GHSA-pw9m-5jxm-xr6h CVE-2026-53512
Better Auth: OAuth refresh-token replay via missing client authentication on oidc-provider and mcp plugins
- Severity
- 0.0 / 10
- Critical · CVSS 4.0
- Exploit chance (EPSS)
- 0.27%
- Chance of exploitation in the next 30 days. Higher than 19.97% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-287 Improper Authentication
- CWE-306 Missing Authentication for Critical Function
- CWE-345 Insufficient Verification of Data Authenticity
- CWE-863 Incorrect Authorization
- Fix
1.6.11- Upgrade better-auth to this version or later
Description
Am I affected?
Users are affected if all of the following are true:
- Their application uses
better-authand has enabled at least one of:oidcProvider()(imported frombetter-auth/plugins/oidc-provider), ormcp()(imported frombetter-auth/plugins/mcp). - Their application has at least one confidential OAuth client registered (any client with
type: "web" | "native" | "user-agent-based"in theoauthApplicationtable, or anytrustedClientsentry withouttype: "public"). Public clients with PKCE are not affected. - Their application uses
better-authat 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:
- Upgrade to
better-auth@1.6.11or later. - Migrate from the deprecated
oidcProvider()to@better-auth/oauth-providerwhen feasible. The new package enforces client authentication on both grants by default. - If developers cannot upgrade their applications, see workarounds below.
Summary
The legacy oidcProvider and mcp plugins each expose an OAuth 2.0 token endpoint whose refresh_token grant authenticates the request entirely on possession of the bound refreshToken row and a matching client_id. Neither plugin verifies the registered confidential client's client_secret on the refresh path. An attacker who obtains any valid refresh_token (via database read, log capture, browser-side XSS, or CORS-amplified script in the mcp case) and the public client_id can mint fresh access tokens and rotated refresh tokens until the chain is revoked.
Details
RFC 6749 §6 and OAuth 2.1 §4.3 require confidential clients to authenticate to the token endpoint on every grant, including refresh. The same plugins' authorization_code grant correctly enforces client_secret (the oidc-provider via verifyStoredClientSecret, the mcp plugin via raw equality), which proves the omission on the refresh path is a regression rather than a design choice.
Token rotation issues a new refresh_token with each call, so a single leaked refresh-token grants indefinite access until the row is revoked or its refreshTokenExpiresAt (default 7 days) passes; rotation refreshes that window each call.
Two adjacent issues on the mcp surface ship in the same patch. The mcp authorization_code grant uses raw === for client-secret comparison and ignores the storeClientSecret: "encrypted" | "hashed" configuration; the fix routes both grants through verifyStoredClientSecret. The mcp /mcp/token endpoint sets Access-Control-Allow-Origin: * unconditionally, which amplifies the refresh bypass in browser contexts; the fix narrows the CORS allowlist.
The newer @better-auth/oauth-provider package routes both grants through validateClientCredentials and is not affected.
Patches
Fixed in better-auth@1.6.11. The legacy oidcProvider and mcp token endpoints now require client_secret on the refresh_token grant for confidential clients, using the same constant-time comparison the authorization_code grant already used. Public clients are unaffected (they have no secret to enforce, and PKCE substitutes on the auth-code grant).
The Authorization: Basic parser is fixed to follow RFC 6749 §2.3.1: the credential is split on the first colon and each half is percent-decoded. Client IDs and secrets that contain reserved characters now authenticate correctly. The /mcp/token endpoint's CORS configuration is narrowed in the same change (the wildcard Access-Control-Allow-Origin: * header is removed), matching the standalone @better-auth/oauth-provider package.
The deprecated oidc-provider plugin remains deprecated. The recommended migration path is @better-auth/oauth-provider.
Workarounds
None of these close the bug fully without a code patch.
- Migrate to
@better-auth/oauth-providerif your deployment can adopt the new plugin. It enforcesclient_secreton both grants. - Force all clients to public + PKCE: set every client's
type: "public"and require PKCE. The bug is unreachable when there is noclient_secretto verify. - Network-layer ingress restriction: limit
/api/auth/oauth2/tokenand/api/auth/mcp/tokento known client IPs at the load balancer. Practical for server-to-server flows, not for end-user-device clients. - Out-of-band refresh-token rotation: on any suspicion of leak, run
db.deleteMany({ model: "oauthAccessToken", where: [{ field: "clientId", value: <id> }] })to invalidate all refresh tokens for the affected client. - For the mcp endpoint specifically: drop the wildcard CORS at an upstream proxy and replace with a tight allowlist.
Impact
- Indefinite confidential-client impersonation: an attacker holding any valid
refresh_tokenand the publicclient_idcan mint access tokens and rotated refresh tokens indefinitely, until the row is revoked. Rotation refreshes the expiration window each call. - Resource access at the user's authorized scope: every minted access token carries the original user's authorization scope, so the attacker reads or writes whatever the resource server grants for that scope.
Credit
Reported by @subhanUmer.
