Critical

Authentication Bypass and Account Takeover in Cal.com NextAuth JWT Callback

IdentifiersCVE-2026-23478CWE-639· Authorization Bypass Through…

CVE-2026-23478 is a critical authentication bypass vulnerability in Cal.com affecting versions 3.1.6 through 6.0.6. The flaw is in a custom NextAuth JWT callback used during session updates. When the callback trigger is set to "update," the application writes client-controlled identity data, specifically the email field, directly into the JSON Web Token without adequate server-side validation. An authenticated attacker can invoke session.update() with a target user's email address, causing the server to issue a JWT that retains the attacker's subject identifier but contains the victim's email. Because subsequent application logic resolves the authenticated user from token.email, later requests are processed as the victim, resulting in full account takeover. The issue has also been associated with CWE-602, but the available content specifically supports an authorization bypass through a user-controlled key.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation gives an attacker full authenticated access to the targeted user's account. This includes access to bookings, event types, integrations, organization memberships, billing information, and any administrative privileges held by the victim. Because the compromise occurs at the session token layer after authentication, protections such as two-factor authentication and external identity provider associations do not prevent the takeover once the attacker has a valid session and can manipulate the update flow. The content states the attack is trivial to execute at scale because it requires only a single API call and knowledge of the victim's email address.

Mitigation

If you can’t patch tonight, do this now.

If immediate upgrade is not possible, reduce exposure by disabling or tightly restricting the vulnerable session update path that permits client-controlled identity field changes, especially email updates through session.update(). Review and harden custom NextAuth JWT callback logic so client-supplied identity attributes are never trusted for token mutation without authoritative server-side validation. Monitor for anomalous session update requests and account activity indicative of email-based impersonation. However, the primary mitigation supported by the content is prompt upgrade to 6.0.7 or later.

Remediation

Patch, then assume compromise.

Upgrade Cal.com to version 6.0.7 or later. The vendor states the vulnerability is fixed in 6.0.7. Hosted Cal.com deployments were patched immediately upon discovery; self-hosted deployments should be updated immediately to a non-vulnerable release.

PUBLIC EXPLOITS

Exploits

No valid public exploits. Mallory filtered out 1 candidate as fakes, detection scripts, or README-only repos.

VALID 0 / 1 TOTALView more in app

All candidate exploits were filtered out by Mallory's validation.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

CalCal.Comapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

7 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

7 SOURCESView more in app

the hacker newsNews

Jan 19, 2026

⚡ Weekly Recap: Fortinet Exploits, RedLine Clipjack, NTLM Crack, Copilot Attack & More

Unknown

cyber security newsNews

Jan 15, 2026

Critical Cal.com Vulnerability Let Attackers bypass authentication and hijack any user account

A critical authentication/authorization bypass in Cal.com caused by improper handling of client-controlled identity fields in a custom NextAuth JWT callback during session updates, allowing account takeover by manipulating JWT claims (notably the email used for user lookup).

security online infoNews

Jan 15, 2026

SHADOW#REACTOR Malware Builds Remcos RAT via Text Files

A critical vulnerability in Cal.com described as allowing a 2FA bypass via a single API call (CVSS 10 per the text).

security online infoNews

Jan 15, 2026

One API Call to Hijack: Critical Cal.com Flaw (CVE-2026-23478, CVSS 10) Bypasses 2FA

Critical authentication bypass / account takeover in Cal.com caused by improper validation of client-controlled identity fields (email) during NextAuth JWT session update, enabling JWT identity overwrite and full user impersonation.

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Exposure mapping

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Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity3