CVE-2026-53965
MCP PHP SDK: Unbounded SSE buffer in HttpTransport enables client-side denial of service
Description
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.
INFO
Published Date :
Aug. 25, 2026, 9:17 p.m.
Last Modified :
Aug. 26, 2026, 4:16 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-53965
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 4.0 | MEDIUM | [email protected] |
Solution
- Update the MCP PHP SDK to version 0.7.1.
- Ensure HttpTransport does not connect to untrusted servers.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-53965.
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-53965 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-53965
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-53965 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-53965 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Modified by [email protected]
Aug. 26, 2026
Action Type Old Value New Value Added Reference https://github.com/modelcontextprotocol/php-sdk/commit/055593f2592e1d7f5f48587c999ca7068a3be90d Added Reference https://github.com/modelcontextprotocol/php-sdk/releases/tag/v0.7.1 Added Reference https://github.com/modelcontextprotocol/php-sdk/security/advisories/GHSA-7m52-jw36-44r3 Removed Reference https://github.com/modelcontextprotocol/php-sdk/commit/055593f2592e1d7f5f48587c999ca7068a3be90d Removed Reference https://github.com/modelcontextprotocol/php-sdk/releases/tag/v0.7.1 Removed Reference https://github.com/modelcontextprotocol/php-sdk/security/advisories/GHSA-7m52-jw36-44r3 -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Aug. 26, 2026
Action Type Old Value New Value Added Reference https://github.com/modelcontextprotocol/php-sdk/security/advisories/GHSA-7m52-jw36-44r3 Added SSVC {'id': 'CVE-2026-53965', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-08-26T14:59:57.576830Z'} -
New CVE Received by [email protected]
Aug. 25, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'modelcontextprotocol', 'product': 'php-sdk', 'versions': [{'status': 'affected', 'version': '>= 0.5.0, < 0.7.1'}]}] Added Description 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. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:H/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 Added CWE CWE-400 Added CWE CWE-770 Added Reference https://github.com/modelcontextprotocol/php-sdk/commit/055593f2592e1d7f5f48587c999ca7068a3be90d Added Reference https://github.com/modelcontextprotocol/php-sdk/releases/tag/v0.7.1 Added Reference https://github.com/modelcontextprotocol/php-sdk/security/advisories/GHSA-7m52-jw36-44r3