Capture-Replay Authentication Bypass in Microsoft Exchange Server
CVSS 3.1
8.0high
EPSS
1%p69
Published
()
Modified
AI analysis
Microsoft Exchange Server contains an authentication bypass flaw (CWE-294) in which captured authentication material can be replayed, allowing an authorized attacker to elevate privileges over a network. Per the CVSS vector, the attack is network-based with low complexity but requires the attacker to already hold low privileges and some user interaction, and success yields high impact on confidentiality, integrity, and availability. Affected products include on-premises Exchange Server and Exchange Server Subscription Edition, though specific vulnerable version ranges are not specified in the available data. Public scans indicate nearly 22,000 Exchange servers remain exposed to the flaw following the August 2026 Patch Tuesday fixes. No in-the-wild exploitation, public proof-of-concept, or KEV listing is known; the bug was demonstrated at Pwn2Own (ZDI-26-534) and carries an EPSS estimate of 1.3% probability of exploitation within 30 days.
What to do: Apply the Exchange Server security updates released in Microsoft's August 2026 Patch Tuesday (refer to Microsoft's advisory for the exact fixed builds) on all on-premises servers, prioritizing internet-facing systems running OWA, EWS, or ActiveSync. Until patched, limit network exposure of Exchange endpoints to trusted networks and monitor authentication logs for replay-style anomalies; per the scan data, roughly 22,000 servers still need the update.
Affected
microsoft exchange server
—
microsoft exchange server subscription edition
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Estimated exposure
large≈22,000 internet-exposed Exchange servers — Based on public internet scan coverage reporting that nearly 22,000 Microsoft Exchange servers remained exposed to CVE-2026-62911 after patching became available.
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
Description
Authentication bypass by capture-replay in Microsoft Exchange Server allows an authorized attacker to elevate privileges over a network.
Microsoft's August 2026 Patch Tuesday fixes 398 CVEs, including an actively exploited WinSock zero-day CVE-2026-68820 and wormable DNS RCE CVE-2026-62878.
Microsoft's August 2026 Patch Tuesday addresses 398 CVEs across Windows, Office, Azure, Exchange Server, SharePoint, Teams, .NET and other products, with 62 rated Critical. The actively exploited CVE-2026-68820 is a use-after-free in the Windows WinSock afd.sys driver that can yield SYSTEM privileges via a race condition. CVE-2026-62878 is a critical, wormable stack buffer overflow in Windows DNS Server allowing unauthenticated, remote code execution without user interaction. Other notable fixes include WDS TFTP RCE CVE-2026-62893, a QUIC flaw affecting roughly 13.5 million sites (CVE-2026-62815), an Exchange authentication bypass EoP demonstrated at Pwn2Own Berlin (CVE-2026-62911), and two TPM 2.0 reference implementation fixes.
High-severity authentication bypass CVE-2026-62911 in Exchange Server has public exploit code; about 22,000 servers remain unpatched and internet-exposed.
Microsoft's August 2026 Patch Tuesday fixed CVE-2026-62911 (CVSS 8.0), an authentication bypass affecting Exchange Server 2016, 2019 and Subscription Edition. Successful exploitation lets an attacker with basic privileges take over all mailboxes on the targeted server, including reading and sending email and downloading attachments. As of September 1, Shadowserver identified roughly 22,000 unpatched, internet-exposed Exchange servers, including about 6,200 in the US and 5,100 in Germany. NCSC-NL confirmed working exploit code is publicly available, while CISA has not yet reported exploitation in the wild.
About 22,000 Microsoft Exchange servers remain unpatched against critical auth bypass CVE-2026-62911 as a working exploit circulates online.
CVE-2026-62911 is a critical authentication bypass by capture-replay in Microsoft Exchange Server that allows an authorized attacker to elevate privileges over the network, with a CVSS score of 8.0. Microsoft patched the flaw on August 11, 2026, but Shadowserver Foundation scans show roughly 22,000 servers unpatched, with the US (6,200) and Germany (5,100) leading; BSI reports 85% of on-premises Exchange servers in Germany are still vulnerable. The Netherlands' NCSC-NL flagged that a working exploit is circulating, and Exchange 2016/2019 require the Extended Security Updates program for fixes.
Pwn2Own-demoed CVE-2026-62911 capture-replay authentication bypass in Microsoft Exchange allows unauthenticated remote attacks, CVSS 8.1.
ZDI-26-534 describes a capture-replay authentication bypass vulnerability in Microsoft Exchange, demonstrated at Pwn2Own, allowing remote attackers to bypass authentication without credentials. ZDI assigned a CVSS score of 8.1, and the flaw is tracked as CVE-2026-62911. Exchange servers are widely deployed enterprise mail infrastructure.
ZDI discloses a Pwn2Own Microsoft Exchange privilege-escalation flaw (CVE-2026-62911, CVSS 8.8) where the required authentication can be bypassed.
ZDI advisory ZDI-26-538 describes CVE-2026-62911, an improper authorization vulnerability in Microsoft Exchange rated CVSS 8.8. Remote attackers can escalate privileges, and while authentication is nominally required, the existing authentication mechanism can be bypassed. The vulnerability was demonstrated at Pwn2Own.
ZDI advisory discloses Microsoft Exchange remote code execution flaw (CVE-2026-62911, CVSS 7.2) with bypassable authentication.
ZDI advisory ZDI-26-535 describes an external control of file path vulnerability in Microsoft Exchange, tracked as CVE-2026-62911 with a CVSS score of 7.2. Remote attackers can execute arbitrary code on affected installations. Although authentication is required, the existing authentication mechanism can be bypassed.
September 2026 Patch Tuesday forecast expects record CVE volume after August's 398 fixes, with SharePoint flaws CVE-2026-55040 and CVE-2026-63520 actively exploited.
This Patch Tuesday forecast column notes August 2026 Patch Tuesday was the second largest ever with 398 resolved CVEs, yet only one was confirmed actively exploited. SharePoint flaws CVE-2026-55040 and CVE-2026-63520 are being chained for authentication bypass and remote code execution in active attacks against unpatched servers. Microsoft Defender's ShieldBreak elevation of privilege flaw (CVE-2026-69414) is publicly disclosed with PoC code and a fix is expected, while Chrome CVE-2026-85046 was reported exploited in the wild. Several products, including Windows 11 24H2 Home/Pro and Exchange Server 2016/2019 ESU, reach end of support in October 2026.
Help Net Security week-in-review roundup: Claude infostealer lockouts, record Patch Tuesday, McKesson breach, SonicWall zero-days, PaperCut attacks.
This Help Net Security weekly roundup aggregates prior stories: Anthropic locking out Claude users after infostealer-compromised login sessions, August 2026 Patch Tuesday's 398 resolved CVEs, ShinyHunters' claim of 284 million stolen patient records from McKesson, active exploitation of SonicWall SMA 1000 zero-days (CVE-2026-83548/83549), Sangoma Switchvox SQL injection exploitation (CVE-2026-9586), and attackers planting RATs on compromised PaperCut servers. It also covers FBI OAuth phishing warnings, a Spring Ring vishing campaign abusing Microsoft Teams, and a fake Claude Opus 5 GitHub repo delivering RevStealer infostealer.