CVE-2026-45364 Better Auth: Rate limiter keys IPv6 addresses individually and is bypassable via prefix rotation
Attackers can bypass rate limiting in Better Auth by rotating IPv6 addresses or varying their textual encoding, enabling brute-force attacks on authentication endpoints.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-45364?
CVE-2026-45364 is a high-severity vulnerability in better-auth, published on 28 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-307
Which versions are affected by CVE-2026-45364?
4 open source packages ship an affected version, including better-auth.
| Package | Affected | Fixed in |
|---|---|---|
better-auth npm | All versions | 1.4.17 |
better-auth npm | All versions | 1.4.17 |
better-auth npm | From 1.5.0-beta.1 | 1.5.0-beta.9 |
better-auth npm | From 1.5.0-beta.1 | 1.5.0-beta.9 |
How do I fix CVE-2026-45364?
Upgrade better-auth to a fixed version for your release line: 1.4.17 or 1.5.0-beta.9.
Is CVE-2026-45364 being exploited?
There is no known exploitation of CVE-2026-45364 in the wild and no public exploit code.
Sources
Also known as GHSA-p6v2-xcpg-h6xw.