Resources
Affected versions
npm
| Package | Vulnerable | Fixed in |
|---|---|---|
| better-auth | < 1.6.11 |
1.6.11 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
better-auth npm → 1.6.11
- npm
npm install better-auth@1.6.11- yarn
yarn add better-auth@1.6.11- pnpm
pnpm add better-auth@1.6.11
Severity breakdown
CVSS 4.0 0.0
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
- Attack vector
- Network
- Attack complexity
- Low
- Attack requirements
- Present
- Privileges required
- None
- User interaction
- None
- Confidentiality (vulnerable system)
- High
- Integrity (vulnerable system)
- High
- Availability (vulnerable system)
- None
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
- Exploit maturity
- Not defined
- Scope
- X
CVSS 3.1 9.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- None
Weakness
CWE-287: Improper Authentication
When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.
How to prevent it
- Architecture and Design: Use an authentication framework or library such as the OWASP ESAPI Authentication feature.
CWE-306: Missing Authentication for Critical Function
The product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources.
How to prevent it
- Architecture and Design: Divide the software into anonymous, normal, privileged, and administrative areas. Identify which of these areas require a proven user identity, and use a centralized authentication capability. Identify all potential communication channels, or other means of interaction with the software, to ensure that all channels are appropriately protected, including those channels that are assumed to be accessible only by authorized parties. Developers sometimes perform authentication at the primary channel, but open up a secondary channel that is assumed to be private. For example, a login mechanism may be listening on one network port, but after successful authentication, it may open up a second port where it waits for the connection, but avoids authentication because it assumes that only the authenticated party will connect to the port. In general, if the software or protocol allows a single session or user state to persist across multiple connections or channels, authentication and appropriate credential management need to be used throughout.
- Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
- Architecture and Design: Where possible, avoid implementing custom, "grow-your-own" authentication routines and consider using authentication capabilities as provided by the surrounding framework, operating system, or environment. These capabilities may avoid common weaknesses that are unique to authentication; support automatic auditing and tracking; and make it easier to provide a clear separation between authentication tasks and authorization tasks. In environments such as the World Wide Web, the line between authentication and authorization is sometimes blurred. If custom authentication routines are required instead of those provided by the server, then these routines must be applied to every single page, since these pages could be requested directly.
- Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using libraries with authentication capabilities such as OpenSSL or the ESAPI Authenticator [REF-45].
- Implementation, System Configuration, Operation: When storing data in the cloud (e.g., S3 buckets, Azure blobs, Google Cloud Storage, etc.), use the provider's controls to require strong authentication for users who should be allowed to access the data [REF-1297] [REF-1298] [REF-1302].
CWE-345: Insufficient Verification of Data Authenticity
The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.
CWE-863: Incorrect Authorization
The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check.
How to prevent it
- Architecture and Design: Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries. Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
- Architecture and Design: Ensure that access control checks are performed related to the business logic. These checks may be different than the access control checks that are applied to more generic resources such as files, connections, processes, memory, and database records. For example, a database may restrict access for medical records to a specific database user, but each record might only be intended to be accessible to the patient and the patient's doctor [REF-7].
- Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using authorization frameworks such as the JAAS Authorization Framework [REF-233] and the OWASP ESAPI Access Control feature [REF-45].
- Architecture and Design: For web applications, make sure that the access control mechanism is enforced correctly at the server side on every page. Users should not be able to access any unauthorized functionality or information by simply requesting direct access to that page. One way to do this is to ensure that all pages containing sensitive information are not cached, and that all such pages restrict access to requests that are accompanied by an active and authenticated session token associated with a user who has the required permissions to access that page.
- System Configuration, Installation: Use the access control capabilities of your operating system and server environment and define your access control lists accordingly. Use a "default deny" policy when defining these ACLs.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-r4xp-prcw-77qf better-auth: OAuth Proxy lets an attacker sign in as another user | Critical | Also affects better-auth |
| GHSA-965c-763c-88jm better-auth: OAuth state can be used as a Magic Link to sign in as another user | Critical | Also affects better-auth |
| GHSA-44jh-23m7-hpcf @better-auth/drizzle-adapter: Concurrent requests can exceed rate limits on PostgreSQL | Medium | Also affects better-auth |
| GHSA-q84f-53jg-9ppm better-auth: Device login approval does not show the requesting client or permissions | High | Also affects better-auth |
| GHSA-qq9h-g4jm-xgf3 Better Auth: Account takeover via pre-account hijacking on magic-link and email-OTP sign-in | High | Also affects better-auth |
| GHSA-2vg6-77g8-24mp Better Auth: Stale sessions persist after user deletion across admin, anonymous, and SCIM flows | Medium | Also affects better-auth |
| GHSA-7w99-5wm4-3g79 @better-auth/oauth-provider's OAuth authorization-code grant allows concurrent redemption when two token requests race the find-then-delete primitive | High | Also affects better-auth |
| GHSA-392p-2q2v-4372 Better Auth: OAuth refresh-token rotation forks the token family on concurrent redemption | High | Also affects better-auth |
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