CVE-2021-21276
CRITICALEPSS 94th pctlDescription
Polr is an open source URL shortener. in Polr before version 2.3.0, a vulnerability in the setup process allows attackers to gain admin access to site instances, even if they do not possess an existing account. This vulnerability exists regardless of users' settings. If an attacker crafts a request with specific cookie headers to the /setup/finish endpoint, they may be able to obtain admin privileges on the instance. This is caused by a loose comparison (==) in SetupController that is susceptible to attack. The project has been patched to ensure that a strict comparison (===) is used to verify the setup key, and that /setup/finish verifies that no users table exists before performing any migrations or provisioning any new accounts. This is fixed in version 2.3.0. Users can patch this vulnerability without upgrading by adding abort(404) to the very first line of finishSetup in SetupController.php.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N
Exploit Intelligence
High risk: more likely to be exploited than 94% of all known CVEs.
References
Related Vulnerabilities
Other CWE-863 (Incorrect Authorization) vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2023-22518 | Critical | 9.8 | 100% | KEV + Ransom | Fix |
| CVE-2023-38035 | Critical | 9.8 | 100% | KEV + Ransom | - |
| CVE-2022-46169 | Critical | 9.8 | 100% | KEV | Fix |
| CVE-2024-38856 | Critical | 9.8 | 99% | KEV | Fix |
| CVE-2025-29927 | Critical | 9.1 | 99% | - | Fix |
| CVE-2020-36289 | Medium | 5.3 | 99% | - | Fix |
Common questions
How do I fix CVE-2021-21276?
Upgrade polr to 2.3.0 or later.
Is CVE-2021-21276 being actively exploited?
Not that we know of. CVE-2021-21276 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 7.2% is the estimated probability that it will be exploited in the next 30 days. That is higher than 94% of all scored CVEs.
How severe is CVE-2021-21276?
CVE-2021-21276 has a CVSS v3 base score of 9.3, rated critical. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2021-21276 affect?
Published advisories record a fix for polr (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2024-11-21.