Description

Authentication bypass by capture-replay in Microsoft Exchange Server allows an authorized attacker to elevate privileges over a network.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.0",
  "pubDate": "2026-08-11T17:18:45.073Z",
  "pubdate": "2026-08-11T17:18:45.073Z",
  "executiveSummary": "This vulnerability involves an authentication bypass by capture-replay weakness affecting Microsoft Exchange Server. The security flaw allows an authorized attacker to execute unauthorized operations and elevate privileges across a network infrastructure. The risk implications are severe, as successful exploitation undermines the foundational authentication mechanisms of the targeted mail server environment, potentially granting malicious actors unauthorized administrative or elevated access.\nThe attack capability requires the adversary to possess initial authorization within the network boundary, enabling them to capture valid authentication exchanges and replay them maliciously. Exploitation occurs over the network, leveraging architectural flaws in how authentication sessions or cryptographic nonces are validated and managed by the affected Microsoft Exchange Server components. Due to the critical nature of Microsoft Exchange Server in enterprise environments, unauthorized privilege elevation can lead to extensive lateral movement, data exfiltration, and total compromise of messaging infrastructure.",
  "technicalDetails": "The vulnerability is rooted in an authentication bypass by capture-replay mechanism within Microsoft Exchange Server. The root cause stems from improper validation, insufficient anti-replay protections, or inadequate session lifecycle enforcement during the authentication handshake or protocol exchange. Specifically, the vulnerable component fails to adequately track, verify, or invalidate previously intercepted cryptographic tokens, session identifiers, or authentication requests, allowing valid credentials or session data captured over the network to be maliciously reused.\nThe attack flow proceeds as follows: an authorized attacker intercepts valid network traffic containing authentication material directed toward Microsoft Exchange Server. Because the system lacks robust replay detection mechanisms—such as strict timestamp verification, single-use nonces, or dynamic challenge-response validation—the attacker can transmit the captured payload back to the server. The targeted Microsoft Exchange Server incorrectly treats the replayed packet sequence as a legitimate, fresh authentication attempt.\nExploitation requires network exposure and an initial baseline level of authorization to capture and inject traffic, although the ultimate outcome is an illegitimate escalation of privileges. Upon successful payload acceptance, the attacker bypasses standard access control enforcement boundaries, effectively inheriting the elevated security context associated with the replayed session. Post-exploitation impact includes unauthorized access to sensitive communications, administrative command execution, and deep systemic compromise of the Microsoft Exchange Server host environment."
}