CVE-2026-45364

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 representation-aliasing vector still applies to IPv6 addresses delivered over IPv4 transport in some edge cases (an upstream proxy carrying an IPv4-mapped IPv6 source), but it is rare in practice.

Fix:

1. Upgrade to [email protected] or later. The current stable line 1.6.x and the pre-release line 1.7.0-beta both carry the fix.
2. If applications cannot upgrade, see workarounds below.

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.

Details

The pre-fix getIp function returned the leftmost x-forwarded-for value verbatim after a single validity check, and onRequestRateLimit constructed the rate-limit key by string concatenation of that value with the request path. Two facts of IPv6 made the key space larger than the population of clients:

• ISPs and cloud providers assign prefixes, not addresses. RFC 6177 recommends /56 for residential users; cloud providers commonly assign /29 to /48. An attacker controlling a single /64 therefore controls 2^64 source addresses without doing anything unusual.
• IPv6 has multiple textual representations for the same address. RFC 5952 specifies a canonical form, but RFC 4291 §2.2 permits the older mixed forms, and ::ffff:0:0/96 IPv4-mapped addresses can be written as either dotted-decimal or hex-encoded.

The fix in [email protected] introduces normalizeIP and applies it to every getIp result. Normalization expands compressed IPv6 forms, lowercases hex digits, collapses IPv4-mapped IPv6 to plain IPv4, and applies a default /64 prefix mask. The rate-limit key construction now uses an explicit | separator to prevent key-construction collisions across address-and-path joins.

The /64 default matches the smallest commonly-allocated IPv6 unit, so a single client cannot use prefix rotation to defeat rate limiting on stock config. Operators who serve clients on coarser allocations (/56 for residential ISPs, larger for cloud) can configure advanced.ipAddress.ipv6Subnet accordingly.

Workarounds

If users cannot upgrade past 1.4.17:

• On >= 1.4.16: set advanced.ipAddress.ipv6Subnet: 64 in the auth configuration. The normalizeIP helper is present at 1.4.16; only the default is wrong. This restores the post-1.4.17 behavior on stock config.
• On < 1.4.16: shift the bypass mitigation upstream. Set the IPv6 prefix length on the app's CDN, WAF, or load balancer rate-limit policy to /64 (or coarser per RFC 6177 if the app serves residential traffic). Cloudflare, Vercel Firewall, AWS WAF, and Google Cloud Armor all support per-prefix rate limiting.
• As a partial mitigation on any version: tighten the customRules window for sign-in, sign-up, and password-reset endpoints. This narrows the abuse window but does not close it.

Credit

Reported by @nexryai on GitHub.

Resources

• CWE-307: Improper Restriction of Excessive Authentication Attempts
• RFC 4291: IP Version 6 Addressing Architecture
• RFC 5952: A Recommendation for IPv6 Address Text Representation
• RFC 6177: IPv6 Address Assignment to End Sites

The bypass enables unbounded authentication attempts from a single IPv6-capable client. Direct consequences:

• Credential-stuffing and brute-force on /sign-in/email are no longer rate-limited per client.
• Account enumeration via response-shape differences becomes faster.
• Password-reset and email-verification email fan-out can be amplified.

The bypass does not directly compromise any account. Successful exploitation still requires the attacker to guess a credential the password store accepts. The rating reflects the loss of one defense-in-depth layer rather than a direct compromise.

CVE-2026-45364 has a CVSS score of 7.3 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.17, 1.5.0-beta.9); upgrading removes the vulnerable code path.

Kodem intelligence

Fixed in [email protected] on the v1.4.x maintenance line and in [email protected] on the v1.5.x line. PR #7470 introduced the normalization primitive (packages/core/src/utils/ip.ts) and applied it to getIp and the rate-limit key. PR #7509 changed the IPv6 prefix-length default from /128 to /64 so that stock config closes the prefix-rotation vector without requiring users to opt in.

After the patch, the rate limiter treats all IPv6 addresses within a /64 allocation as a single client, all textual encodings of one IPv6 address as the same address, and all IPv4-mapped IPv6 addresses as their underlying IPv4 form.

1. What is CVE-2026-45364? CVE-2026-45364 is a high-severity security vulnerability in better-auth (npm), affecting versions < 1.4.17. It is fixed in 1.4.17, 1.5.0-beta.9.
2. How severe is CVE-2026-45364? CVE-2026-45364 has a CVSS score of 7.3 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of better-auth are affected by CVE-2026-45364? better-auth (npm) versions < 1.4.17 is affected.
4. Is there a fix for CVE-2026-45364? Yes. CVE-2026-45364 is fixed in 1.4.17, 1.5.0-beta.9. Upgrade to this version or later.
5. Is CVE-2026-45364 exploitable, and should I be worried? Whether CVE-2026-45364 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-45364 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-45364?
   • Upgrade better-auth to 1.4.17 or later
   • Upgrade better-auth to 1.5.0-beta.9 or later