CVE-2021-31196Known exploitationPublic exploitNetwork attack vector

Credential disclosure via cookie padding oracle

Published Jul 14, 2021 · Updated Aug 10, 2026

Padding oracle in Microsoft Exchange Server 2013, 2016, and 2019 allows remote attackers to recover OWA credentials through crafted cookies. FbaModule.ParseCadataCookies returns different redirect reasons for invalid CBC padding and validly padded data, exposing a decryption oracle. Exploitation requires a victim's encrypted FBA cookies; the demonstrated ProxyOracle chain obtains them through user interaction and recovers the username and password.

CVSS severity7.2
High
EPSS probability54.06%
Next 30 days · Sep 16, 2026
Known exploitationReported
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

Padding oracle in Microsoft Exchange Server 2013, 2016, and 2019 allows remote attackers to recover OWA credentials through crafted cookies. FbaModule.ParseCadataCookies returns different redirect reasons for invalid CBC padding and validly padded data, exposing a decryption oracle. Exploitation requires a victim's encrypted FBA cookies; the demonstrated ProxyOracle chain obtains them through user interaction and recovers the username and password.

The record

CVE
CVE-2021-31196
Published
Jul 14, 2021
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2016 Cumulative Update 21
Classifications
T1555
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jul 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 21, 2024Exploitation reportedCISA Known Exploited Vulnerabilities catalog listing
  3. 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=15.01.0 <15.01.2308.014

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2016 Cumulative Update 21version=15.01.0 <15.01.2308.014
Microsoft · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.023
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 10version=15.02.0 <15.02.0922.013
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 9version=15.02.0 <15.02.0858.015
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 20version=15.01.0 <15.01.2242.012

Published CVSS scores

7.2MicrosoftCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H

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 exploitationReported
Public exploitPublished

Weakness, pattern, technique

T1555Credentials from Password Stores

Public exploit references

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

Reported exploitation

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