Mark3 Labs mcp-go up to 1.2.0 OAuth Metadata OAuthHandler.fetchServerMetadata input validation

CVSS Meta Temp Score
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Current Exploit Price (≈)
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CTI Interest Score
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7.5$0-$5k0.87+

Summaryinfo

A vulnerability was found in Mark3 Labs mcp-go up to 1.2.0 and classified as critical. The affected element is the function OAuthHandler.fetchServerMetadata of the component OAuth Metadata Handler. Such manipulation leads to input validation. This vulnerability is traded as CVE-2026-123272. The attack may be launched remotely. There is no exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, has been found in Mark3 Labs mcp-go up to 1.2.0. Affected by this issue is the function OAuthHandler.fetchServerMetadata of the component OAuth Metadata Handler. The manipulation with an unknown input leads to a input validation vulnerability. Using CWE to declare the problem leads to CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Impacted is confidentiality, integrity, and availability. CVE summarizes:

Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability in OAuthHandler.fetchServerMetadata that allows malicious MCP servers to obtain tokens for other resources by declaring arbitrary resources in well-known metadata. Attackers can declare a different MCP server as the RFC 8707 resource indicator, causing the authorization server to mint tokens the attacker replays against that resource.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-123272 since 11/05/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available.

Upgrading to version 1.2.1 eliminates this vulnerability.

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Productinfo

Type

Vendor

Name

Version

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA CVSS-B Score: 🔒
CNA CVSS-BT Score: 🔒
CNA Vector: 🔒

CVSSv3info

VulDB Meta Base Score: 7.6
VulDB Meta Temp Score: 7.5

VulDB Base Score: 7.3
VulDB Temp Score: 7.0
VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA Base Score: 8.0
CNA Vector (VulnCheck): 🔒

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Input validation
CWE: CWE-20
CAPEC: 🔒
ATT&CK: 🔒

Physical: No
Local: No
Remote: Yes

Availability: 🔒
Status: Not defined
Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: mcp-go 1.2.1

Timelineinfo

11/05/2026 Advisory disclosed
11/05/2026 +0 days CVE reserved
11/05/2026 +0 days VulDB entry created
11/05/2026 +0 days VulDB entry last update

Sourcesinfo

Advisory: github.com
Status: Confirmed

CNA: VulnCheck
CVE: CVE-2026-123272 (🔒)

GCVE (CVE): GCVE-0-2026-123272
GCVE (VulDB): GCVE-100-431624

Entryinfo

Created: 11/05/2026 20:07
Changes: 11/05/2026 20:07 (78)
Complete: 🔍
Cache ID: 216::103

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