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Auth.js: Email normalizer validates the address before Unicode normalization, allowing a homoglyph @ bypass

Critical severity GitHub Reviewed Published Jul 20, 2026 in nextauthjs/next-auth

Package

npm @auth/core (npm)

Affected versions

>= 0.1.0, < 0.41.3

Patched versions

0.41.3
npm next-auth (npm)
>= 4.10.3, < 4.24.15
>= 5.0.0-beta.1, <= 5.0.0-beta.31
4.24.15
5.0.0-beta.32

Description

Summary

The default email-address normalizer used by the email/magic-link sign-in flow validates the address before applying Unicode normalization. An address can contain a Unicode character that is not an ASCII @ (U+0040) but canonicalizes to one under NFKC/NFKD normalization (the normalization commonly applied by mail libraries and services for internationalized email). Such an address passes the normalizer's single-@ check, but a downstream mail library that normalizes the string then sees two @ separators and may deliver the passwordless sign-in link to a different recipient than intended. This is an instance of validating before canonicalizing.

Am I affected?

You may be affected if all of the following hold:

  • You use next-auth >= 4.0.0, < 4.24.14, or @auth/core >= 0.1.0, < 0.41.3.
  • You have the email / magic-link (passwordless) provider enabled.
  • You rely on the built-in default identifier normalizer (you have not supplied your own normalizeIdentifier).
  • Your sendVerificationRequest implementation uses a mail library or delivery service that applies Unicode normalization to recipient addresses (most internationalized-email/SMTPUTF8-capable senders do).

You are not affected if you do not use the email provider, or if your normalizer/mailer rejects or canonicalizes non-ASCII addresses before they are validated.

Impact

  • Account takeover: an attacker who knows a victim's email address can request a magic link that is delivered to an attacker-controlled mailbox, then use it to sign in as the victim.
  • No victim interaction is required to misroute the link; the attacker initiates the flow.

Patched version

The fix applies Unicode (NFKC) normalization before the address is validated, so homoglyph separators are collapsed and rejected up front. Upgrade to the first release containing this fix (pending; this advisory will be updated with the exact patched version before publication). No application code changes are required after upgrading.

Workarounds

If you cannot upgrade immediately:

  • Supply a custom normalizeIdentifier on the email provider that calls identifier.normalize("NFKC") (and lower-cases/trims) before any validation, and rejects addresses that do not contain exactly one @ after normalization.
  • Or reject any address whose local part or domain contains non-ASCII characters, if your user base does not require internationalized email addresses.

Credit

Reported by @kakashi-kx. Thank you for the responsible disclosure.

References

@gustavovalverde gustavovalverde published to nextauthjs/next-auth Jul 20, 2026
Published to the GitHub Advisory Database Jul 23, 2026
Reviewed Jul 23, 2026

Severity

Critical

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 None
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:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Incorrect Behavior Order: Validate Before Canonicalize

The product validates input before it is canonicalized, which prevents the product from detecting data that becomes invalid after the canonicalization step. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-7rqj-j65f-68wh

Source code

Credits

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