Cyber Pulse: CVEs Diagnosed by a Paramedic Turned Analyst – Microsoft Teams “Chat with Anyone” Feature: Hidden Attack Surface

Intro

The upcoming rollout of the Microsoft Teams “Chat with Anyone” feature – which allows users to initiate chats using only an email address, even if the recipient isn’t a Teams user – is a productivity win for some but a **massive security liability** for many. Without proper guardrails, this capability opens a wide door for phishing lures, malware drops, and credential harvesting inside the collaboration fabric of your organisation.

📌 CVE Context

– Products & versions affected: Microsoft Teams (cloud service) and associated external access/federation settings
– Disclosure timeline: Feature planned for targeted release in early November 2025, general worldwide release by January 2026
– Attack vector, auth level, impact: Chat initiation via email-only address, external user (potentially unmanaged) initiation, social engineering + payload delivery capability


CVSS Metric Breakdown (v4.0) – No formal CVE registered yet (feature‑driven risk)  
Attack Vector (AV): N (Network)  
Attack Complexity (AC): H (High) – Requires social engineering but infrastructure supported  
Privileges Required (PR): N (None) – External can initiate chat with internal user  
User Interaction (UI): R (Required) – Internal user must click/accept/link  
Confidentiality Impact (VC): L (Low) – May gain chat access but not full environment  
Integrity Impact (VI): L (Low) – Chat messages tampered or impersonated  
Availability Impact (VA): N (None) – Does not directly crash service  
Scope Changed (SC): N (None)  
Safety Impact (SI): N (None)  
Automation (SA): H (High) – Attack could be scripted at scale  
Exploit Maturity: M (Medium) – Threat actors already using Teams for phishing  
Base Score: ~6.0 (Medium)

– Exploit tools/payloads observed: Phishing messages via Teams chats targeting credential prompts; ransomware gangs using Teams invitations as entry vectors :contentReference[oaicite:1]{index=1} – Confirmed victims/global exposure: While no public CVE yet, external‑chat spam and impersonation via Teams is reported and increasing.

🔬 Exploitation Detail

– Step‑by‑step breakdown:

Attacker crafts a plausible email address impersonating a vendor/partner.
Using the new Teams “Chat with Anyone” feature, they send a chat invite to the target’s email.
Target clicks “Accept” or opens chat; attacker sends malicious link, attachment, or prompts MFA/credential share.
Payload executes (malware, ransomware, spyware) or credentials are harvested; lateral movement begins.

– Where it lives: The risk is in the external access/federation layer of Teams (not a vulnerability in code, but a **design‑configuration risk**). Attack surface is the chat‑initiation channel, external unmanaged identity, and user trust boundary.


[Sample chat invite text]  
From: “vendor.partnerContracts@trusted‑corp.com”  
To: employee@yourcompany.com  
Subject: “Please join me in Teams to finalise Q4 invoice”  
Message: “Click here to open chat and sign the document securely.”

📎 Attacker Behavior Snapshot

– What the attacker sends: Chat invite from an email only identity not previously in organization; message bypasses “we only chat internally” policies.

– What the system does: Accepts invite (feature enabled by default) → opens chat window with internal user, external user appears as guest/unmanaged.

– What leaks back: Potential credential prompt, malicious link click, download of malware, Teams payload delivery via Teams file share, or escalation to other collaboration apps.

🧩 Why This Matters

This design shift highlights how **collaboration platforms are not just messaging apps—they are new attack surfaces**. When external identities are given chat‑entry rights, the boundary between “trusted internal” and “unverified external” collapses. Attackers can exploit the trust in the tool rather than a code bug.

Exploitation results in:

Social engineering via chat as initial vector (less suspicious than email).
Payload delivery inside the corporate chat context which may bypass email protections.
Rapid lateral movement leveraging collaboration fabric rather than isolated endpoints.

🧩 MITRE ATT&CK Mapping

Initial Access: T1190 – Exploit Public‑Facing Application (chat feature)
Execution: T1059.003 – Command Shell / scripting (via payload)
Persistence: T1505.003 – Server Software Component (malicious Teams session or bot)

🧪 Detection Rules

YARA Rule (Memory/Doc/PCAP)


rule Teams_ExternalChat_Invite  
{  
   meta:  
      description = "Detects unusual Teams chat invite from unmanaged external address"  
   strings:  
      $emailpattern = /[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.(com|net|biz)/  
      $teamsheader = "Teams Chat request from"  
   condition:  
      $teamsheader and $emailpattern and external_source=true  
}

Suricata or Zeek (Network)


alert tcp any any -> any 443 (msg:"Teams external chat initiation suspicious"; tls.subject contains "teams.microsoft.com"; content:"/chat/invite"; flow:established; sid:1000001; rev:1;)

