ZDI-26-534: (Pwn2Own) Microsoft Exchange Capture-Replay Authentication Bypass Vulnerability
Admiralty grading (A–F · 1–6)
Source reliability
- A Completely reliable
- B Usually reliable
- C Fairly reliable
- D Not usually reliable
- E Unreliable
- F Cannot be judged
Information credibility
- 1 Confirmed
- 2 Probably true
- 3 Possibly true
- 4 Doubtful
- 5 Improbable
- 6 Cannot be judged
NATO Admiralty (AJP-2.1) grades confidence, independent of the risk score. Cross-source corroboration isn't tracked for non-CVE news, so single-source items are capped at a lower credibility number; a low number does not imply low quality.
Key metrics
- CVSS
- 8.0
- EPSS
- 1%
Affected versions
Key insight
The vulnerability was demonstrated as a zero-day at Pwn2Own and allows unauthenticated bypass of Exchange authentication , an indication that patches may only become available with a delay.
Description
CVE-2026-62911 is a capture-replay vulnerability in Microsoft Exchange reported by ZDI as part of Pwn2Own. A remote attacker can bypass authentication without prior login by replaying previously captured authentication data. ZDI rates the risk at CVSS 8.1; exploitation requires network access to the affected service but no valid credentials. At the time of publication there were no indications of active exploitation in the wild; a PoC was demonstrated during the competition. Whether and in what form a vendor patch is available is not stated in the advisory.
Risk score
- cvss base
- 80.00
- kev bonus
- 0.00
- epss bonus
- 0.00
- poc bonus
- 15.00
- raw before weight
- 95.00
- industry weight
- 1.21
- freshness factor
- 0.50
- exploitability factor
- 1.00
- days old
- 31.00
- vendor mismatch penalty
- 0.00
Path: operational