Better Auth: Rate limiter keys IPv6 addresses individually and is bypassable via prefix rotation

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Summary

CVECVE-2026-45364
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-28 22:17:00 UTC
Updated2026-06-01 18:41:24 UTC
DescriptionBetter Auth is an authentication and authorization library for TypeScript. Prior to 1.4.17 and 1.5.0-beta.9, Better Auth's HTTP rate limiter keyed each request by the exact textual IP address it received in x-forwarded-for (or the configured IP-bearing header). IPv6 clients controlling a typical /64 allocation could rotate through 2^64 distinct source addresses without exhausting the per-address counter, defeating rate limiting on /sign-in/email, /sign-up/email, /forget-password, and every other path the limiter protects. The same bug allowed a single client to vary the textual encoding of one IPv6 address (uppercase, compression, IPv4-mapped, hex-encoded IPv4-in-IPv6) and produce multiple distinct keys. This vulnerability is fixed in 1.4.17 and 1.5.0-beta.9.

Risk And Classification

Primary CVSS: v3.1 7.3 HIGH from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

EPSS: 0.000700000 probability, percentile 0.214440000 (date 2026-06-01)

Problem Types: CWE-307 | CWE-307 CWE-307: Improper Restriction of Excessive Authentication Attempts

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.3HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
3.1CNADECLARED7.3HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Better-auth Better-auth affected < 1.4.17 Not specified
CNA Better-auth Better-auth affected >= 1.5.0-beta.1, < 1.5.0-beta.9 Not specified

References

ReferenceSourceLinkTags
github.com/better-auth/better-auth/pull/7470 [email protected] github.com
github.com/better-auth/better-auth/commit/57af0f7b910dcf7b1a5c0615d10b9b... [email protected] github.com
github.com/better-auth/better-auth/security/advisories/GHSA-p6v2-xcpg-h6xw [email protected] github.com
github.com/better-auth/better-auth/commit/43e719bcc0c223c7079fa0c611a9cf... [email protected] github.com
github.com/better-auth/better-auth/pull/7509 [email protected] github.com
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