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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-45364: 0.29% estimated exploitation activity in the next 30 days; percentile 22.19 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Am I affected? Users are affected if all of the following are true: - Their app uses better-auth at a version < 1.4.17, or at a v1.5 prerelease tagged <= 1.5.0-beta.8. - The apps authentication endpoints serve clients reachable over IPv6. Most managed hosts including Cloudflare, Vercel, Fly.io, AWS Application Load Balancer, and Google Cloud Load Balancing advertise IPv6 by default. - The app's rate-limit configuration is enabled (the production default) and relies on the leftmost x-forwarded-for value (the stock setup) or any other configured IP-bearing header. If users are on 1.4.16 specifically, the normalizeIP helper exists in your version but the IPv6 prefix length defaults to /128. Stock config still permits prefix rotation because no prefix mask is applied. Either upgrade to 1.4.17 or set advanced.ipAddress.ipv6Subnet: 64 in the config. If applications do not use the rate limiter, or if the deployment serves only IPv4 clients, the prefix-rotation vector does not apply. The…

Published by the source

15 May 2026, 17:41 UTC

Last modified by the source

9 Jun 2026, 10:30 UTC

Source last checked

26 Oct 2026, 12:53 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-p6v2-xcpg-h6xw

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-p6v2-xcpg-h6xw

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# Patch Next advisory brief: CVE-2026-45364

Advisory: Better Auth: Rate limiter keys IPv6 addresses individually and is bypassable via prefix rotation
IDs: CVE-2026-45364, GHSA-p6v2-xcpg-h6xw
Record: https://patchnext.com/advisories/cve-2026-45364 (revision 2, source checked 26 Oct 2026, 12:53 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: better-auth 1.5.0-beta.9, 1.4.17. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm better-auth: affected < 1.4.17; >= 1.5.0-beta.1, < 1.5.0-beta.9; fixed in 1.4.17, 1.5.0-beta.9

## Suggested inspection (read-only)
- `npm ls better-auth --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on better-auth.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-p6v2-xcpg-h6xw (OSV record GHSA-p6v2-xcpg-h6xw lists affected ranges: npm better-auth >=0 =1.5.0-beta.1 <1.5.0-beta.9 (fixed))
- Verified: https://github.com/advisories/GHSA-p6v2-xcpg-h6xw (GitHub-reviewed advisory GHSA-p6v2-xcpg-h6xw lists affected ranges: npm better-auth >=0 =1.5.0-beta.1 <1.5.0-beta.9)

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