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CVE Record

CVE-2021-23435: Open Redirect

This affects the package clearance before 2.5.0. The vulnerability can be possible when users are able to set the value of session[:return_to]. If the value used for return_to contains multiple leading slashes (/////example.com) the user ends up being redirected to the external domain that comes after the slashes (http://example.com).

HighCVSS 7.6Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2021-23435 is an open redirect in the Ruby clearance authentication package before 2.5.0. If an application lets users influence the post-login return destination, crafted values with repeated leading slashes can send users to an external site, enabling phishing or trust abuse.

Executive priority

Treat as high priority for internet-facing apps that use clearance. This is less likely to enable direct system takeover, but it can damage user trust and support phishing. Patch or validate exposure during the next security maintenance window.

Technical view

The issue is in handling of session[:return_to]. When user-controlled input reaches that session value, clearance before 2.5.0 may treat a malformed path as safe and redirect outside the application. The source bundle does not identify affected CPEs or enumerate exact vulnerable releases beyond before 2.5.0.

Likely exposure

Exposure is likely limited to Ruby applications using clearance before 2.5.0 with login or authentication flows that store user-influenced return paths in session[:return_to]. Public-facing authentication routes increase business relevance. The bundle lists versions as unspecified apart from the before-2.5.0 description.

Exploitation context

The CVSS vector indicates network reachability, low complexity, no privileges, and required user interaction. The bundle does not report active exploitation, and KEV is false. Impact is mainly phishing, redirect-based trust abuse, and possible disruption depending on application flow.

Researcher notes

Key uncertainty is application-specific reachability: the bug requires user influence over session[:return_to]. Validate vulnerable package versions and redirect handling in the actual app. Do not assume active exploitation from the provided evidence.

Mitigation direction

  • Upgrade clearance to 2.5.0 or later where supported.
  • Check vendor guidance and the linked pull request before production rollout.
  • Restrict return destinations to same-origin relative paths.
  • Reject malformed redirect paths with repeated leading slashes.
  • Add tests covering unsafe return_to values in authentication flows.

Validation and detection

  • Inventory applications and lockfiles for the clearance package version.
  • Review code assigning user-controlled input to session[:return_to].
  • Confirm authentication redirects cannot leave the trusted application origin.
  • Verify unit or e2e tests cover malformed return destinations.
  • Check security monitoring for suspicious external redirects from login flows.
Prepared
Confidence
medium
Sources
4

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.

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CVE-2021-23435 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.6 (3.1)
Known Exploited
No
Published

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

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
3Source 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
7.6CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H2.84.7Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

7.6High
CVSS 3.1 vector shape for CVE-2021-23435Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

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
n/aclearanceunspecifiedListed
Weakness

CWE details

No CWE listed

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