for providing the behavioral telemetry (process chains, network data) that your *human* MDR team needs to hunt.Edureka — Threat Hunting Training
Your SOC team can’t find what they don’t know. Train them *now* on PowerShell Threat Hunting and LotL TTPs.TurboVPN
The phish often lands on a *remote* device on *public Wi-Fi*. A VPN encrypts this initial access channel.

Alibaba Cloud (VDI)
A key mitigation. Use Virtual Desktop Infrastructure (VDI). If the VDI is popped, you *burn it* and re-image in seconds. The host is safe.AliExpress (Hardware Keys)
*Mandate* this for all Domain Admins. Get FIDO2/YubiKey-compatible keys. They stop the *initial phish* from succeeding.Rewardful
Run a bug bounty program. Pay white-hats to find flaws *before* APTs do.

CyberDudeBivash Services & Apps

We don’t just report on these threats. We hunt them. We are the “human-in-the-loop” that your automated EDR is missing.

SessionShield — Our flagship app. This is the *only* solution designed to *behaviorally* detect and *instantly* kill a hijacked Slack/M365 session. It stops the *result* of the breach.
Managed Detection & Response (MDR): This is the *solution*. Our 24/7 SOC team becomes your Threat Hunters, watching your EDR logs for these *exact* “wscript -> powershell” TTPs.
Emergency Incident Response (IR): You found this TTP? Call us. Our 24/7 team will hunt the attacker and eradicate them.
PhishRadar AI — Stops the phishing attacks that *initiate* the infostealer breach.

Book Your FREE 30-Min Assessment Explore 24/7 MDR Services Subscribe to ThreatWire

FAQ

Q: What is a “Trusted Partner” Attack?
A: This is a Supply Chain attack. The attacker *breaches* a “trusted” partner (like a hotel) and *uses their legitimate account* (like the Booking.com chat) to send *you* malware. Your defenses *trust* the message because it’s from a *legitimate* source.

Q: Why does my EDR/Antivirus miss this attack?
A: Because your EDR is *configured to trust* `wscript.exe` and `powershell.exe`. This is a “Trusted Process” bypass. The EDR sees a ‘trusted’ Microsoft process running and *ignores* it. You *must* have a *human* MDR team hunting for the *behavioral* anomalies.

Q: What is the #1 fix for the Gootloader .JS attack?
A: You must HARDEN your endpoints. The #1 fix is to *de-weaponize* JavaScript files. Use a Group Policy (GPO) to *change the default file handler* for `.JS` and `.VBS` files from `wscript.exe` (Execute) to `notepad.exe` (View). This *instantly* neutralizes the threat.

Q: How do I protect my personal data on Booking.com?
A: 1) Use a Virtual Credit Card. 2) *Never* download attachments from a hotel, even on the “real” app. Call the hotel *directly* to confirm any payment issue. 3) Install a *real* antivirus (like Kaspersky) on your PC.

Timeline & Credits

This “Gootloader/BEC 2.0” TTP (T1566.001 / T1059) is an active, ongoing campaign by multiple APTs and RaaS groups.
Credit: This analysis is based on active Incident Response engagements by the CyberDudeBivash threat hunting team.

References

Affiliate Disclosure: We may earn commissions from partner links at no extra cost to you. These are tools we use and trust. Opinions are independent.

CyberDudeBivash — Global Cybersecurity Apps, Services & Threat Intelligence.

cyberdudebivash.com · cyberbivash.blogspot.com · cryptobivash.code.blog

#BEC #Booking #Gootloader #LNKexploit #ZIP #FilelessMalware #PowerShell #EDRBypass #Ransomware #CyberDudeBivash #IncidentResponse #MDR #ThreatHunting #LotL #C2

Cyberdudebivash

The “Slack Data Breach” Explained: (How 1 Employee’s Account Leaked 17,000 Customer Records).

CISO Briefing: The “Slack Data Breach” Explained. (How 1 Employee’s Stolen Cookie Bypassed MFA & Leaked 17,000 Records) — by CyberDudeBivash

Phase 1: The “MFA Bypass” (How They Steal the “Key”)

Phase 2: The “Zero-Trust Fail” (How They Become Your Employee)

Stage 1: The “Session Replay”

Stage 2: The “Zero-Trust” Failure

Stage 3: Data Exfiltration (The “17,000 Records”)

Stage 4: The “Trusted” Pivot (Ransomware)

Exploit Chain (Engineering)

Reproduction & Lab Setup (Safe)

Detection & Hunting Playbook (The New SOC Mandate)

Mitigation & Hardening (The CISO Mandate)

Audit Validation (Blue-Team)

Recommended by CyberDudeBivash (Partner Links)

CyberDudeBivash Services & Apps

FAQ

Timeline & Credits

References

Leave a comment Cancel reply

The “Slack Data Breach” Explained: (How 1 Employee’s Account Leaked 17,000 Customer Records).

CISO Briefing: The “Slack Data Breach” Explained. (How 1 Employee’s Stolen *Cookie* Bypassed MFA & Leaked 17,000 Records) — by CyberDudeBivash

Phase 1: The “MFA Bypass” (How They Steal the “Key”)

Phase 2: The “Zero-Trust Fail” (How They Become Your Employee)

Stage 1: The “Session Replay”

Stage 2: The “Zero-Trust” Failure

Stage 3: Data Exfiltration (The “17,000 Records”)

Stage 4: The “Trusted” Pivot (Ransomware)

Exploit Chain (Engineering)

Reproduction & Lab Setup (Safe)

Detection & Hunting Playbook (The *New* SOC Mandate)

Mitigation & Hardening (The CISO Mandate)

Audit Validation (Blue-Team)

Recommended by CyberDudeBivash (Partner Links)

CyberDudeBivash Services & Apps

FAQ

Timeline & Credits

References

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Leave a comment Cancel reply

CISO Briefing: The “Slack Data Breach” Explained. (How 1 Employee’s Stolen Cookie Bypassed MFA & Leaked 17,000 Records) — by CyberDudeBivash

Detection & Hunting Playbook (The New SOC Mandate)