CVE-2026-33640 Outline has a rate limit bypass that allows brute force of email login OTP

Critical 9.1 Published 26 Mar 2026 Updated 1 Apr 2026

Attackers can bypass rate limits to brute force email OTP codes, leading to account takeover in Outline collaborative documentation service.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-33640?

CVE-2026-33640 is a critical vulnerability in outline, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Outline is a service that allows for collaborative documentation. Outline implements an Email OTP login flow for users not associated with an Identity Provider. Starting in version 0.86.0 and prior to version 1.6.0, Outline does not invalidate OTP codes based on amount or frequency of invalid submissions, rather it relies on the rate limiter to restrict attempts. Consequently, identified bypasses in the rate limiter permit unrestricted OTP code submissions within the codes lifetime. This allows attackers to perform brute force attacks which enable account takeover. Version 1.6.0 fixes the issue.

Weakness: CWE-307

Which versions are affected by CVE-2026-33640?

It affects 1 product.

Products include Getoutline Outline.

How do I fix CVE-2026-33640?

No fix has been published for CVE-2026-33640 yet. Limit exposure of the affected software until one is available.

Is CVE-2026-33640 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33640 exists.

Sources

Also known as GHSA-cwhc-53hw-qqx6.