CVE-2026-53965: MCP PHP SDK unbounded SSE buffer memory exhaustion

CVE-2026-53965 · Severity: high · CVSS 4 · Published 2026-08-25

The MCP PHP SDK's HTTP client transport accumulates Server-Sent-Events (SSE) stream data in an unbounded in-memory buffer without enforcing any size limit. A remote server or network attacker that withholds the event delimiter (`\n\n`) can cause the buffer to grow indefinitely, exhausting the client's available memory and crashing the process. This is a denial-of-service attack against any client connecting to a malicious or compromised MCP server.

Vendors: Anthropic, Packagist.

Quick answers

Is CVE-2026-53965 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-53965 being exploited in the wild.
How severe is CVE-2026-53965?
High severity, with a CVSS score of 4, published 25 August 2026.
Which products and versions are affected?
Anthropic Model Context Protocol PHP SDK >=0.5.0, <0.7.1; mcp/sdk (Packagist) >= 0.5.0, < 0.7.1, ECOSYSTEM:introduced=0.5.0,fixed=0.7.1.
Is there a fix for CVE-2026-53965?
Yes. mcp/sdk (Packagist): upgrade to 0.7.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-53965 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-53965?
Check whether you run Anthropic Model Context Protocol PHP SDK or mcp/sdk (Packagist) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • mcp/sdk (Packagist): upgrade to 0.7.1 or later
    composer require mcp/sdk:^0.7.1

Technical details

The vulnerability exists in the `HttpTransport::processSSEStream()` method (line 203 of `src/Client/Transport/HttpTransport.php`), which reads up to 4096-byte chunks from an SSE response stream and appends them directly to `$this->sseBuffer` with no length validation. The buffer is only flushed when a complete SSE event delimiter (`\n\n`) is encountered; if the server never sends this delimiter, the buffer grows without bound across successive `tick()` loop iterations. A remote MCP server (or a network attacker intercepting plaintext responses) can stream arbitrary data without delimiters to force unbounded buffer accumulation. The attack requires network-level access to the server's SSE response body but no prior authentication. Proof-of-concept testing confirms that a 400 MB delimiter-free stream exhausts a 256 MB PHP memory limit. The fix, provided in version 0.7.1, bounds the SSE buffer length and aborts the stream when it exceeds a configured maximum (e.g., 8 MiB) without receiving a complete event.

Affected products

  • modelcontextprotocol php-sdk >=0.5.0, <0.7.1

Identifiers

  • CVE-2026-53965
  • GHSA-7m52-jw36-44r3

Timeline

  • 2026-08-14: disclosed
  • 2026-08-19: advisory: GHSA-7m52-jw36-44r3 published
  • 2026-09-01: patched: Fix released in version 0.7.1

References

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