CVE-2026-33640
Published: 26 March 2026
Summary
CVE-2026-33640 is a critical-severity Improper Restriction of Excessive Authentication Attempts (CWE-307) vulnerability in Getoutline Outline. Its CVSS base score is 9.1 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 36.9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 AC-7 (Unsuccessful Logon Attempts) and IA-5 (Authenticator Management).
Deeper analysis
CVE-2026-33640 is a high-severity vulnerability (CVSS 3.1 score of 9.8) affecting the Outline collaborative documentation service, specifically its Email OTP login flow for users not associated with an Identity Provider. In versions 0.86.0 through 1.5.x, Outline fails to invalidate OTP codes based on the number or frequency of invalid submissions, instead relying on a rate limiter with identified bypasses. This permits unrestricted OTP code submissions within the code's lifetime, enabling brute-force attacks that lead to account takeover (CWE-307: Improper Restriction of Excessive Authentication Attempts).
The vulnerability is exploitable remotely over the network by unauthenticated attackers (AV:N/AC:L/PR:N/UI:N) with no privileges or user interaction required. Attackers can brute-force Email OTP codes to gain unauthorized access to user accounts, achieving high impacts on confidentiality, integrity, and availability (C:H/I:H/A:H), including full account takeover.
Outline version 1.6.0 addresses the issue by fixing the OTP invalidation logic and rate limiter bypasses. Security practitioners should upgrade to this version immediately, as detailed in the official release notes (https://github.com/outline/outline/releases/tag/v1.6.0) and GitHub security advisory (https://github.com/outline/outline/security/advisories/GHSA-cwhc-53hw-qqx6).
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-16415
Vulnerability details
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…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Vulnerability in public-facing Outline service directly enables remote exploitation (T1190) via unrestricted OTP brute-forcing (T1110) for account takeover.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
AC-7 enforces limits on consecutive invalid logon attempts and automatic account locking or delays, directly preventing brute-force attacks on the Email OTP login flow.
IA-5 mandates disabling authenticators like OTP codes after a defined number of unsuccessful authentication attempts, addressing the failure to invalidate OTPs despite invalid submissions.
SI-2 requires timely identification, reporting, and correction of software flaws such as the OTP invalidation defect and rate limiter bypasses, mitigating exploitation through patching.