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GHSA-q382-vc8q-7jhj
No affected components available
The Go SDK recently transitioned to the segmentio/encoding library for JSON parsing in version 1.3.1. While this change addressed both case-insensitivity and ASCII folding issues, the new parser implemented aggressive key matching that treated keys with null Unicode characters appended at the end as equivalent to their base strings.
Impact
When combined with duplicate keys, the described behavior leads to a "last key wins" resolution that could override the intended MCP message. This had the potential for:
- Bypassing intermediary inspection: Proxies or policy layers that matched on exact field names may have failed to detect or filter these messages.
- Cross-implementation inconsistency: Other MCP SDKs (TypeScript, Python) use case-sensitive parsing and would reject the same messages, creating potential security-boundary confusion.
Fix:
The segmentio/encoding package was patched with a fix in https://github.com/segmentio/encoding/commit/7d5a25dbc5da13aed3cb047a127e4d0e96f536fb and a new version of the package was released (v0.5.4). The SDK switched to the patched version of the dependency in 724dd47aa. Users are advised to update to v1.4.1 to resolve this issue.
Credits:
Thank you to Francesco Lacerenza (Doyensec) for reporting this issue.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
Probability that this vulnerability will be exploited in the wild within the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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