CVE-2026-27896
Lfprojects Mcp Go Sdk ≤ 1.3.1
Raw vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:XSummary
CVE-2026-27896 is a high-severity Improper Handling of Case Sensitivity (CWE-178) vulnerability in Lfprojects Mcp Go Sdk. Its CVSS base score is 7.0 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Masquerading (T1036); ranked at the 17th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
This vulnerability is AI-related — categorised as AI Agent Protocols and Integrations; in the Protocol-Specific Risks risk domain.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-27896 is a vulnerability in the Go MCP SDK versions prior to 1.3.1, stemming from the use of Go's standard encoding/json.Unmarshal for parsing JSON-RPC and MCP protocol messages. This library performs case-insensitive matching of JSON keys to struct field tags, meaning a field tagged "json:\"method\"" would accept "Method", "METHOD", or similar variations. This violates the JSON-RPC 2.0 specification, which mandates exact field names, and is associated with CWE-178 (Improper Handling of Case Sensitivity) and CWE-436 (Interpretation Conflict).
A malicious MCP peer can exploit this over the network by sending protocol messages with non-standard field casing, which the SDK silently accepts. Attackers require no privileges or user interaction (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N, score 7.5), potentially bypassing intermediary inspection tools expecting strict JSON-RPC compliance and causing cross-implementation inconsistencies.
The vulnerability is fixed in SDK version 1.3.1 by replacing Go's standard JSON unmarshaling with a case-sensitive decoder, as detailed in commit 7b8d81c264074404abdf5aa16e2cf0c2d9c64cc0. Users should update to v1.3.1, with additional guidance in the GitHub security advisory at GHSA-wvj2-96wp-fq3f.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8792
Vulnerability Data
The Go MCP SDK used Go's standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing in versions prior to 1.3.1. Go's standard library performs case-insensitive matching of JSON keys to struct field tags — a field tagged json:"method" would also…
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match "Method", "METHOD", etc. This violated the JSON-RPC 2.0 specification, which defines exact field names. A malicious MCP peer may have been able to send protocol messages with non-standard field casing that the SDK would silently accept. This had the potential for bypassing intermediary inspection and coss-implementation inconsistency. Go's standard JSON unmarshaling was replaced with a case-sensitive decoder in commit 7b8d81c. Users are advised to update to v1.3.1 to resolve this issue.
- CWE(s)
AI Security AnalysisAI
- AI Category
- AI Agent Protocols and Integrations
- Risk Domain
- Protocol-Specific Risks
- OWASP Top 10 for LLMs 2025
- None mapped
- Classification Reason
- Matched keywords: mcp
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation can discover cases where two products interpret the same inputs or state transitions differently.
Strict, consistently applied input validation reduces the chance that one product will accept data the other product rejects or interprets differently.
Applying access control decisions to each request depends on accurate property determination that accounts for case differences.
Correct enforcement of authorizations for resource access structurally requires consistent case-sensitive handling of identifiers to avoid incorrect decisions.
Applying security engineering principles during design can require unambiguous protocol and data-format specifications that eliminate divergent interpretations between products.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly reduce the chance of introducing parser or state-machine inconsistencies.
Correlating logs from multiple products can surface discrepancies caused by interpretation conflicts.
Runtime monitoring of software behavior can detect adverse outcomes stemming from differing interpretations.
Supplier risk assessments can identify products whose differing interpretations create systemic exposure.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can detect and correct cases where one component misinterprets another’s state or messages.
Secure SDLC practices should include case-sensitivity requirements in design and coding standards.
Application security requirements must specify case handling for identifiers and paths.
Architecture principles should enforce canonical, case-aware resource naming.
Secure coding guidelines must mandate explicit case handling to prevent inconsistent lookups.
Access-control rules that ignore case can allow unintended resource access.