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Security and Microsoft Teams

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    Microsoft Teams

Important

The Teams service model is subject to change in order to improve customer experiences. For example, the default access or refresh token expiration times may be subject to modification in order to improve performance and authentication resiliency for those using Teams. Any such changes would be made with the goal of keeping Teams secure and Trustworthy by Design.

Microsoft Teams, as part of the Microsoft 365 and Office 365 services, follows all the security best practices and procedures such as service-level security through defense-in-depth, customer controls within the service, security hardening, and operational best practices. For full details, see the Microsoft Trust Center.

Trustworthy by design

Teams is designed and developed in compliance with the Microsoft Trustworthy Computing Security Development Lifecycle (SDL), which is described at Microsoft Security Development Lifecycle (SDL). The first step in creating a more secure unified communications system was to design threat models and test each feature as it was designed. Multiple security-related improvements were built into the coding process and practices. Build-time tools detect buffer overruns and other potential security threats before the code is checked in to the final product. It's impossible to design against all unknown security threats. No system can guarantee complete security. However, because product development embraced secure design principles from the start, Teams incorporates industry standard security technologies as a fundamental part of its architecture.

Trustworthy by default

Network communications in Teams are encrypted by default. By requiring all servers to use certificates and by using OAUTH, Transport Layer Security (TLS), and Secure Real-Time Transport Protocol (SRTP), all Teams data is protected on the network.

How Teams handles common security threats

This section identifies the more common threats to the security of the Teams Service and how Microsoft mitigates each threat.

Compromised-key attack

Teams uses the PKI features in the Windows Server operating system to protect the key data used for encryption for the TLS connections. The keys used for media encryptions are exchanged over TLS connections.

Network denial-of-service attack

A distributed denial-of-service (DDOS) attack occurs when the attacker prevents normal network use and function by valid users. By using a denial-of-service attack, the attacker can:

  • Send invalid data to applications and services running in the attacked network to disrupt their normal function.
  • Send a large amount of traffic, overloading the system until it stops responding or responds slowly to legitimate requests.
  • Hide the evidence of the attacks.
  • Prevent users from accessing network resources.

Teams mitigates against these attacks by running Azure DDOS network protection and by throttling client requests from the same endpoints, subnets, and federated entities.

Eavesdropping

Eavesdropping occurs when an attacker gains access to the data path in a network and has the ability to monitor and read the traffic. Eavesdropping is also called sniffing or snooping. If the traffic is in plain text, the attacker can read the traffic when the attacker gains access to the path. An example is an attack performed by controlling a router on the data path.

Teams uses mutual TLS (MTLS) and Server to Server (S2S) OAuth (among other protocols) for server communications within Microsoft 365 and Office 365, and also uses TLS from clients to the service. All traffic on the network is encrypted.

These methods of communication make eavesdropping difficult or impossible to achieve within the time period of a single conversation. TLS authenticates all parties and encrypts all traffic. While TLS doesn't prevent eavesdropping, the attacker can't read the traffic unless the encryption is broken.

The Traversal Using Relays around NAT (TURN) protocol is used for real-time media purposes. The TURN protocol doesn't mandate the traffic to be encrypted and the information that it's sending is protected by message integrity. Although it's open to eavesdropping, the information it's sending, that is, IP addresses and port, can be extracted directly by looking at the source and destination addresses of the packets. The Teams service ensures that the data is valid by checking the Message Integrity of the message using the key derived from a few items including a TURN password, which is never sent in clear text. SRTP is used for media traffic and is also encrypted.

Identity spoofing (IP address spoofing)

Spoofing occurs when the attacker identifies and then uses an IP address of a network, computer, or network component without being authorized to do so. A successful attack allows the attacker to operate as if the attacker is the entity normally identified by the IP address.

TLS authenticates all parties and encrypts all traffic. Using TLS prevents an attacker from performing IP address spoofing on a specific connection (for example, mutual TLS connections). An attacker could still spoof the address of the Domain Name System (DNS) server. However, because authentication in Teams is performed with certificates an attacker would not have a valid information required to spoof one of the parties in the communication.

