How severe is CVE-2021-23435?

CVE-2021-23435 has a CVSS 3.x base score of 7.6, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is low, integrity low, and availability high.

Is CVE-2021-23435 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (49th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-23435?

CVE-2021-23435 affects Thoughtbot Clearance. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-23435?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-23435 published?

CVE-2021-23435 was published on 2021-09-12 and last updated on 2026-06-17.

CVE-2021-23435

CVE-2021-23435 is a high-severity vulnerability in Thoughtbot Clearance with a CVSS 3.x base score of 7.6. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-601.

Key facts

Severity: High (CVSS 3.x base score 7.6)
CVSS v2: 5.8
EPSS exploit prediction: 1% (49th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-601
Affected product: Thoughtbot Clearance
Published: 2021-09-12
Last modified: 2026-06-17

Description

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).

Frequently asked questions

What is CVE-2021-23435?: 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).
How severe is CVE-2021-23435?: CVE-2021-23435 has a CVSS 3.x base score of 7.6, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is low, integrity low, and availability high.
Is CVE-2021-23435 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (49th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-23435?: CVE-2021-23435 affects Thoughtbot Clearance. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-23435?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-23435 published?: CVE-2021-23435 was published on 2021-09-12 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:thoughtbot:clearance:*:*:*:*:*:*:*:*

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