CVE-2026-53965 modelcontextprotocol CVE debrief

Who should care

Defenders and developers using the MCP PHP SDK, especially those with clients using HttpTransport against untrusted server endpoints, should be aware of this vulnerability and take action to mitigate it.

Why it matters

The MCP PHP SDK has a denial-of-service vulnerability that can be exploited by a remote server, potentially leading to memory exhaustion and client process termination. Defenders should prioritize updating to version 0.7.1 and assess exposure for clients using HttpTransport against untrusted server endpoints.

• Denial of service through memory exhaustion
• Potential for fatal allocation errors or OS out-of-memory kills
• Need for verification of client process's memory usage
• Importance of updating to version 0.7.1 for mitigation

Technical summary

The MCP PHP SDK (Composer package mcp/sdk) has a vulnerability in versions 0.5.0 through 0.7.0 that allows a remote server to cause a denial of service by exhausting the client's memory. The issue arises from the HTTP client transport reading Server-Sent Events response streams without a proper buffer limit, leading to potential fatal allocation errors or OS out-of-memory kills. This vulnerability affects clients using HttpTransport against untrusted server endpoints and does not require authentication or user interaction beyond initiating the connection. The fix is included in version 0.7.1.

Defensive priority

Defenders should prioritize updating to version 0.7.1 of the MCP PHP SDK to mitigate this denial-of-service vulnerability, especially for clients using HttpTransport against untrusted server endpoints.

Recommended defensive actions

• Update to version 0.7.1 of the MCP PHP SDK
• Assess exposure for clients using HttpTransport against untrusted server endpoints
• Monitor for potential denial-of-service attacks
• Review compensating controls for exposed systems
• Check relevant monitoring, detection, and logs for exposed assets
• Track exceptions and retest remediated assets
• Confirm whether affected product deployments exist in managed environments

Evidence notes

The CVE record and NVD detail page provide information on the vulnerability, its impact, and the fix in version 0.7.1 of the MCP PHP SDK. The vulnerability arises from the HTTP client transport reading Server-Sent Events response streams without a proper buffer limit, which can lead to memory exhaustion. Defenders should verify client process memory usage and review compensating controls. Evidence is limited to public CVE and NVD records, and further verification is recommended.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.