NextAuth.js @auth/core/next-auth Email Normalization defaultNormalizer improper authentication

Summaryinfo

A vulnerability, which was classified as critical, has been found in NextAuth.js @auth, core and next-auth. This issue affects the function defaultNormalizer of the component Email Normalization. Performing a manipulation results in improper authentication. This vulnerability was named CVE-2026-73420. The attack may be initiated remotely. There is no available exploit.

Detailsinfo

A vulnerability classified as critical has been found in NextAuth.js @auth, core and next-auth (affected version unknown). This affects the function defaultNormalizer of the component Email Normalization. The manipulation with an unknown input leads to a improper authentication vulnerability. CWE is classifying the issue as CWE-287. When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

NextAuth.js provides authentication for Next.js. Prior to @auth/core 0.41.3 and next-auth 4.24.15 and 5.0.0-beta.32, the defaultNormalizer used by the email and magic-link sign-in flow validates an address before applying Unicode normalization. An address can contain a Unicode character such as U+FF20 FULLWIDTH COMMERCIAL AT that is not ASCII at-sign but canonicalizes to an ASCII at-sign under NFKC or NFKD normalization. The address passes the normalizer's single-at-sign check, but a downstream sendVerificationRequest mail library or delivery service that normalizes the address can then see two at-sign separators and deliver the passwordless sign-in link to an attacker-controlled recipient. Applications are affected when the email provider uses the built-in normalizer rather than a custom normalizeIdentifier and the downstream sender applies Unicode normalization. An attacker who knows a victim's email address can request the misrouted magic link and sign in as the victim without victim interaction. This issue is fixed in @auth/core 0.41.3 and next-auth 4.24.15 and 5.0.0-beta.32.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-73420 since 08/12/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 08/14/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Type

• JavaScript Library

Vendor

• NextAuth.js

Name

• @auth
• core
• next-auth

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