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-2026-53518 — @better-auth / oauth-provider

Authentication Bypass by Capture-replay

Am I affected?

Users are affected if all of the following are true:

Their project depends on @better-auth/oauth-provider at a version >= 1.6.0, < 1.6.11, or uses the embedded plugin in better-auth >= 1.4.8-beta.7, < 1.6.0, or enables the legacy oidc-provider or mcp plugins from better-auth/plugins.
Their application exposes /api/auth/oauth2/token (or the legacy plugins' /oauth2/token and /mcp/token) as a token endpoint to OAuth/OIDC clients, including internal MCP clients (Claude Desktop, custom MCP tool callers, AI agents).
Their application has not implemented an external mitigation: a load-balancer-level idempotency cache keyed by code, a database trigger that rejects duplicate token issuance for the same authorization code, or a custom adapter override that performs an atomic compare-and-delete.

Fix:

Upgrade to @better-auth/[email protected] or later. If developers use the legacy plugin paths from better-auth/plugins, upgrade better-auth to 1.6.11 or later.
If developers cannot upgrade, see workarounds below.

Summary

The OAuth provider's POST /oauth2/token endpoint, on the authorization_code grant, redeems a single-use authorization code through a non-atomic find-then-delete sequence. Two concurrent requests with the same code value both pass the read step before either delete completes, then both proceed to PKCE verification and createUserTokens. Each surviving request mints a fresh access token, refresh token, and id token. RFC 6749 §4.1.2 requires authorization codes to be single-use; this primitive does not enforce that under concurrency.

Details

The same architectural primitive (find a single-use verification row, then delete it, then trust the row to authorize) is used in 20 other call sites across the codebase. The deletion primitive returns Promise<void>, discarding the row count surfaced by adapter.deleteMany, so no call site can detect "another caller already claimed this row". The fix lands at the primitive layer rather than at any individual call site.

The fix introduces a claimVerificationByIdentifier primitive at the internal-adapter layer that performs an atomic claim-and-return, replaces the find-then-delete pair at this call site, and migrates the highest-impact variant sites in the same release.

Patches

Fixed in @better-auth/[email protected] and [email protected] for the legacy oidc-provider and mcp plugin paths. All three token-exchange call sites now consume the verification row through internalAdapter.consumeVerificationValue, an atomic claim primitive that deletes the row and returns its prior value in one operation. The first request to arrive takes the row and mints tokens; concurrent racers observe an empty result and return invalid_grant.

Error-code consistency is also tightened on the @better-auth/oauth-provider token endpoint: the malformed-verification-value branches previously returned a project-specific invalid_verification code, which is not part of RFC 6749 §5.2's response error set. Both branches now return invalid_grant so spec-compliant clients can branch on the standard code without a special case.

Workarounds

None of these close the bug fully without a code patch. Upgrading is the only good path.

Network-layer: deploy an authorization-server-aware reverse proxy (Envoy, NGINX with Lua, custom Cloudflare Worker) that holds an in-flight registry keyed by the code parameter and serializes concurrent requests for the same code. Fragile under multi-instance deployments unless the registry is shared (Redis-backed).
Database-layer: add a SQL or Mongo uniqueness constraint that prevents two oauthAccessToken rows from being created with the same upstream code reference. Adapter-specific and not always feasible since the schema does not currently store the source code.
Application-layer: wrap deleteVerificationByIdentifier with a custom hook that uses adapter.deleteMany and surfaces the count, then injects an invalid_grant rejection when the count is zero. Requires forking the internal adapter.

Impact

Multiple independent token sets from a single authorization: forked access tokens, refresh tokens, and id tokens issued from the same code, all valid for the original user's authorization scope.
Detection bypass: standard OAuth single-use enforcement does not fire for the second redemption when both requests interleave through the read step.
Legacy-plugin reach: oidc-provider and mcp plugins share the primitive on the same surface, so deployments using them inherit the same impact.

Credit

Reported by @chdanielmueller.

Resources

Published: Jul 7, 2026
Updated: Jul 8, 2026
CVE: CVE-2026-53518
GHSA: GHSA-7w99-5wm4-3g79
Severity: High
Exploit:
CISA KEV:

CVSS v3:

Severity: High
Score: 8.1
AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

CWEs:

Affected Software
References

Software	From	Fixed in
@better-auth / oauth-provider	1.6.0	1.6.11
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.