CVE-2026-116685VulnerabilityPublished 27 Oct 2026

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

Affects mcpSource GCVE Database

Authoritative record:CVE-2026-116685· Medium 5.9

Not known to be exploited · CVSS 5.9 Medium : upgrade to 2.2.0 when convenient.

Data as of 27 Oct 2026, 22:27 UTC

CVSS 3.1 base score

5.9Medium

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

Patch: Available – fixed in 2.2.0, 1.30.0

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
Weaponised: 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 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 to 2.2.0; mcp to 1.30.0.

Affected software

Product	Affected	Fixed in
PyPImcp	Affected >=2.0.0 <2.2.0	Fixed in 2.2.0
PyPImcp	Affected >=1.8.0 <1.30.0	Fixed in 1.30.0

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

AI-suggested

Upgrade MCP SDK to version 1.30.0 (1.x) or 2.2.0 (2.x) or later, where redirects are restricted to the same origin and cross-origin redirects cause request failure. No code changes are required. If immediate upgrade is not possible, configure the HTTP client to disable automatic redirect following (follow_redirects=False). If credentials may have been exposed due to prior redirect following, rotate those credentials. Avoid relying on redirects to different hosts; instead, update endpoint URLs to the final target. The SDK provides an unsupported option to restore old behavior by supplying a custom HTTP client that follows all redirects, but this is discouraged due to security risks.

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 behavior can expose sensitive headers such as API keys if the trusted endpoint redirects to an attacker-controlled domain. The SDK's OAuth client providers are similarly affected. The lone Authorization bearer header is not exposed due to existing HTTP library protections. The issue is fixed in MCP versions 1.30.0 and 2.2.0 by restricting redirects to the same origin only and failing otherwise.

Impact

Sensitive custom headers such as API keys and request bodies can be leaked to unintended, potentially malicious hosts if a trusted MCP endpoint issues a redirect to a different origin. This can lead to credential exposure and unauthorized access. The Authorization bearer token is not leaked due to HTTP library protections. The vulnerability does not affect MCP servers or clients that do not follow redirects. There is no indication of active exploitation in the wild.

Summary

In affected MCP Python SDK versions, the Streamable HTTP and SSE client transports follow HTTP redirects to any host, sending custom headers (e.g., X-API-Key) and, on 307 or 308 redirects, the request body to the redirected origin. OAuth client providers also follow redirects similarly. The Authorization bearer header is not forwarded on cross-origin redirects. An attacker able to cause a trusted endpoint to redirect to a malicious host can capture these headers and request bodies. The vulnerability affects SDK usage with HTTP clients configured to follow redirects. The issue is resolved in MCP 1.30.0 and 2.2.0 by limiting redirects to the same origin or HTTP to HTTPS upgrades on the same host, and by failing requests that redirect cross-origin. Users must upgrade or disable automatic redirect following to mitigate exposure.

CWE-200Exposure of Sensitive Information to an Unauthorized Actor (opens in a new tab)
Class weaknessLikelihood of exploit: HighCWE Top 25 (2025): #20OWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design · Separation of Privilege
Compartmentalize the system to have "safe" areas where trust boundaries can be unambiguously drawn. Do not allow sensitive data to go outside of the trust boundary and always be careful when interfacing with a compartment outside of the safe area.
OWASP Cheat Sheets
Error Handling (opens in a new tab)
Logging (opens in a new tab)
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V8.3.4 (opens in a new tab)
V14.3.3 (opens in a new tab)
V7.3.3 (opens in a new tab)
Attack patterns
Subverting Environment Variable Values (opens in a new tab)
ICMP Echo Request Ping (opens in a new tab)
TCP SYN Scan (opens in a new tab)
Enumerate Mail Exchange (MX) Records (opens in a new tab)
DNS Zone Transfers (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 27 Oct 2026, 22:27 UTC

WeaknessesCWE-200

Technical details

References

Community reviews

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