CVE-2026-33946Public exploitNetwork attack vector

SSE stream hijacking via session object overwrite

Published Mar 27, 2026 · Updated Mar 27, 2026

Session hijacking in Model Context Protocol MCP Ruby SDK before 0.9.2 allows remote attackers to intercept SSE data using a valid session ID. The store_stream_for_session method overwrites the sole stream object stored for a session without rejecting a second connection or validating ownership. Possession of a live session ID is required; replay redirects subsequent tool responses and notifications from the victim to the attacker.

CVSS severity8.2
High
EPSS probability0.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Session hijacking in Model Context Protocol MCP Ruby SDK before 0.9.2 allows remote attackers to intercept SSE data using a valid session ID. The store_stream_for_session method overwrites the sole stream object stored for a session without rejecting a second connection or validating ownership. Possession of a live session ID is required; replay redirects subsequent tool responses and notifications from the victim to the attacker.

The record

CVE
CVE-2026-33946
Published
Mar 27, 2026
Updated
Mar 27, 2026
Vendor
Model Context Protocol
Product
MCP Ruby SDK
Classifications
CWE-384, CWE-639, T1550
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 27, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 27, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.9.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Model Context Protocol · MCP Ruby SDKversion=< 0.9.2

Published CVSS scores

5.9NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
8.2GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-384Session Fixation
CWE-639Authorization Bypass Through User-Controlled Key
T1550Use Alternate Authentication Material

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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