{"id":"CVE-2026-53518","aliases":["EUVD-2026-44734","GHSA-7w99-5wm4-3g79"],"status":"published","title":"Better Auth OAuth Provider: Race Condition in Authorization Code Exchange Enables Multi-Use Code Redemption","description":"Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.","severity":{"version":"4.0","score":7.6,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00409,"percentile":0.35508,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-294","CWE-362","CWE-367"],"published":"2026-07-07T20:56:35Z","modified":"2026-10-28T02:30:53Z","fixes":[{"ecosystem":"npm","package":"@better-auth/oauth-provider","fixed_in":["1.6.11"],"sources":["ghsa","osv"]},{"ecosystem":"npm","package":"better-auth","fixed_in":["1.6.11"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"@better-auth/oauth-provider","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"1.6.0"},{"fixed":"1.6.11"}]}]},{"source":"ghsa","ecosystem":"npm","package":"better-auth","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.6.11"}]}]},{"source":"osv","ecosystem":"npm","package":"@better-auth/oauth-provider","ranges":[{"type":"SEMVER","events":[{"introduced":"1.6.0"},{"fixed":"1.6.11"}]}]},{"source":"osv","ecosystem":"npm","package":"better-auth","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"1.6.11"}]}]}],"products":[{"vendor":"better-auth","product":"better-auth"},{"vendor":"better-auth","product":"better-auth\\/oauth-provider"},{"vendor":"better-auth","product":"better_auth"}],"references":[{"url":"https://github.com/better-auth/better-auth/security/advisories/GHSA-7w99-5wm4-3g79","tags":["x_refsource_CONFIRM","Mitigation","Vendor Advisory","advisory","web"]},{"url":"https://github.com/better-auth/better-auth/commit/b4bc65a007784b2eb0efb459e5fa6fd8055d3ec9","tags":["x_refsource_MISC","Patch","fix","web"]},{"url":"https://github.com/better-auth/better-auth/releases/tag/v1.6.11","tags":["x_refsource_MISC","Release Notes","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-53518","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-7w99-5wm4-3g79"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53518.json","tags":["advisory"]},{"url":"https://github.com/better-auth/better-auth","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-53518","title":"Better Auth OAuth Provider: Race Condition in Authorization Code Exchange Enables Multi-Use Code Redemption","description":"Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.","status":"published","published":"2026-07-15T17:17:06.41Z","modified":"2026-07-15T18:07:49.19Z","cvss":[{"version":"4.0","score":7.6,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N","level":"high","assessor":"cna"}],"cwes":["CWE-362","CWE-367"],"updated_at":"2026-11-02T14:19:59.761005Z"}],"euvd":[{"id":"EUVD-2026-44734","description":"Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.","status":"published","published":"2026-07-15T17:17:06Z","modified":"2026-07-15T18:07:49Z","cvss":[{"version":"4.0","score":7.6,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N","level":"high"}],"updated_at":"2026-11-02T18:19:52.35951Z"}],"ghsa":[{"id":"GHSA-7w99-5wm4-3g79","title":"@better-auth/oauth-provider's OAuth authorization-code grant allows concurrent redemption when two token requests race the find-then-delete primitive","description":"### Am I affected?\n\nUsers are affected if all of the following are true:\n\n- Their project depends on `@better-auth/oauth-provider` at a version `\u003e= 1.6.0, \u003c 1.6.11`, or uses the embedded plugin in `better-auth \u003e= 1.4.8-beta.7, \u003c 1.6.0`, or enables the legacy `oidc-provider` or `mcp` plugins from `better-auth/plugins`.\n- 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).\n- 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.\n\nFix:\n\n1. Upgrade to `@better-auth/oauth-provider@1.6.11` or later. If developers use the legacy plugin paths from `better-auth/plugins`, upgrade `better-auth` to `1.6.11` or later.\n2. If developers cannot upgrade, see workarounds below.\n\n### Summary\n\nThe 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.\n\n### Details\n\nThe 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\u003cvoid\u003e`, 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.\n\nThe 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.\n\n### Patches\n\nFixed in `@better-auth/oauth-provider@1.6.11` and `better-auth@1.6.11` 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`.\n\nError-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.\n\n### Workarounds\n\nNone of these close the bug fully without a code patch. Upgrading is the only good path.\n\n- **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).\n- **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.\n- **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.\n\n### Impact\n\n- **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.\n- **Detection bypass**: standard OAuth single-use enforcement does not fire for the second redemption when both requests interleave through the read step.\n- **Legacy-plugin reach**: `oidc-provider` and `mcp` plugins share the primitive on the same surface, so deployments using them inherit the same impact.