Track vulnerabilities like this one.
Sign up free to get alerted when software you use is affected.
7.5
CVE-2026-27896: MCP Go SDK Fails to Validate Field Names Correctly
CVE-2026-27896 · published 8 months ago
Summary
The MCP Go SDK doesn't correctly check the names of fields in certain types of messages. This could allow a malicious user to send messages that would be blocked by other systems, creating confusion. To fix this, update to MCP Go SDK v1.3.1.
What to do
- Update github.com modelcontextprotocol to version 1.3.1.
- Update lfprojects mcp_go_sdk to version 1.3.1 or later.
Affected software
| Ecosystem | Vendor | Product | Affected versions |
|---|---|---|---|
| go | github.com | modelcontextprotocol |
< 1.3.1 Fix: upgrade to 1.3.1
|
| – | lfprojects | mcp_go_sdk |
< 1.3.1 cpe:2.3:a:lfprojects:mcp_go_sdk:*:*:*:*:*:*:*:* |
| – | modelcontextprotocol | go-sdk | < 1.3.1 |
Original advisory text
MCP Go SDK Vulnerable to Improper Handling of Case Sensitivity
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 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.
References
- https://github.com/advisories/GHSA-wvj2-96wp-fq3f
- https://access.redhat.com/security/cve/CVE-2026-27896 Vendor Advisory
- https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27896... Vendor Advisory
- https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-wvj2-96w... Vendor Advisory
- https://nvd.nist.gov/vuln/detail/CVE-2026-27896 Vendor Advisory
- https://bugzilla.redhat.com/show_bug.cgi?id=2442903 Third Party Advisory
- https://github.com/modelcontextprotocol/go-sdk/commit/7b8d81c264074404abdf5aa16e... Patch
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27896.json URL
Internet-facing
14 days
Internal
60 days
- Not known to be exploited
- Can be exploited automatically
- Gives an attacker partial control
Severity
7.5
High
Type
CWE-178Improper Handling of Case Sensitivity
CWE-436Interpretation Conflict
Timeline
Published26 Feb 2026
Updated3 Nov 2026
First seen6 Mar 2026
Track software like this
Free during beta