CVE-2026-53512: Better Auth: OAuth refresh-token replay via missing client authentication on oidc-provider and mcp plugins

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.3%, percentile 19: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 19% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes better-auth (npm) < 1.6.11, fixed in 1.6.11. Check your lockfile, including transitive dependencies.

What it is

Better Auth is an authentication and authorization library for TypeScript. Prior to 1.6.11, the legacy oidcProvider and mcp plugins expose OAuth token endpoints whose refreshtoken grant authenticates only possession of the bound refreshToken row and matching clientid, without verifying the confidential client's clientsecret, allowing an attacker with a valid refreshtoken to mint access tokens and rotated refresh tokens through /api/auth/oauth2/token or /api/auth/mcp/token. The @better-auth/oauth-provider package is not affected. This issue is fixed in version 1.6.11.

From the GitHub advisory

Am I affected?

Users are affected if all of the following are true:

- Their application uses better-auth and has enabled at least one of: oidcProvider() (imported from better-auth/plugins/oidc-provider), or mcp() (imported from better-auth/plugins/mcp). - Their application has at least one confidential OAuth client registered (any client with type: "web" | "native" | "user-agent-based" in the oauthApplication table, or any trustedClients entry without type: "public"). Public clients with PKCE are not affected. - Their application uses better-auth at a version below the patched release.

If an application only uses @better-auth/oauth-provider (the canonical replacement for oidc-provider) and the mcp plugin is not enabled, it is not affected.

Fix:

Weakness type: CWE-287, CWE-306, CWE-345, CWE-863.

Product: better-auth (npm) · Assigned by: GitHub_M · Published: 2026-07-15

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.27% probability of exploitation activity in the next 30 days; percentile 19.

Timeline

• 2026-07-07 — GitHub-reviewed advisory GHSA-pw9m-5jxm-xr6h published (critical)
• 2026-07-15 — CVE record published by GitHub_M
• 2026-07-18 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-pw9m-5jxm-xr6h github.com
2. CVE.org record CVE-2026-53512 cve.org
3. NVD entry CVE-2026-53512 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

Critical npm advisories · Today’s Security Digest