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CVE-2021-21276

Status: ModifiedCritical (9.3)—Exploit
Severity9.3Critical · CVSS 3.1
Exploitation probability7.16 %higher than 94% of CVEs
Active exploitationNonot in the CISA catalog
Public exploitAvailablePublished on Exploit-DB

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.

Read full descriptionShow less

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.

CVSS

Base score
9.3
Version
3.1
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N

Exploitation probability (EPSS)

Probability of exploitation in the next 30 days
7.16 %
Percentile among all scored CVEs
94
Score date
10/21/2026

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

Public exploits

Exploit code or detection templates are publicly available. This is not the same as confirmed active exploitation (KEV), but it raises the risk: patch with priority.

Affected technologies (1)

Polrproject — Polr

CWEs

References

Show 8 references

Raw JSON (NVD)

Show
{
  "id": "CVE-2021-21276",
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      "value": "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."
    },
    {
      "lang": "es",
      "value": "Polr es un acortador de URL de código abierto.&#xa0;en Polr anterior a la versión 2.3.0, una vulnerabilidad en el proceso de configuración permite a atacantes obtener acceso de administrador a las instancias del sitio, incluso si no poseen una cuenta existente.&#xa0;Esta vulnerabilidad existe independientemente de la configuración de los usuarios.&#xa0;Si un atacante crea una petición con encabezados de cookies específicas para el endpoint /setup/finish, es posible que pueda obtener privilegios de administrador en la instancia.&#xa0;Esto es causado por una comparación imprecisa (==) en SetupController que es susceptible de ataque.&#xa0;El proyecto ha sido parcheado para garantizar que se utilice una comparación estricta (===) para verificar la clave de configuración, y que /setup/finish verifique que no exista ninguna tabla de usuarios antes de realizar algunas migraciones o aprovisionar cuentas nuevas.&#xa0;Esto se corrige en la versión 2.3.0. Los usuarios pueden parchear esta vulnerabilidad sin actualizar agregando abort (404) a la primera línea de finishSetup en SetupController.php"
    }
  ],
  "lastModified": "2026-06-17T03:35:10.593",
  "configurations": [
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}