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CVE Record

CVE-2021-39191: URL Redirection to Untrusted Site ('Open Redirect') in mod_auth_openidc

mod_auth_openidc is an authentication/authorization module for the Apache 2.x HTTP server that functions as an OpenID Connect Relying Party, authenticating users against an OpenID Connect Provider. In versions prior to 2.4.9.4, the 3rd-party init SSO functionality of mod_auth_openidc was reported to be vulnerable to an open redirect attack by supplying a crafted URL in the `target_link_uri` parameter. A patch in version 2.4.9.4 made it so that the `OIDCRedirectURLsAllowed` setting must be applied to the `target_link_uri` parameter. There are no known workarounds aside from upgrading to a patched version.

MediumCVSS 4.7Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This flaw could let an attacker lure a user through a vulnerable login flow and redirect them to an untrusted site. The main business risk is phishing or trust abuse around SSO, not direct server takeover. Upgrade priority is moderate, especially for public-facing Apache SSO gateways.

Executive priority

Treat as a moderate SSO trust issue. Patch public authentication gateways promptly in the next maintenance window; accelerate if the service protects high-value apps or customer-facing login journeys.

Technical view

mod_auth_openidc before 2.4.9.4 did not apply OIDCRedirectURLsAllowed to target_link_uri in third-party initiated SSO. This allowed open redirect behavior. CVSS 3.1 is 4.7: network reachable, low complexity, no privileges, user interaction required, changed scope, low integrity impact.

Likely exposure

Exposure is likely where Apache 2.x uses zmartzone mod_auth_openidc versions up to 2.4.9.3, especially public SSO entry points using third-party initiated login flows.

Exploitation context

The bundle does not cite active exploitation, and KEV is false. Exploitation requires a user to interact with a crafted login URL, so risk centers on phishing, brand abuse, and redirecting users away from trusted SSO pages.

Researcher notes

The fix specifically ties target_link_uri enforcement to OIDCRedirectURLsAllowed. Evidence is strong for affected versions and patch direction, but the supplied sources do not establish real-world exploitation or broader affected products.

Mitigation direction

  • Upgrade mod_auth_openidc to version 2.4.9.4 or later.
  • Apply relevant Fedora or Debian package security updates where distro packages are used.
  • Review OIDCRedirectURLsAllowed after upgrading to ensure only trusted redirect destinations are allowed.
  • Do not rely on a documented workaround; sources state none aside from upgrading.

Validation and detection

  • Inventory Apache hosts loading mod_auth_openidc.
  • Check installed mod_auth_openidc package or module version is not 2.4.9.3 or older.
  • Confirm patched builds include the 2.4.9.4 fix or downstream security update.
  • Review SSO logs for unusual external target_link_uri destinations, without testing offensive redirects.
Prepared
Confidence
high
Sources
8

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · medium confidence lookup

CWE-601: User-session and phishing behavior lookup

Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2021-39191 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
Medium
CVSS
4.7 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
8Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
4.7CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N2.81.4Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

4.7Medium
CVSS 3.1 vector shape for CVE-2021-39191Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
zmartzonemod_auth_openidc<=2.4.9.3Listed
Weakness

CWE details

CWE-601

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-601 · source CWE mapping

URL Redirection to Untrusted Site ('Open Redirect')

URL Redirection to Untrusted Site ('Open Redirect') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs