mcp-router: Missing Authentication for Critical Functionality
CVE-2026-81094 is a critical-severity missing authentication for critical functionality vulnerability in mcp-router, with a CVSS 3.1 base score of 9.1 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 0.6.3 as the first without the flaw, so updating mcp-router to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.61% 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
- Update mcp-router to 0.6.3 or a later version.
- 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
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Mcp-router | mcp-router | before 0.6.3 | 0.6.3 |
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
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N. Scored by CNA (vulncheck.com).
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 9.3 | Critical | CNA (vulncheck.com) |
| CVSS 3.1 | 9.1 | Critical | CNA (vulncheck.com) |
Official description
The mcp-router CLI served its MCP aggregator on every interface and enforced authentication only when the operator asked for it. The serve command in apps/cli/src/commands/serve.ts defaulted its host to the all-interfaces address on a fixed port, and required a token only when the corresponding flag was supplied, so a default invocation exposed the aggregator, and every MCP server it fronted, to anyone able to reach the port. Release 0.6.3 defaults the host to the loopback address and refuses to start without a token whenever the host it is given is not a loopback address; no earlier release carries either check.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Missing Authentication for Critical Functionality
The product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources.
How developers prevent this class of flaw
- For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
- Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using libraries with authentication capabilities such as OpenSSL or the ESAPI Authenticator.
- When storing data in the cloud (e.g., S3 buckets, Azure blobs, Google Cloud Storage, etc.), use the provider's controls to require strong authentication for users who should be allowed to access the data.
More Missing Authentication for Critical Functionality vulnerabilities (CWE-306)
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
Patches and release notes
Other references
Recent Missing Authentication for Critical Functionality vulnerabilities
- 7.5 High severity
- 10.0 Critical severity
- 9.1 Critical severity
- 8.2 High severity
- 10.0 Critical severity
- 9.8 Critical severity
Spotted an error? Tell us.