CVE-2021-33779Network attack vector

Token replay via weak Primary Refresh Token encryption

Published Jul 14, 2021 · Updated Aug 10, 2026

Token replay in AD FS on Microsoft Windows Server 2016, 2019, version 2004, and 20H2 allows remote authenticated users to reuse PRTs. AD FS uses the original key derivation function to protect Primary Refresh Tokens without strong encryption, permitting captured tokens to be replayed. A valid token must first be obtained, and reuse remains possible until that token expires or is renewed; no service interruption is reported.

CVSS severity8.1
High
EPSS probability2.39%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Token replay in AD FS on Microsoft Windows Server 2016, 2019, version 2004, and 20H2 allows remote authenticated users to reuse PRTs. AD FS uses the original key derivation function to protect Primary Refresh Tokens without strong encryption, permitting captured tokens to be replayed. A valid token must first be obtained, and reuse remains possible until that token expires or is renewed; no service interruption is reported.

The record

CVE
CVE-2021-33779
Published
Jul 14, 2021
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Windows Server 2019
Classifications
T1550.001
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jul 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 10, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.0.17763.0 <10.0.17763.2061

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Microsoft · Windows Server 2019version=10.0.17763.0 <10.0.17763.2061
Microsoft · Windows Server 2016 (Server Core installation)version=10.0.14393.0 <10.0.14393.4530
Microsoft · Windows Server, version 2004version=10.0.0 <10.0.19041.1110
Microsoft · Windows Server 2016version=10.0.14393.0 <10.0.14393.4530
Microsoft · Windows Server 2019 (Server Core installation)version=10.0.17763.0 <10.0.17763.2061
Microsoft · Windows Server version 20H2version=10.0.0 <10.0.19042.1110

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:N
5.5NIST NVDAV:N/AC:L/Au:S/C:P/I:P/A:N
8.1Microsoft CorporationCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1550.001Application Access Token

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo