Vulnerability-Lookup

CVE-2026-45364 (GCVE-0-2026-45364)

Vulnerability from cvelistv5 – Published: 2026-05-28 21:34 – Updated: 2026-05-29 19:05

Title

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

Summary

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.

Severity

7.3 (High)


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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-05-29 19:05 UTC

CWE

CWE-307 - Improper Restriction of Excessive Authentication Attempts

Assigner

GitHub_M mitre root

References

5 references

URL	Tags
https://github.com/better-auth/better-auth/securi…	x_refsource_CONFIRM
https://github.com/better-auth/better-auth/pull/7470	x_refsource_MISC
https://github.com/better-auth/better-auth/pull/7509	x_refsource_MISC
https://github.com/better-auth/better-auth/commit…	x_refsource_MISC
https://github.com/better-auth/better-auth/commit…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
better-auth	better-auth	Affected: < 1.4.17 Affected: >= 1.5.0-beta.1, < 1.5.0-beta.9	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-45364

Vulnerability from fkie_nvd - Published: 2026-05-28 22:17 - Updated: 2026-07-21 10:10

Severity

7.3 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

Summary

References

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

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "better-auth",
          "vendor": "better-auth",
          "versions": [
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              "status": "affected",
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            },
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            }
          ]
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      ],
      "source": "security-advisories@github.com"
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    },
    {
      "lang": "es",
      "value": "Better Auth es una biblioteca de autenticaci\u00f3n y autorizaci\u00f3n para TypeScript. Antes de 1.4.17 y 1.5.0-beta.9, el limitador de tasa HTTP de Better Auth identificaba cada solicitud por la direcci\u00f3n IP textual exacta que recib\u00eda en x-forwarded-for (o el encabezado configurado que conten\u00eda la IP). Clientes IPv6 que controlaban una asignaci\u00f3n /64 t\u00edpica pod\u00edan rotar a trav\u00e9s de 2^64 direcciones de origen distintas sin agotar el contador por direcci\u00f3n, eludiendo la limitaci\u00f3n de tasa en /sign-in/email, /sign-up/email, /forget-password y cualquier otra ruta que el limitador protegiera. El mismo error permit\u00eda a un \u00fanico cliente variar la codificaci\u00f3n textual de una direcci\u00f3n IPv6 (may\u00fasculas, compresi\u00f3n, mapeada a IPv4, IPv4 codificada en hexadecimal en IPv6) y producir m\u00faltiples claves distintas. Esta vulnerabilidad est\u00e1 corregida en 1.4.17 y 1.5.0-beta.9."
    }
  ],
  "id": "CVE-2026-45364",
  "lastModified": "2026-07-21T10:10:00.103",
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GHSA-P6V2-XCPG-H6XW

Vulnerability from github – Published: 2026-05-15 17:41 – Updated: 2026-06-09 10:26

Summary

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

Details

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:

Upgrade to better-auth@1.4.17 or later. The current stable line 1.6.x and the pre-release line 1.7.0-beta both carry the fix.
If applications cannot upgrade, see workarounds below.

Summary

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 better-auth@1.4.17 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.

Patches

Fixed in better-auth@1.4.17 on the v1.4.x maintenance line and in better-auth@1.5.0-beta.9 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.

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.

Impact

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.

Credit

Reported by @nexryai on GitHub.

Resources

Severity

7.3 (High)


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

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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "better-auth"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.4.17"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "better-auth"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.5.0-beta.1"
            },
            {
              "fixed": "1.5.0-beta.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-45364"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-307"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-15T17:41:37Z",
    "nvd_published_at": "2026-05-28T22:17:00Z",
    "severity": "HIGH"
  },
  "details": "### Am I affected?\n\nUsers are affected if all of the following are true:\n\n- Their app uses `better-auth` at a version `\u003c 1.4.17`, or at a v1.5 prerelease tagged `\u003c= 1.5.0-beta.8`.\n- 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.\n- The app\u0027s 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.\n\nIf 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.\n\nIf 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.\n\nFix:\n\n1. Upgrade to `better-auth@1.4.17` or later. The current stable line `1.6.x` and the pre-release line `1.7.0-beta` both carry the fix.\n2. If applications cannot upgrade, see workarounds below.\n\n### Summary\n\nBetter Auth\u0027s 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.\n\n### Details\n\nThe 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:\n\n- 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.\n- IPv6 has multiple textual representations for the same address. RFC 5952 specifies a canonical form, but RFC 4291 \u00a72.2 permits the older mixed forms, and `::ffff:0:0/96` IPv4-mapped addresses can be written as either dotted-decimal or hex-encoded.\n\nThe fix in `better-auth@1.4.17` 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.\n\nThe `/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.\n\n### Patches\n\nFixed in `better-auth@1.4.17` on the v1.4.x maintenance line and in `better-auth@1.5.0-beta.9` 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.\n\nAfter 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.\n\n### Workarounds\n\nIf users cannot upgrade past `1.4.17`:\n\n- **On `\u003e= 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.\n- **On `\u003c 1.4.16`**: shift the bypass mitigation upstream. Set the IPv6 prefix length on the app\u0027s 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.\n- **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.\n\n### Impact\n\nThe bypass enables unbounded authentication attempts from a single IPv6-capable client. Direct consequences:\n\n- Credential-stuffing and brute-force on `/sign-in/email` are no longer rate-limited per client.\n- Account enumeration via response-shape differences becomes faster.\n- Password-reset and email-verification email fan-out can be amplified.\n\nThe 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.\n\n### Credit\n\nReported by `@nexryai` on GitHub.\n\n### Resources\n\n- [CWE-307: Improper Restriction of Excessive Authentication Attempts](https://cwe.mitre.org/data/definitions/307.html)\n- [RFC 4291: IP Version 6 Addressing Architecture](https://datatracker.ietf.org/doc/html/rfc4291)\n- [RFC 5952: A Recommendation for IPv6 Address Text Representation](https://datatracker.ietf.org/doc/html/rfc5952)\n- [RFC 6177: IPv6 Address Assignment to End Sites](https://datatracker.ietf.org/doc/html/rfc6177)",
  "id": "GHSA-p6v2-xcpg-h6xw",
  "modified": "2026-06-09T10:26:37Z",
  "published": "2026-05-15T17:41:37Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/better-auth/better-auth/security/advisories/GHSA-p6v2-xcpg-h6xw"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-45364"
    },
    {
      "type": "WEB",
      "url": "https://github.com/better-auth/better-auth/pull/7470"
    },
    {
      "type": "WEB",
      "url": "https://github.com/better-auth/better-auth/pull/7509"
    },
    {
      "type": "WEB",
      "url": "https://github.com/better-auth/better-auth/commit/43e719bcc0c223c7079fa0c611a9cf7ea1188254"
    },
    {
      "type": "WEB",
      "url": "https://github.com/better-auth/better-auth/commit/57af0f7b910dcf7b1a5c0615d10b9bd56bb69bef"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/better-auth/better-auth"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Better Auth: Rate limiter keys IPv6 addresses individually and is bypassable via prefix rotation"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

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Action not permitted

CVE-2026-45364 (GCVE-0-2026-45364)

FKIE_CVE-2026-45364

GHSA-P6V2-XCPG-H6XW

Am I affected?

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