Man-in-the-middle attack

A man-in-the-middle attack occurs when an attacker reroutes communication between two users through the attacker's computer without the knowledge of the two communicating users. The attacker can monitor and read the traffic before sending it on to the intended recipient. Each user in the communication unknowingly sends traffic to and receives traffic from the attacker, all while thinking they are communicating only with the intended user. This scenario can happen if an attacker can modify Active Directory Domain Services to add their server as a trusted server, or modify DNS configuration or use other means to get clients to connect through the attacker on their way to the server.

Man-in-the-middle attacks on media traffic between two endpoints participating in Teams audio, video, and application sharing, is prevented by using Secure Real-Time Transport Protocol (SRTP) to encrypt the media stream. Cryptographic keys are negotiated between the two endpoints over a proprietary signaling protocol (Teams Call Signaling protocol) which uses TLS 1.2 and AES-256 (in GCM mode) encrypted UDP or TCP channel.

Real-time Transport Protocol (RTP) replay attack

A replay attack occurs when a valid media transmission between two parties is intercepted and retransmitted for malicious purposes. Teams uses SRTP with a secure signaling protocol that protects transmissions from replay attacks by enabling the receiver to maintain an index of already received RTP packets and compare each new packet with packets already listed in the index.

Spim

Spim is unsolicited commercial instant messages or presence subscription requests, like spam, but in instant message form. While not by itself a compromise of the network, it's annoying in the least, can reduce resource availability and production, and can possibly lead to a compromise of the network. An example is users spimming each other by sending requests. Users can block each other to prevent spimming, but with federation, if a malicious actor establishes a coordinated spim attack, it can be difficult to overcome unless you disable federation from the partner.

Viruses and worms

A virus is a unit of code whose purpose is to reproduce more, similar code units. To work, a virus needs a host, such as a file, email, or program. Like a virus, a worm is a unit of code that reproduces more, similar code units, but that unlike a virus doesn't need a host. Viruses and worms primarily show up during file transfers between clients or when URLs are sent from other users. If a virus is on your computer, it can, for example, use your identity and send instant messages on your behalf. Standard client security best practices such as periodically scanning for viruses can mitigate this issue.

Phishing attempts

Phishing attacks in Teams are costly monetarily and to peace of mind. These attacks operate by means of tricking users into revealing information such as passwords, codes, credit card numbers, and other critical information, through fake website links, and attachments that appear innocuous but can download dangerous software on click. Because many of these attacks target users, even high value targets with a lot of access, they can be pervasive. However, there are anti-phishing strategies for both Teams administrators and users.

Security Framework for Teams

Teams endorses security ideas like Zero Trust, and principles of Least Privilege access. This section gives an overview of fundamental elements that form a security framework for Microsoft Teams.

Core elements are:

  • Microsoft Entra ID, which provides a single trusted back-end repository for user accounts. User profile information is stored in Microsoft Entra ID through the actions of Microsoft Graph.
    • There may be multiple tokens issued which you may see if tracing your network traffic. Including Skype tokens you might see in traces while looking at chat and audio traffic.
  • Transport Layer Security (TLS) encrypts the channel in motion. Authentication takes place using either mutual TLS (MTLS), based on certificates, or using Service-to-Service authentication based on Microsoft Entra ID.
  • Point-to-point audio, video, and application sharing streams are encrypted and integrity checked using Secure Real-Time Transport Protocol (SRTP).
  • You will see OAuth traffic in your trace, particularly around token exchanges and negotiating permissions while switching between tabs in Teams, for example to move from Posts to Files. For an example of the OAuth flow for tabs, see this document.
  • Teams uses industry-standard protocols for user authentication, wherever possible.

The next sections discuss some of these core technologies.

Microsoft Entra ID

Microsoft Entra ID functions as the directory service for Microsoft 365 and Office 365. It stores all user and application directory information and policy assignments.

Traffic Encryption in Teams

This table shows the main Traffic types and what protocol is used for encryption.

Traffic type Encrypted by
Server-to-server TLS (with MTLS or Service-to-Service OAuth)
Client-to-server, for example, instant messaging and presence TLS
Media flows, for example, audio and video sharing of media TLS
Audio and video sharing of media SRTP/TLS
Signaling TLS
Client-to-client enhanced encryption (for example, end-to-end encryption calls) SRTP/DTLS

Certificate Revocation List (CRL) Distribution Points

Microsoft 365 and Office 365 traffic takes place over TLS/HTTPS encrypted channels, meaning that certificates are used for encryption of all traffic. Teams requires all server certificates to contain one or more CRL distribution points. CRL distribution points (CDPs) are locations from which CRLs can be downloaded for purposes of verifying that the certificate hasn't been revoked since the time it was issued and the certificate is still within the validity period. A CRL distribution point is noted in the properties of the certificate as a URL and is secure HTTP. The Teams service checks CRL with every certificate authentication.