\n\n### Credit\n\nReported by @chdanielmueller.\n\n### Resources\n\n- [CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition)](https://cwe.mitre.org/data/definitions/362.html)\n- [CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition](https://cwe.mitre.org/data/definitions/367.html)\n- [CWE-294: Authentication Bypass by Capture-replay](https://cwe.mitre.org/data/definitions/294.html)\n- [RFC 6749 §4.1.2: Authorization Response](https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.2)\n- [OAuth 2.1 §4.1: Authorization Code Grant](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1#section-4.1)","status":"published","published":"2026-07-07T20:56:35Z","modified":"2026-07-20T19:24:53Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high"},{"version":"4.0","score":7.6,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N","level":"high"}],"cwes":["CWE-294","CWE-362","CWE-367"],"updated_at":"2026-11-02T14:40:47.245721Z"}],"nvd":[{"id":"CVE-2026-53518","description":"Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.","status":"published","published":"2026-07-15T18:16:48.24Z","modified":"2026-07-21T16:03:12.347Z","cvss":[{"version":"4.0","score":7.6,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/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","level":"high","assessor":"secondary"},{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high","assessor":"primary"}],"cwes":["CWE-362","CWE-367"],"updated_at":"2026-11-02T14:12:29.879716Z"}],"osv":[{"id":"CVE-2026-53518","title":"Better Auth OAuth Provider: Race Condition in Authorization Code Exchange Enables Multi-Use Code Redemption","description":"Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.","status":"published","published":"2026-07-15T17:17:06.41Z","modified":"2026-10-28T02:30:53.619666967Z","cvss":[{"version":"4.0","score":7.6,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N","level":"high"}],"cwes":["CWE-362","CWE-367"],"updated_at":"2026-11-02T15:45:08.132336Z"},{"id":"GHSA-7w99-5wm4-3g79","title":"@better-auth/oauth-provider's OAuth authorization-code grant allows concurrent redemption when two token requests race the find-then-delete primitive","description":"### Am I affected?\n\nUsers are affected if all of the following are true:\n\n- Their project depends on `@better-auth/oauth-provider` at a version `\u003e= 1.6.0, \u003c 1.6.11`, or uses the embedded plugin in `better-auth \u003e= 1.4.8-beta.7, \u003c 1.6.0`, or enables the legacy `oidc-provider` or `mcp` plugins from `better-auth/plugins`.\n- 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).\n- 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.\n\nFix:\n\n1. Upgrade to `@better-auth/oauth-provider@1.6.11` or later. If developers use the legacy plugin paths from `better-auth/plugins`, upgrade `better-auth` to `1.6.11` or later.\n2. If developers cannot upgrade, see workarounds below.\n\n### Summary\n\nThe 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.\n\n### Details\n\nThe 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\u003cvoid\u003e`, 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.\n\nThe 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.\n\n### Patches\n\nFixed in `@better-auth/oauth-provider@1.6.11` and `better-auth@1.6.11` 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`.\n\nError-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.\n\n### Workarounds\n\nNone of these close the bug fully without a code patch. Upgrading is the only good path.\n\n- **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).\n- **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.\n- **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.\n\n### Impact\n\n- **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.\n- **Detection bypass**: standard OAuth single-use enforcement does not fire for the second redemption when both requests interleave through the read step.\n- **Legacy-plugin reach**: `oidc-provider` and `mcp` plugins share the primitive on the same surface, so deployments using them inherit the same impact.\n\n### Credit\n\nReported by @chdanielmueller.\n\n### Resources\n\n- [CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition)](https://cwe.mitre.org/data/definitions/362.html)\n- [CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition](https://cwe.mitre.org/data/definitions/367.html)\n- [CWE-294: Authentication Bypass by Capture-replay](https://cwe.mitre.org/data/definitions/294.html)\n- [RFC 6749 §4.1.2: Authorization Response](https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.2)\n- [OAuth 2.1 §4.1: Authorization Code Grant](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1#section-4.1)","status":"published","published":"2026-07-07T20:56:35Z","modified":"2026-07-20T19:30:24.125273382Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high"},{"version":"4.0","score":7.6,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N","level":"high"}],"cwes":["CWE-294","CWE-362","CWE-367"],"updated_at":"2026-11-02T15:46:41.349637Z"}]}}