Yes, phishing can bypass some forms of multifactor authentication (MFA)—not necessarily by cracking the second factor, but by relaying a real sign-in and stealing the authenticated session. Microsoft says phishing-as-a-service (PhaaS) kits are increasing the reach of these adversary-in-the-middle (AiTM) attacks. The strongest direct defense is phishing-resistant authentication, such as passkeys or FIDO2 security keys, combined with access controls and monitoring.
What PhaaS and AiTM mean
Phishing-as-a-service is a model in which operators obtain phishing infrastructure or kits from a provider. That can lower the technical and operational barriers to running credential-phishing campaigns. It does not mean every kit works the same way: some campaigns may simply collect credentials, while AiTM-capable setups can relay a live authentication flow.
An AiTM attack places an attacker-controlled proxy between a person and a legitimate service. The proxy presents a convincing sign-in page, forwards the person’s activity to the real service, and passes the service’s prompts and responses back and forth. If the flow succeeds, the attacker may capture a session cookie or token that represents an already authenticated session.
How an AiTM attack can get past MFA
- A lure brings the user to a fake page. It may resemble a shared-document notice, payment request, or account-verification prompt.
- The proxy relays the sign-in. The user enters credentials into the phishing page; the proxy forwards them to the real identity service.
- The real service requests MFA. The proxy relays the prompt to the user and sends the response to the service.
- The service authenticates the session. Its response can include a session cookie or token, which the attacker may capture and use to access the authenticated session.
Microsoft’s Digital Defense Report 2023 describes this reverse-proxy sequence. In this context, “MFA bypass” often means the attacker relays a phishable factor and then steals or replays the resulting session. It does not establish that every second factor has been cryptographically defeated. Microsoft Learn cautions that “Traditional MFA methods remain vulnerable to adversary-in-the-middle attacks and social engineering” in its identity-protection guidance.
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What Microsoft has reported about the threat
In a May 29, 2025 threat-intelligence article, Microsoft said AiTM credential phishing was increasing as MFA adoption grew and that PhaaS kits had increased its impact. This is Microsoft’s qualitative assessment, not a numeric estimate of how common the attacks are across all organizations. The same article identifies Evilginx as an AiTM-capable framework used by multiple actors, including Storm-0485 and Star Blizzard. Microsoft also described lures involving payment remittance, shared documents, and fake LinkedIn account verification, distributed through email, Teams, social media, and QR codes. Microsoft Security Blog, May 29, 2025.
Microsoft’s November 21, 2024 article reported a 146% rise in AiTM attacks, attributing the figure to the 2024 Microsoft Digital Defense Report. It also said the fraudulent ONNX operation was among the top five phishing-kit providers by email volume in the first half of 2024. The first figure is Microsoft’s reported increase; the second is a ranking by email volume for that specific period, not a ranking of providers by every measure. Microsoft On the Issues, November 21, 2024.
Microsoft says threat actors have used large language models to support social-engineering operations. That observation concerns creating or improving lures; it does not mean AI performs the AiTM proxy technique itself.
Which authentication methods are more resistant?
Passkeys and FIDO2 security keys are phishing-resistant options Microsoft recommends. Unlike a password or one-time code that can be entered into a convincing fake page and relayed, phishing-resistant authentication is designed to bind authentication to the legitimate service. Microsoft names these methods in its identity-protection guidance.
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| Approach | Resistance to relay phishing | What to check |
|---|---|---|
| Traditional phishable MFA, such as codes or prompts that can be relayed | Can be exposed to AiTM relay and subsequent session theft. | Use where necessary as a layer rather than treating it as a guarantee against phishing. |
| Passkeys or FIDO2 security keys | Phishing-resistant authentication is the stronger direct mitigation in Microsoft’s guidance. | Confirm support for the identity provider, applications, accounts, devices, and organizational policy; plan enrollment and recovery. |
Microsoft’s documentation supports this comparison, but it is not an independent cross-vendor product test or ranking. Compatibility and recovery arrangements vary by platform and organization. A physical security key may be one option, but buying one does not by itself protect an account or tenant; confirm that the account and organization allow it.
How organizations can reduce AiTM risk
- Prioritize phishing-resistant authentication. Start with privileged accounts and extend coverage to other users, including external users where appropriate. Check actual enrollment and application support rather than assuming a method is available everywhere.
- Apply risk-aware access controls. Use Conditional Access or an equivalent policy to consider sign-in risk, location, and device status. Monitor sign-in and token-risk signals. Microsoft’s May 2025 guidance recommends these controls alongside MFA, not as a replacement for phishing-resistant authentication.
- Strengthen email and link defenses. Apply safe-link handling to internal as well as external messages where supported. Train users to report unexpected login prompts, shared-document lures, QR-code messages, and unusual requests from familiar accounts.
- Reduce adjacent identity-phishing paths. Restrict device-code authentication flow when it is unnecessary, and limit user consent to untrusted applications. These controls address other identity risks; they do not substitute for phishing-resistant sign-in.
- Make reporting straightforward. Ensure staff know how to report suspicious messages and prompts quickly, including messages appearing to come from colleagues or known services.
What to do if a session may have been stolen
Treat suspected AiTM activity as an identity and session incident, not only as a password problem. A password reset alone may not invalidate an already stolen session in every platform. Follow your identity provider’s current incident procedures; the actions below are general priorities, not a universal runbook.
- Investigate relevant sign-ins and identity alerts, including unfamiliar locations, devices, and activity after the suspected login.
- Revoke active sessions or tokens using the controls available in your identity platform.
- Reset affected credentials and confirm the user is enrolled in approved authentication methods.
- Review for persistence, including newly added authentication methods and application grants or consent.
- Preserve relevant evidence and escalate through your organization’s incident-response process.
Does MFA still help?
Yes. MFA remains useful, but its protection depends on the method and the attack path. Relay-based phishing shows why organizations should not treat every MFA prompt as equally resistant to phishing. Phishing-resistant methods are the stronger authentication control; conditional access, monitoring, email defenses, and prompt reporting add layers around it.
Microsoft’s internal rollout illustrates one organization’s adoption, not a universal benchmark: Microsoft Learn said 92% of Microsoft employee productivity accounts were protected with phishing-resistant methods as part of its Secure Future Initiative rollout, on a page accessed October 5, 2026. Microsoft Learn: Phishing-resistant MFA.
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