CVE-2021-21276: Privilege escalation in Polr

Severity: critical (CVSS 9.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 7.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2021-02-01 · updated 2026-06-17
Weakness: CWE-863

Description

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.

Affected products

cydrobolt polr: before 2.3.0 — fixed in 2.3.0

How to fix

Upgrade to a fixed version: cydrobolt polr 2.3.0. Check the vendor advisory for mitigations.

References

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