Enhanced Key Usage

All components of the Teams service require all server certificates to support Enhanced Key Usage (EKU) for server authentication. Configuring the EKU field for server authentication means that the certificate is valid for authenticating servers. This EKU is essential for MTLS.

TLS for Teams

Teams data is encrypted in transit and at rest in Microsoft services, between services, and between clients and services. Microsoft does this using industry standard technologies such as TLS and SRTP to encrypt all data in transit. Data in transit includes messages, files, meetings, and other content. Enterprise data is also encrypted at rest in Microsoft services so that organizations can decrypt the content if needed, to meet security and compliance obligations through methods such as eDiscovery. For more information about encryption in Microsoft 365, see Encryption in Microsoft 365

TCP data flows are encrypted using TLS, and MTLS and Service-to-service OAuth protocols provide endpoint authenticated communications between services, systems, and clients. Teams uses these protocols to create a network of trusted systems and to ensure that all communication over that network is encrypted.

On a TLS connection, the client requests a valid certificate from the server. To be valid, the certificate must have been issued by a Certificate Authority (CA) that is also trusted by the client and the DNS name of the server must match the DNS name on the certificate. If the certificate is valid, the client uses the public key in the certificate to encrypt the symmetric encryption keys to be used for the communication, so only the original owner of the certificate can use its private key to decrypt the contents of the communication. The resulting connection is trusted and from that point is not challenged by other trusted servers or clients.

Using TLS helps prevent both eavesdropping and man-in-the middle attacks. In a man-in-the-middle attack, the attacker reroutes communications between two network entities through the attacker's computer without the knowledge of either party. TLS and Teams' specification of trusted servers mitigate the risk of a man-in-the middle attack partially on the application layer by using encryption that is coordinated using the Public Key cryptography between the two endpoints. An attacker would have to have a valid and trusted certificate with the corresponding private key and issued to the name of the service to which the client is communicating to decrypt the communication.

Encryption in Teams and Microsoft 365

There are multiple layers of encryption at work within Microsoft 365. Encryption in Teams works with the rest of Microsoft 365 encryption to protect your organization's content. This article describes encryption technologies that are specific to Teams. For an overview of encryption in Microsoft 365, see Encryption in Microsoft 365.

Media encryption

Call flows in Teams are based on the Session Description Protocol (SDP) RFC 8866 offer and answer model over HTTPS. Once the callee accepts an incoming call, the caller and callee agree on the session parameters.

Media traffic is encrypted by, and flows between, the caller and callee using Secure RTP (SRTP), a profile of Real-time Transport Protocol (RTP) that provides confidentiality, authentication, and replay attack protection to RTP traffic. SRTP uses a session key generated by a secure random number generator and exchanged using the signaling TLS channel. In most cases, client to client media traffic is negotiated through client to server connection signaling, and is encrypted using SRTP when going directly from client to client.

In normal call flows, negotiation of the encryption key occurs over the call signaling channel. In an end-to-end encrypted call, the signaling flow is the same as a regular one-to-one Teams call. However, Teams uses DTLS to derive an encryption key based on per-call certificates generated on both client endpoints. Since DTLS derives the key based on the client certificates, the key is opaque to Microsoft. Once both clients agree upon the key, the media begins to flow using this DTLS-negotiated encryption key over SRTP.

To protect against a man-in-the-middle attack between the caller and callee, Teams derives a 20-digit security code from the SHA-256 thumbprints of the caller’s and callee’s endpoint call certificates. The caller and callee can validate the 20-digit security codes by reading them to each other to see if they match. If the codes don’t match, then the connection between the caller and callee has been intercepted by a man-in-the-middle attack. If the call has been compromised, users can end the call manually.

Teams uses a credentials-based token for secure access to media relays over TURN. Media relays exchange the token over a TLS-secured channel.

Federal Information Processing Standard (FIPS)

Teams uses FIPS compliant algorithms for encryption key exchanges. For more information on the implementation of FIPS, see Federal Information Processing Standard (FIPS) Publication 140-2.

