CVE-2021-21276

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in. High risk of data exposure.

Exploit odds

7.2% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-02-01 15:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 03:35 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 7.2% in the next 30 days (EPSS).
• Severity: 9.3 CRITICAL (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-02-01 15:15 UTC

Last modified

2026-06-17 03:35 UTC

Exploit probability

7.2% · EPSS, next 30 days

Weakness

CWE-863 · Incorrect authorization

Record status

Modified

Affected

• Polrproject Polr < 2.3.0

Attack profile CVSS 3.1 · 9.3

Attack vector

Network

Complexity

Low

Privileges needed

None

User interaction

None

Scope

Changed

Confidentiality

High

Integrity

Low

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-21276.md and follow it. Check whether this project is affected by CVE-2021-21276. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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