CVE-2021-39191Network attack vector

Open redirect via unchecked target_link_uri

Published Sep 3, 2021 · Updated Aug 4, 2024

Open redirect in mod_auth_openidc before 2.4.9.4 allows remote attackers to send users to an attacker-controlled URL through third-party init SSO. The 3rd-party initiated SSO handler accepts target_link_uri without applying the OIDCRedirectURLsAllowed allowlist, so a crafted destination reaches the redirect response. The attack requires a victim to follow the crafted SSO link; impact is redirection to an attacker-selected site, with no reported direct server data modification or service interruption.

CVSS severity6.1
Medium
EPSS probability1.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Open redirect in mod_auth_openidc before 2.4.9.4 allows remote attackers to send users to an attacker-controlled URL through third-party init SSO. The 3rd-party initiated SSO handler accepts target_link_uri without applying the OIDCRedirectURLsAllowed allowlist, so a crafted destination reaches the redirect response. The attack requires a victim to follow the crafted SSO link; impact is redirection to an attacker-selected site, with no reported direct server data modification or service interruption.

The record

CVE
CVE-2021-39191
Published
Sep 3, 2021
Updated
Aug 4, 2024
Vendor
OpenIDC
Product
mod_auth_openidc
Classifications
CWE-601, T1608.005
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 3, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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

  1. Affected versionversion=<=2.4.9.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

OpenIDC · mod_auth_openidcversion=<=2.4.9.3

Published CVSS scores

4.7GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N
5.8NVDAV:N/AC:M/Au:N/C:P/I:P/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-601URL Redirection to Untrusted Site ('Open Redirect')
T1608.005Link Target

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

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Your stack

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