CVE-2021-20323Public exploitNetwork attack vector

Script execution via unescaped registration errors

Published Mar 25, 2022 · Updated Aug 3, 2024

Reflected XSS in Keycloak before 18.0.0 allows remote attackers to execute scripts in a user's browser through a crafted POST request. The clients-registrations endpoint returns an attacker-controlled unknown JSON property in a text/html error response without proper output escaping. Exploitation requires a user to render the response, and cross-origin delivery of the JSON POST depends on explicitly permissive CORS.

CVSS severity6.1
Medium
EPSS probability37.25%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Reflected XSS in Keycloak before 18.0.0 allows remote attackers to execute scripts in a user's browser through a crafted POST request. The clients-registrations endpoint returns an attacker-controlled unknown JSON property in a text/html error response without proper output escaping. Exploitation requires a user to render the response, and cross-origin delivery of the JSON POST depends on explicitly permissive CORS.

The record

CVE
CVE-2021-20323
Published
Mar 25, 2022
Updated
Aug 3, 2024
Vendor
Keycloak
Product
Keycloak
Classifications
CWE-79, CWE-116, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 25, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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=keycloak 17.0.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Keycloak · Keycloakversion=keycloak 17.0.0

Published CVSS scores

6.1Red HatCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
6.1NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-116Improper Encoding or Escaping of Output
T1190Exploit Public-Facing Application

Public exploit references

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

Technologies

Your stack

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Your stack

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