cal.com: Client-Side Enforcement of Server-Side Security
CVE-2026-23478 is a critical-severity client-side enforcement of server-side security vulnerability in cal.com from Calcom, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected cal.com versions: >= 3.1.6, < 6.0.7. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.47% chance of exploitation within 30 days, which is low compared with other CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Check which version of cal.com you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 10.0 | Critical | CNA (github.com) |
| CVSS 3.1 | 9.8 | Critical | NVD |
Official description
Cal.com is open-source scheduling software. From 3.1.6 to before 6.0.7, there is a vulnerability in a custom NextAuth JWT callback that allows attackers to gain full authenticated access to any user's account by supplying a target email address via session.update(). This vulnerability is fixed in 6.0.7.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Client-Side Enforcement of Server-Side Security
The product includes a server that relies on the client to implement a protection mechanism that is intended to protect the server.
How developers prevent this class of flaw
- If some degree of trust is required between the two entities, then use integrity checking and strong authentication to ensure that the inputs are coming from a trusted source. Design the product so that this trust is managed in a centralized fashion, especially if there are complex or numerous communication channels, in order to reduce the risks that the implementer will mistakenly omit a check in a single code path.
More Client-Side Enforcement of Server-Side Security vulnerabilities (CWE-602)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Advisories
- github.com/calcom/cal.com/security/advisories/GHSA-7hg4-x4pr-3hrgThird Party Advisory
Other vulnerabilities in cal.com
- 9.8 Critical severity
Recent Client-Side Enforcement of Server-Side Security vulnerabilities
- 9.8 Critical severity
- 7.5 High severity
- 7.4 High severity
- 7.5 High severity
- 9.8 Critical severity
- 8.7 High severity
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