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Better Auth: OAuth refresh-token rotation forks the token family on concurrent redemption

High severity GitHub Reviewed Published May 31, 2026 in better-auth/better-auth • Updated Jul 20, 2026

Package

npm @better-auth/oauth-provider (npm)

Affected versions

>= 1.6.0, < 1.6.11

Patched versions

1.6.11
npm better-auth (npm)
>= 1.4.8-beta.7, < 1.6.0
1.6.0

Description

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.
  • At least one OAuth client served by their application's authorization server requests the offline_access scope, so refresh tokens are minted.
  • Concurrent redemption of the same refresh token is reachable: an SPA shares one refresh token across browser tabs without a mutex, a mobile client retries after a transient failure, an attacker who has stolen a refresh token times two requests, or a service worker queues offline requests.

If developer applications do not request offline_access for any client, no refresh tokens are minted and they are not exposed.

Fix:

  1. Upgrade to @better-auth/oauth-provider@1.6.11 or later.
  2. If developers cannot upgrade, see workarounds below.

Summary

The OAuth provider's POST /oauth2/token endpoint, on the refresh_token grant, performs a non-atomic read / validate / revoke / mint sequence on the oauthRefreshToken row. Two concurrent requests presenting the same parent refresh token both pass the revocation check before either revoke completes, so each mints a fresh refresh token. The replay-detection branch only fires when revoked is already truthy at read time, which is exactly the state concurrent attackers race past. The result is a forked refresh-token family from a single parent token.

Details

The adapter.update predicate on the parent row is keyed on id only; it does not include revoked IS NULL, so two concurrent updates both succeed (last-write-wins, no error path). The schema does not declare unique on oauthRefreshToken.token, so concurrent creates do not collide on a unique-key violation either.

RFC 9700 §4.14 (OAuth Security Best Current Practice) prescribes refresh-token family invalidation on detected reuse; this implementation tries to enforce that contract through the revoked check, but the check is not atomic with the consumption step. Token rotation issues a new refresh token with each call, so a single stolen refresh token grants indefinite access until the row is revoked or its refreshTokenExpiresAt (default 7 days) passes. Rotation refreshes that window each call.

The fix lands an atomic compare-and-swap on the parent row inside the rotation primitive (UPDATE ... WHERE id = ? AND revoked IS NULL with a rowcount check), so the losing rotation fails closed with invalid_grant and the parent row stays marked revoked. Subsequent replay of the original refresh token then trips the existing family-invalidation guard. The schema gains a unique constraint on oauthRefreshToken.token for parity with oauthAccessToken.token.

Patches

Fixed in @better-auth/oauth-provider@1.6.11. The refresh-token rotation primitive now performs an atomic compare-and-swap on the parent row, and the explicit revokeRefreshToken path uses the same CAS. On a contested rotation, exactly one caller wins and mints a fresh refresh token; the loser receives invalid_grant. Subsequent replay of the original refresh token trips the existing family-invalidation guard because the parent row stays marked revoked.

@better-auth/memory-adapter@1.6.11 ships a compatibility fix in the same wave: the in-memory where clause now treats undefined and null as equivalent under an eq null predicate, mirroring SQL IS NULL and Mongo's missing-or-null semantics. Without this change, the CAS predicate WHERE revoked IS NULL falls through on every call against a row whose optional revoked field is absent (the adapter factory's transformInput skips writing undefined when no default exists), so the rotation above is broken for any deployment using the in-memory adapter.

Strict refresh-token family invalidation on a contested rotation, per RFC 9700 §4.14 (which calls for invalidating the winner's tokens too when reuse is detected at rotation time), is deferred to a follow-up minor on the next channel. Closing it cleanly requires an opt-in transactional rotation in the adapter contract so the family-delete cannot interleave with the winner's in-flight access-token insert. The deferred site carries a FIXME(strict-family-invalidation) marker.

Schema-migration note: the better-auth migration generator only emits UNIQUE for newly-created columns. Existing installs will not pick up the new oauthRefreshToken.token unique constraint from migrate / generate; add it manually if an application's operational tooling depends on it (CREATE UNIQUE INDEX oauth_refresh_token_token_uniq ON "oauthRefreshToken" (token);). The CAS fix above does not depend on the database-level constraint to be correct; the constraint is defense-in-depth so collisions from a buggy custom generateRefreshToken callback fail loudly.

Workarounds

None of these close the bug fully without a code patch.

  • Adapter-level: configure the database adapter to run the OAuth refresh handler under serializable isolation, or wrap the adapter.update on oauthRefreshToken with a row-level pessimistic lock (SELECT ... FOR UPDATE). Narrows the window without closing it.
  • Token lifetime: pass oauthProvider({ refreshTokenExpiresIn: 60 }) to expire forked families within one minute. Trades attacker persistence for shorter user sessions.
  • Client-side single-flight: serialize refresh-token usage in the client SDK with a mutex. Mitigates honest concurrency but does nothing against an attacker with a stolen refresh token.
  • Disable refresh tokens: do not request the offline_access scope. Closes the surface but breaks long-lived sessions.

Impact

  • Indefinite access from a single stolen refresh token: forked refresh-token families grant access at the original user's authorization scope, surviving past any single revocation if an attacker holds any branch.
  • Detection bypass: legitimate users whose refresh token has been forked do not trip family invalidation when they refresh, because the attacker's branch already swapped the parent row out from under the legitimate user's check.

Credit

Reported by @chdanielmueller.

Resources

References

@gustavovalverde gustavovalverde published to better-auth/better-auth May 31, 2026
Published to the GitHub Advisory Database Jul 7, 2026
Reviewed Jul 7, 2026
Published by the National Vulnerability Database Jul 15, 2026
Last updated Jul 20, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(15th percentile)

Weaknesses

Authentication Bypass by Capture-replay

A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes). Learn more on MITRE.

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently. Learn more on MITRE.

Time-of-check Time-of-use (TOCTOU) Race Condition

The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. Learn more on MITRE.

Insufficient Session Expiration

According to WASC, Insufficient Session Expiration is when a web site permits an attacker to reuse old session credentials or session IDs for authorization. Learn more on MITRE.

CVE ID

CVE-2026-53517

GHSA ID

GHSA-392p-2q2v-4372

Source code

Credits

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