User and Client Authentication

A trusted user is one whose credentials have been authenticated by Microsoft Entra ID in Microsoft 365 or Office 365.

Authentication is the provision of user credentials to a trusted server or service. Teams uses the following authentication protocols, depending on the status and location of the user.

  • Modern Authentication (MA) is the Microsoft implementation of OAUTH 2.0 for client to server communication. It enables security features such as multifactor authentication and Conditional Access. To use MA, both the online tenant and the clients need to be enabled for MA. The Teams clients across PC and mobile, and the web client, all support MA.

Note

If you want more information on Microsoft Entra authentication and authorization methods, this article's Introduction and 'Authentication basics in Microsoft Entra ID' sections will help.

Teams authentication is accomplished through Microsoft Entra ID and OAuth. The process of authentication can be simplified to:

  • User sign in > token issuance > next request use issued token.

Requests from client to server are authenticated and authorized by Microsoft Entra ID with the use of OAuth. Users with valid credentials issued by a federated partner are trusted and pass through the same process as native users. However, further restrictions can be put into place by administrators.

For media authentication, the ICE and TURN protocols also use the Digest challenge as described in the IETF TURN RFC.

Windows PowerShell and Team Management Tools

In Teams, IT Admins can manage their service via the Microsoft 365 admin center or by using Tenant Remote PowerShell (TRPS). Tenant admins use Modern Authentication to authenticate to TRPS.

Configuring access to Teams at your Internet boundary

For Teams to function properly, for example, for users to be able to join meetings, customers need to configure their internet access such that outbound UDP and TCP traffic to services in the Teams cloud is allowed. For more information, see Office 365 URLs and IP address ranges.

UDP 3478-3481 and TCP 443

The UDP 3478-3481 and TCP 443 ports are used by clients to request service for audio visuals. A client uses these two ports to allocate UDP and TCP ports respectively to enable these media flows. The media flows on these ports are protected with a key that is exchanged over a TLS protected signaling channel.

Federation safeguards for Teams

Federation provides your organization with the ability to communicate with other organizations to share IM and presence. In Teams federation is on by default. However, tenant admins have the ability to control federation through the Microsoft 365 admin center.

Addressing threats to Teams Meetings

Enterprise users can create and join real-time meetings and invite external users who don't have a Microsoft Entra ID, Microsoft 365, or Office 365 account, to participate in these meetings.

Letting external users participate in Teams meetings can be useful, but also brings up some security risks. To address these risks, Teams uses these safeguards:

Before the meeting:

  1. Decide which external participant types will be allowed to join your meetings:

    • Anonymous access allows for meeting join of (1) unauthenticated users that are not signed in Team (typically joining through the meeting link in browser) and (2) authenticated users from external tenants that don’t have established External access with the organizer and your org.

    • Through External access you can decide which authenticated external users and organizations will be able to join your meetings with more privileges. These users are considered to belong to trusted organizations.

    • Guest access allows you create guest accounts for people outside of your org to have access to teams, documents in channels, resources, chats, and applications while maintaining control over your corporate data.

Note

Users and orgs that don’t have external access with your org will be considered anonymous. In case you’ve blocked anonymous join they won’t be able to join your meetings.

External access needs to be enabled bi-directionally, both organizations need to allow for mutual External access.

Note

For more information on Guest and External Access in Teams, see this article. It covers what features guest or external users can expect to see and use when they login to Teams.

  1. Decide who can join the meeting directly and who will need to wait in the Lobby to be admitted by Organizer, co-organizer or authenticated users with Presenter meeting role:

  2. Decide if anonymous users and dial-in callers can start a meeting before users from your org, users from trusted org and users with guest access joins the call.

Note

Scheduling meetings is restricted to authenticated users from your org or users with guest access to your org.

During the meeting:

  1. Assign specific participant meeting roles to determine meeting control privileges. Meeting participants fall into groups, each with its own privileges and restrictions, outlined here.

Note

If you're recording meetings and want to see a permissions matrix around accessing the content, consult this article and its matrix.

Modify while the meeting is running:

  • You can modify the meeting options while a meeting is on-going. The change, when it's saved, will be noticeable in the running meeting within seconds. It also affects any future occurrences of the meeting. For details on how to assign these roles, read this article.

Note

Changes in Teams admin settings can take up to 24 hours.

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