MCP PHP SDK: Unbounded SSE buffer in HttpTransport enables client-side denial of service
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Active
- Attack complexity
- Low
- Attack requirements
- None
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Fixed
Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub. Impact type from CWE-400.
Summary
The MCP PHP SDK (Composer package mcp/sdk) is the official Model Context Protocol SDK for PHP. In versions 0.5.0 through 0.7.0, the HTTP client transport reads a Server-Sent Events response stream incrementally and appends each chunk to an in-memory buffer with no upper bound. The buffer is only flushed when an SSE event delimiter, a double newline, is found, so a remote MCP server that streams response bytes without ever sending the delimiter causes the buffer to grow without limit. A malicious, compromised, or man-in-the-middle-controlled server that the client connects to over the HTTP transport can exploit this to exhaust the client process's memory, triggering a fatal allocation error or OS out-of-memory kill and denying service to the MCP client. The issue affects any client using HttpTransport against an untrusted server endpoint and does not require authentication or user interaction beyond initiating the connection. This issue is fixed in version 0.7.1.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- Poc
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| MLflow | php-sdk | 0.5.0 to before 0.7.1 | 0.7.1 | Patch available | CNA |
| — | mcp/sdk (Packagist) | 0.5.0 to before 0.7.1 | 0.7.1 | Patch available | GHSA-7m52-jw36-44r3 |
- php-sdk VendorMLflow Affected 0.5.0 to before 0.7.1 Fixed in 0.7.1 Patch available CNA
- mcp/sdk (Packagist) Vendor— Affected 0.5.0 to before 0.7.1 Fixed in 0.7.1 Patch available GHSA-7m52-jw36-44r3
Remediation
- Fix · CNA php-sdk (modelcontextprotocol) Fixed in 0.7.1
- Fix · GHSA-7m52-jw36-44r3 mcp/sdk (Packagist) Fixed in 0.7.1 Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2026-53965
- 2026-10-03 Alias added: GHSA-7m52-jw36-44r3
References
Other 5
- https://github.com/modelcontextprotocol/php-sdk/security/advisories/GHSA-7m52-jw36-44r3
- https://github.com/modelcontextprotocol/php-sdk/commit/055593f2592e1d7f5f48587c999ca7068a3be90d
- https://github.com/modelcontextprotocol/php-sdk/releases/tag/v0.7.1
- https://github.com/FriendsOfPHP/security-advisories/blob/master/mcp/sdk/CVE-2026-53965.yaml
- https://github.com/modelcontextprotocol/php-sdk
More MLflow vulnerabilities
- TI-2026-46366 Server-side request forgery in mlflow KEVCVSS 3.1CRITICAL 9.3
- TI-2023-03518 Absolute Path Traversal in mlflow/mlflow CVSS 3.1CRITICAL 10.0
- TI-2023-01113 Path Traversal: '\..\filename' in mlflow/mlflow CVSS 3.1CRITICAL 9.8
- TI-2025-31759 Redis Lua Use-After-Free may lead to remote code execution CVSS 3.1CRITICAL 10.0
- TI-2026-81527 Code execution via scorer deserialization in MLflow servers CVSS 3.1CRITICAL 9.8
- TI-2025-14028 Command Injection in mlflow/mlflow CVSS 3.1CRITICAL 10.0
Sources
- CVE-2026-53965 · CVE Program
- CVE-2026-53965 · NVD
- FIRST EPSS
- GHSA-7m52-jw36-44r3 · GitHub Advisory Database · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences