Vulnerability-Lookup

CVE-2026-67431 (GCVE-0-2026-67431)

Vulnerability from cvelistv5 – Published: 2026-07-29 19:15 – Updated: 2026-07-30 14:35

Title

MCP Ruby SDK: Ruby SSE Session Poisoning

Summary

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. This issue is fixed in version 0.23.0.

Severity

8.3 (High)


                        
                          CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N

SSVC

Exploitation: poc Automatable: yes Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-07-30 14:34 UTC

CWE

CWE-284 - Improper Access Control

Assigner

GitHub_M mitre root

References

3 references

URL	Tags
https://github.com/modelcontextprotocol/ruby-sdk/…	x_refsource_CONFIRM
https://github.com/modelcontextprotocol/ruby-sdk/…	x_refsource_MISC
https://github.com/modelcontextprotocol/ruby-sdk/…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
modelcontextprotocol	ruby-sdk	Affected: < 0.23.0

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-67431",
                "options": [
                  {
                    "Exploitation": "poc"
                  },
                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "total"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-07-30T14:34:47.124282Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-07-30T14:35:45.395Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "references": [
          {
            "tags": [
              "exploit"
            ],
            "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "ruby-sdk",
          "vendor": "modelcontextprotocol",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.23.0"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim\u0027s session. This issue is fixed in version 0.23.0."
        }
      ],
      "metrics": [
        {
          "cvssV4_0": {
            "attackComplexity": "LOW",
            "attackRequirements": "PRESENT",
            "attackVector": "NETWORK",
            "baseScore": 8.3,
            "baseSeverity": "HIGH",
            "privilegesRequired": "NONE",
            "subAvailabilityImpact": "NONE",
            "subConfidentialityImpact": "NONE",
            "subIntegrityImpact": "NONE",
            "userInteraction": "NONE",
            "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N",
            "version": "4.0",
            "vulnAvailabilityImpact": "LOW",
            "vulnConfidentialityImpact": "NONE",
            "vulnIntegrityImpact": "HIGH"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-284",
              "description": "CWE-284: Improper Access Control",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-07-29T19:15:32.340Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv"
        },
        {
          "name": "https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356"
        },
        {
          "name": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0"
        }
      ],
      "source": {
        "advisory": "GHSA-5p9g-j988-pcwv",
        "discovery": "UNKNOWN"
      },
      "title": "MCP Ruby SDK: Ruby SSE Session Poisoning"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-67431",
    "datePublished": "2026-07-29T19:15:32.340Z",
    "dateReserved": "2026-07-29T15:07:24.991Z",
    "dateUpdated": "2026-07-30T14:35:45.395Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-67431",
      "date": "2026-10-03",
      "epss": "0.0048",
      "percentile": "0.3924"
    },
    "vulnrichment": {
      "containers": {
        "adp": [
          {
            "metrics": [
              {
                "other": {
                  "content": {
                    "id": "CVE-2026-67431",
                    "options": [
                      {
                        "Exploitation": "poc"
                      },
                      {
                        "Automatable": "yes"
                      },
                      {
                        "Technical Impact": "total"
                      }
                    ],
                    "role": "CISA Coordinator",
                    "timestamp": "2026-07-30T14:34:47.124282Z",
                    "version": "2.0.3"
                  },
                  "type": "ssvc"
                }
              }
            ],
            "providerMetadata": {
              "dateUpdated": "2026-07-30T14:35:38.999Z",
              "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "shortName": "CISA-ADP"
            },
            "references": [
              {
                "tags": [
                  "exploit"
                ],
                "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv"
              }
            ],
            "title": "CISA ADP Vulnrichment"
          }
        ],
        "cna": {
          "affected": [
            {
              "product": "ruby-sdk",
              "vendor": "modelcontextprotocol",
              "versions": [
                {
                  "status": "affected",
                  "version": "\u003c 0.23.0"
                }
              ]
            }
          ],
          "descriptions": [
            {
              "lang": "en",
              "value": "MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim\u0027s session. This issue is fixed in version 0.23.0."
            }
          ],
          "metrics": [
            {
              "cvssV4_0": {
                "attackComplexity": "LOW",
                "attackRequirements": "PRESENT",
                "attackVector": "NETWORK",
                "baseScore": 8.3,
                "baseSeverity": "HIGH",
                "privilegesRequired": "NONE",
                "subAvailabilityImpact": "NONE",
                "subConfidentialityImpact": "NONE",
                "subIntegrityImpact": "NONE",
                "userInteraction": "NONE",
                "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N",
                "version": "4.0",
                "vulnAvailabilityImpact": "LOW",
                "vulnConfidentialityImpact": "NONE",
                "vulnIntegrityImpact": "HIGH"
              }
            }
          ],
          "problemTypes": [
            {
              "descriptions": [
                {
                  "cweId": "CWE-284",
                  "description": "CWE-284: Improper Access Control",
                  "lang": "en",
                  "type": "CWE"
                }
              ]
            }
          ],
          "providerMetadata": {
            "dateUpdated": "2026-07-29T19:15:32.340Z",
            "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
            "shortName": "GitHub_M"
          },
          "references": [
            {
              "name": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv",
              "tags": [
                "x_refsource_CONFIRM"
              ],
              "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv"
            },
            {
              "name": "https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356",
              "tags": [
                "x_refsource_MISC"
              ],
              "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356"
            },
            {
              "name": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0",
              "tags": [
                "x_refsource_MISC"
              ],
              "url": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0"
            }
          ],
          "source": {
            "advisory": "GHSA-5p9g-j988-pcwv",
            "discovery": "UNKNOWN"
          },
          "title": "MCP Ruby SDK: Ruby SSE Session Poisoning"
        }
      },
      "cveMetadata": {
        "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "assignerShortName": "GitHub_M",
        "cveId": "CVE-2026-67431",
        "datePublished": "2026-07-29T19:15:32.340Z",
        "dateReserved": "2026-07-29T15:07:24.991Z",
        "dateUpdated": "2026-07-29T19:15:32.340Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}

