What’s Vulnerable › Vulnerabilities

CVE-2026-45364: Better Auth: Rate limiter keys IPv6 addresses individually and is bypassable via prefix rotation

• Severity: high (CVSS 7.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-05-28 · updated 2026-07-21
• Weakness: CWE-307

Description

Better 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.

Affected products

• better-auth: before 1.4.17; from 1.5.0-beta.1, < 1.5.0-beta.9

References

• https://github.com/better-auth/better-auth/commit/43e719bcc0c223c7079fa0c611a9cf7ea1188254
• https://github.com/better-auth/better-auth/commit/57af0f7b910dcf7b1a5c0615d10b9bd56bb69bef
• https://github.com/better-auth/better-auth/pull/7470
• https://github.com/better-auth/better-auth/pull/7509
• https://github.com/better-auth/better-auth/security/advisories/GHSA-p6v2-xcpg-h6xw

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