Sigma Rule (SIEM/EDR)


title: Teams external chat initiation from unmanaged email  
logsource:  
    product: microsoft‑teams  
detection:  
    selection:  
        EventID: "ChatInviteSent"  
        InitiatorUserType: "External"  
        TargetUserDomain: "yourcompany.com"  
        InitiatorEmailAddress NOT LIKE "*.yourcompany.com"  
    condition: selection  
level: high

🔎 Detection Strategies

✅ Network Detection:

Look for inbound Teams chat initiation attempts from domains not in your allowlist.
Detect unexpected “Invite to chat” flows from unknown/unmanaged identities.
Monitor for rapid file or link sharing within chats initiated by external identities.

✅ Endpoint Detection:

Teams desktop/web client spawning unexpected PowerShell or download processes after external chat acceptance.
Unexpected file downloads or links from chat windows whose initiator is external/unverified.
Alert on guest/unmanaged user added to chat and rapidly sending attachments/links.

⚡ Splunk Query


index=teams_logs sourcetype="teams:chat"  
| where initiator_email NOT LIKE "%@yourcompany.com%"  
| where external_invite = true  
| stats count by initiator_email, target_user, chat_id  
| where count > 3

🛠️ SOC Detection Strategy

– Triage: Prioritise chats where initiator is external/unverified and invited internal user accepts.

– Log sources: Teams Admin logs (chat invite events), Azure AD sign‑ins, DLP & file share logs in Teams/SharePoint.

– Alert logic: External chat invited → file shared or link clicked → internal user downloads/executed payload.

– Real‑world alerts: “External email x@unknown.com initiated chat with user y@yourcompany.com. Attachment downloaded within chat session.”

🛠️ Tools & Techniques

Tool | Usage
Sysmon | Detect parent–child anomalies when Teams process spawns powershell/download.
Velociraptor | Endpoint hunting for file creation via Teams chat directory.
Zeek | Monitor TLS flows to teams.microsoft.com with unexpected SNI/chat patterns.
Sigma/YARA | Create detection rules for known malicious file names shared via Teams chats.

🛡️ Mitigation & Response

– Patch info: While this isn’t a code CVE, treat the feature rollout like a security change. Review config when rollout occurs.

– Temporary mitigations:

In the Teams Admin Center, navigate to Users → External access → restrict or turn off “People in my organization can communicate with Teams users whose accounts aren’t managed by an organization”. :contentReference[oaicite:2]{index=2}
Allow only specific external domains your org collaborates with (Allow‑list) instead of default open. :contentReference[oaicite:3]{index=3}
Enable external chat/spam/impersonation detection in Microsoft Defender for Office 365 for Teams (Safe Links / malicious‑URL scanning) :contentReference[oaicite:5]{index=5}
Apply Conditional Access policies, MFA enforcement, endpoint protection for systems with Teams clients. :contentReference[oaicite:6]{index=6}

– Config changes: Use PowerShell to set federation configuration:


Set‑CsTenantFederationConfiguration ‑AllowTeamsConsumer $false ‑AllowPublicUsers $false

– Credential rotation, audit of guest sign‑ins, enforcement of least‑privilege guest/chat access.

– Monitor for lateral movement post‑compromise: unusual Teams channel creation, file uploads to SharePoint via a guest, new external user additions.

📋 Incident Response Snippets

– Log query example (Azure AD):


SigninLogs  
| where AppDisplayName == "Microsoft Teams"  
| where UserPrincipalName endswith "@yourcompany.com"  
| where ExternalUserPresent == true

– IR questions to ask:

Which internal users accepted chat invites from external/unverified identities?
Were file shares or links exchanged in those chats?
Did any secondary credentials get used after the chat session?
Was any unusual account creation (guest accounts) or lateral movement observed?

– Cleanup & movement checks: Revoke guest/unverified accounts, quarantine shared files, reset credentials if suspicious activity found, enhance monitoring for that user for next 30 days.

📚 Suggested Reading & External References

– IT Admins – Manage external meetings and chat with people and organisations using Microsoft identities
– Prevent spam or phishing attempts from external chats in Microsoft Teams
– Safeguarding Microsoft Teams with Microsoft Defender for Office 365

🗾️ Final Thoughts

This feature adds a **new attack vector** where none existed—or at least wasn’t as exposed—before: external email → Teams chat invite → internal user engagement → phishing/malware. The most effective action _now_ is to **restrict external chat initiation, enable threat scanning, and train your users**. Detection isn’t a checkbox—it’s the frontline triage in a system that just got a little more open.

Published: November 8, 2025

Ramblings of a CyberSecurity Nerd