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CVE-2026-73420 – Authentication Bypass / Account Takeover – NextAuth.js (next-auth) and @auth/core

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Summary

CVE-2026-73420 is an authentication bypass vulnerability in NextAuth.js (next-auth and @auth/core) caused by improper validation-before-canonicalization of email addresses used in the passwordless/email magic-link sign-in flow. The library’s default email normalizer validates an address for a single "@" before Unicode normalization is applied, allowing a crafted address to canonicalize differently downstream and misdirect the sign-in link. The issue is rated CRITICAL with a 9.1 base score.

Technical details

  • Root cause: the default email normalizer (defaultNormalizer) checks for a single "@" separator before applying Unicode normalization (CWE-180: Incorrect Behavior Order: Validate Before Canonicalize).
  • Trigger: an attacker submits an email address containing the Unicode character U+FF20 (FULLWIDTH COMMERCIAL AT), which is not an ASCII "@" and passes the normalizer’s single-at-sign validation.
  • Downstream effect: if the mail-sending library or delivery service used in sendVerificationRequest applies Unicode (NFKC/NFKD) normalization, the fullwidth "@" collapses to a standard ASCII "@", causing the address to resolve with two effective separators and deliver the passwordless sign-in link to an attacker-controlled recipient.
  • Attack vector: network, no authentication or user interaction beyond submitting a sign-in request; requires the application to use the built-in email normalizer rather than a custom normalizeIdentifier.
  • Impact: bypass of email ownership verification, enabling an attacker to obtain a valid magic-link session for a victim account — high confidentiality and integrity impact.

Affected software

  • next-auth versions < 4.24.15
  • next-auth versions >= 5.0.0-beta.4 and < 5.0.0-beta.32
  • @auth/core versions >= 0.1.0 and < 0.41.3

Severity

CVSS v4.0 Base Score: 9.1 (Critical)
Vector: 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

Mitigation and recommended actions

  • Immediate: upgrade to @auth/core 4.24.15, next-auth 4.24.15, or next-auth 5.0.0-beta.32 or later, whichever release track applies.
  • If immediate patching is not possible: implement a custom normalizeIdentifier for the email provider that rejects non-ASCII "@"-like characters (e.g. restrict to ASCII-only local-part/domain validation) prior to any downstream normalization, and verify that the mail delivery pipeline does not apply Unicode canonicalization to recipient addresses after NextAuth.js validation.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Set-Cookie header: next-auth.csrf-token, next-auth.session-token, next-auth.callback-url, next-auth.pkce.code_verifier, next-auth.state, authjs.csrf-token, authjs.session-token, authjs.callback-url, authjs.pkce.code_verifier, authjs.state

References

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