EUVD-2026-81652

European Vulnerability Database identifier assigned by ENISA

Reserved

2026-10-03 10:53

Assigner

ENISA

Alias of

a CVE record, shown under related vulnerabilities. This identifier carries no description, severity or references of its own: they belong to that CVE.

JSON

To clipboard

{
  "assigner": "ENISA",
  "date_reserved": "2026-10-03T10:53:53.546904+00:00",
  "id": "EUVD-2026-81652"
}

FKIE_CVE-2026-67431

Vulnerability from fkie_nvd - Published: 2026-07-29 20:17 - Updated: 2026-07-30 19:30

Severity

8.3 (High) - CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356
	security-advisories@github.com	https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0
	security-advisories@github.com	https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "ruby-sdk",
          "vendor": "modelcontextprotocol",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.23.0"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim\u0027s session. This issue is fixed in version 0.23.0."
    }
  ],
  "id": "CVE-2026-67431",
  "lastModified": "2026-07-30T19:30:33.710",
  "metrics": {
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "PRESENT",
          "attackVector": "NETWORK",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 8.3,
          "baseSeverity": "HIGH",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "NONE",
          "subIntegrityImpact": "NONE",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/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:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "LOW",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "HIGH",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-67431",
          "options": [
            {
              "exploitation": "poc"
            },
            {
              "automatable": "yes"
            },
            {
              "technicalImpact": "total"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-07-30T14:34:47.124282Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-07-29T20:17:12.120",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-284"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-5P9G-J988-PCWV

Vulnerability from github – Published: 2026-07-30 14:44 – Updated: 2026-07-30 14:44

Summary

MCP Ruby SDK: Ruby SSE Session Poisoning

Details

Summary

Vulnerability: Missing Session Ownership Validation in the Ruby MCP SDK's Streamable and SSE HTTP transport implementation. Any attacker with a stolen session ID can execute tools with the victim's session. This is a silent attack - the victim's session is compromised and being used for unauthorized actions, but it is hard to know for the victim

Details

https://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb#L260-L278

Victim starts a legitimate MCP session and receives session ID abc-123 Attacker obtains the session ID (various means - network sniffing, logs, etc. out of scope for this analysis) Attacker sends POST to /messages/abc-123 with a tool call Server accepts the request (no ownership validation!) Server executes the tool and sends response to victim's SSE stream Victim receives attacker's response, thinking it's legitimate

PoC

attacker_client.py legitimate_client.py

Prerequisites

Python 3.8+
Install dependencies: requests

Running the Demo

Terminal 1: Start the Ruby MCP Server

ruby streamable_http_server.rb

Makes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb This server has a tool call notification_tool which the clients call

Terminal 2: Start Victim Client

python3 legitimate_client.py

Terminal 3 - Attacker Client:

Copy the session ID from Terminal 1 and run:

python3 attacker_client.py abc-123-def-456

Back to Terminal 2 - Victim sees the injected response:

Impact

Integrity: HIGH - Attacker can execute unauthorized tools and modify state
Availability: LOW - Attacker can disrupt victim's session with injected responses

Additional Details

Session Hijacking Protection in MCP Implementations

The MCP specification recommends - "MCP servers SHOULD bind session IDs to user-specific information".

