CVE-2026-116685VulnerabilityPublished 29 Oct 2026

fastmcp · Model Context Protocol Python SDK: HTTP client transports followed cross-origin redirects carrying custom headers and

Affects fastmcpSource GCVE Database

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Authoritative record:CVE-2026-116685· Medium 5.9

Not known to be exploited · CVSS 5.9 Medium : upgrade to 4.0.4 when convenient.

Data as of 30 Oct 2026, 10:15 UTC

CVSS 3.1 base score

5.9Medium

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

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	5.9

Patch: Available – fixed in 4.0.4

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

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: None
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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
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: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 27 Oct 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 29 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 to 4.0.4.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=2.12.3 <4.0.4	Fixed in 4.0.4

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

AI-suggested

Upgrade the MCP Python SDK to version 1.30.0 (1.x) or 2.2.0 (2.x) or later, which restricts redirects to the same origin or http-to-https on the same host and fails otherwise. No code changes are needed. Until upgrading, disable redirect following by supplying an HTTP client with follow_redirects set to False. If a client may have already followed a redirect to an untrusted host, rotate any credentials that were sent. Avoid relying on redirects to different hosts as the SDK will no longer follow them after the fix.

Description

The Model Context Protocol (MCP) Python SDK versions 1.8.0 through 1.29.1 and 2.0.0 through 2.1.1 have a vulnerability where their HTTP client transports follow cross-origin HTTP redirects and send custom headers and request bodies to the redirected host. This can expose sensitive headers such as API keys if the trusted endpoint redirects to an attacker-controlled domain. The SDK now restricts redirects to the same origin or an http-to-https upgrade on the same host. Fixed in versions 1.30.0 and 2.2.0 and later.

Impact

Sensitive custom headers and request bodies can be leaked to unintended origins if a trusted MCP endpoint redirects to a different host. This could expose API keys or other credentials to attackers controlling the redirect target. The Authorization bearer token is not leaked. The vulnerability does not affect MCP servers or clients not using HTTP or with redirect following disabled. The impact is limited to confidentiality of custom headers and request bodies during cross-origin redirects.

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 10:15 UTC

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