How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.

357,869

Total vulnerabilities in the database

CVE-2026-53512 — better-auth

Improper Authentication

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:

Upgrade to [email protected] or later.
Migrate from the deprecated oidcProvider() to @better-auth/oauth-provider when 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 [email protected]. 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-provider if your deployment can adopt the new plugin. It enforces client_secret on both grants.
Force all clients to public + PKCE: set every client's type: "public" and require PKCE. The bug is unreachable when there is no client_secret to verify.
Network-layer ingress restriction: limit /api/auth/oauth2/token and /api/auth/mcp/token to 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_token and the public client_id can 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

Published: Jul 7, 2026
Updated: Jul 8, 2026
CVE: CVE-2026-53512
GHSA: GHSA-pw9m-5jxm-xr6h
Severity: Critical
Exploit:
CISA KEV:

CVSS v3:

Severity: Critical
Score: 9.1
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

CWEs:

OWASP TOP 10:

A2 - Broken Authentication

Affected Software
References

Software	From	Fixed in
better-auth	-	1.6.11

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.