VulnerabilityPublished 12 Sep 2026

CVE-2026-59973: FrontMCP and mcp-from-openapi have bypass of OpenAPI external $ref SSRF fix

Affects mcp-from-openapiSource GCVE Database

Authoritative record:CVE-2026-59973· High 8.5

Not known to be exploited · CVSS 8.5 High : upgrade to 2.5.0 soon.

Data as of 27 Oct 2026, 05:11 UTC

CVSS 3.1 base score

8.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 33% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.5.0, 1.5.0

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:L/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Changed
Confidentiality: High
Integrity: Low
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 15 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: evidence found.
CISA SSVC: proof of concept public
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

CVE ID reserved
by GitHub, Inc.
7 Jul 202619:41 UTC, 70 days before disclosure
CVE published
on cve.org
15 Sep 202615:13 UTC, t = 0
CISA SSVC assessment
proof of concept public
16 Sep 202616:06 UTC, 25 hours after disclosure

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CISA SSVC, the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade mcp-from-openapi to 2.5.0; @frontmcp/adapters and frontmcp to 1.5.0.

Affected software

Product	Affected	Fixed in
npmmcp-from-openapi	Affected >=2.3.0 <2.5.0	Fixed in 2.5.0
npm@frontmcp/adapters	Affected >=1.2.1 <1.5.0	Fixed in 1.5.0
npmfrontmcp	Affected >=1.2.1 <1.5.0	Fixed in 1.5.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A patch is available for this vulnerability. Users should upgrade mcp-from-openapi to a fixed version beyond 2.3.0 and FrontMCP to versions beyond 1.2.1 that update this dependency and improve the SSRF protections. Until patched, avoid loading untrusted OpenAPI specifications that may contain external $ref references. The vendor advisory confirms a fix is available; apply official updates accordingly.

Description

FrontMCP is a TypeScript-first framework for the Model Context Protocol (MCP). From mcp-from-openapi 2.3.0 until 2.5.0 and from frontmcp and @frontmcp/adapters 1.2.1 until 1.5.0, libs/adapters/src/openapi/openapi.adapter.ts loadOpenAPISpec() forwards untrusted OpenAPI url and spec inputs and loadOptions.refResolution to OpenAPIToolGenerator.fromURL() and OpenAPIToolGenerator.fromJSON(). The external $ref guard checks parsed hostname strings without resolving addresses, pinning validated addresses, revalidating redirect targets, or normalizing IPv4-mapped IPv6. An authenticated user who can import or configure an OpenAPI specification in a hosted or multi-user deployment can use DNS-to-loopback resolution, redirect-to-loopback behavior, or IPv4-mapped IPv6 loopback forms to cause backend-origin requests to internal services. This can expose internal administrative APIs, metadata-like services, and other private network endpoints. The practical impact is lower when only a trusted local administrator can configure OpenAPI specs, and disabling external reference protocols prevents the external $ref request. This issue is fixed in mcp-from-openapi 2.5.0 and frontmcp and @frontmcp/adapters 1.5.0.

Impact

An attacker able to supply an untrusted OpenAPI specification to a FrontMCP deployment can cause the backend server to make HTTP requests to localhost or other private network services. This SSRF bypass can lead to unauthorized access to internal services or data exposure. The vulnerability has a CVSS score of 8.5 (high severity) with high confidentiality impact, low integrity impact, and no availability impact. There are no known exploits in the wild at this time.

Summary

The vulnerability is a bypass of the OpenAPI external $ref SSRF fix in mcp-from-openapi 2.3.0 and FrontMCP integration. The previous fix added a hostname denylist to block direct local IP addresses in external $ref dereferencing, but it does not resolve hostnames before allow/deny checks, does not revalidate redirects, and misses IPv4-mapped IPv6 loopback addresses. As a result, requests to loopback addresses can be triggered via hostnames like 127.0.0.1.nip.io, redirects to 127.0.0.1, or IPv4-mapped IPv6 addresses. FrontMCP versions 1.2.1 and current main still use mcp-from-openapi 2.3.0 and call vulnerable methods when loading OpenAPI specs. The vulnerability allows an attacker who can supply an untrusted OpenAPI spec to cause backend-origin requests to localhost or private services during tool generation, bypassing the previous SSRF protections.

CWE-918Server-Side Request Forgery (SSRF) (opens in a new tab)
Base weaknessCWE Top 25 (2025): #22OWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.
OWASP Cheat Sheets
Server Side Request Forgery Prevention (opens in a new tab)
Unvalidated Redirects and Forwards (opens in a new tab)
Cross Site Scripting Prevention (opens in a new tab)
DOM based XSS Prevention (opens in a new tab)
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V12.6.1 (opens in a new tab)
V5.2.6 (opens in a new tab)
Attack patterns
Server Side Request Forgery (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 19 Sep 2026, 22:14 UTC

WeaknessesCWE-918

Technical details

References

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