User Binding - Comparison other SDKs

csharp-sdk

https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L36
https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L95-L98

Go-sdk

https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L66
https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L316-L320

Comparison with the Stream Replacement vulnerability

Stream Replacement https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35

Target: SSE stream connection
Attack Vector: GET /mcp
What happens to Victim: The connection is disconnected and doesn't receive reponses

Session Poisoning

Target: Tool execution
Attack Vector: POST /mcp
What happens to Victim: The connection stays connected and receives responses of tool calls by attacker

Severity

8.3 (High)


                  
                    CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.22.0"
      },
      "package": {
        "ecosystem": "RubyGems",
        "name": "mcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-67431"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-284"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-30T14:44:28Z",
    "nvd_published_at": "2026-07-29T20:17:12Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n**Vulnerability**: Missing Session Ownership Validation in the Ruby MCP SDK\u0027s Streamable and SSE HTTP transport implementation. Any attacker with a stolen session ID can execute tools with the victim\u0027s session. This is a silent attack - the victim\u0027s session is compromised and being used for unauthorized actions, but it is hard to know for the victim\n\n### Details\nhttps://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb#L260-L278\n\n**Victim** starts a legitimate MCP session and receives session ID abc-123\n**Attacker** obtains the session ID (various means - network sniffing, logs, etc. out of scope for this analysis)\n**Attacker** sends POST to /messages/abc-123 with a tool call\n**Server** accepts the request (no ownership validation!)\n**Server** executes the tool and sends response to victim\u0027s SSE stream\n**Victim** receives attacker\u0027s response, thinking it\u0027s legitimate\n\n### PoC\n[attacker_client.py](https://github.com/user-attachments/files/26044817/attacker_client.py)\n[legitimate_client.py](https://github.com/user-attachments/files/26044818/legitimate_client.py)\n\n**Prerequisites**\n\n1. Python 3.8+\n2. Install dependencies: `requests`\n\n**Running the Demo**\n\n1. **Terminal 1:** Start the Ruby MCP Server\n\n`ruby streamable_http_server.rb`\n\nMakes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb\nThis server has a tool call notification_tool which the clients call\n\n2. **Terminal 2:**  Start Victim Client\n\n`python3 legitimate_client.py`\n\n3.  **Terminal 3** - Attacker Client:\n\nCopy the session ID from Terminal 1 and run:\n\n```bash\npython3 attacker_client.py abc-123-def-456\n```\n4. **Back to Terminal 2** - Victim sees the injected response:\n\n### Impact\n\n- **Integrity:** HIGH - Attacker can execute unauthorized tools and modify state\n- **Availability:** LOW - Attacker can disrupt victim\u0027s session with injected responses\n\n### Additional Details\nSession Hijacking Protection in MCP Implementations\n\nThe MCP specification recommends - \"[MCP servers SHOULD bind session IDs to user-specific information](https://modelcontextprotocol.io/docs/tutorials/security/security_best_practices#mitigation-4)\".\n\n#### User Binding - Comparison other SDKs\n**csharp-sdk**\n\n- https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L36\n- https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L95-L98\n\n**Go-sdk**\n\n- https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L66\n- https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L316-L320\n\n#### Comparison with the Stream Replacement vulnerability \n\n**Stream Replacement**\nhttps://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35\n\n- **Target**: SSE stream connection\n- **Attack Vector**: GET /mcp\n- **What happens to Victim**: The connection is disconnected and doesn\u0027t receive reponses \n\n**Session Poisoning**\n\n- **Target**: Tool execution\n- **Attack Vector**: POST /mcp\n- **What happens to Victim**: The connection stays connected and receives responses of tool calls by attacker",
  "id": "GHSA-5p9g-j988-pcwv",
  "modified": "2026-07-30T14:44:28Z",
  "published": "2026-07-30T14:44:28Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-67431"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "MCP Ruby SDK: Ruby SSE Session Poisoning"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-67431 (GCVE-0-2026-67431)

EUVD-2026-81652

FKIE_CVE-2026-67431

GHSA-5P9G-J988-PCWV

Summary

Details

PoC

Impact

Additional Details

User Binding - Comparison other SDKs

Comparison with the Stream Replacement vulnerability

Tags

Sightings

Nomenclature

Related by attack behaviour