# CVE-2026-53965: MCP PHP SDK: Unbounded SSE buffer in HttpTransport enables client-side denial of service

> A malicious server can exhaust client memory via unbounded SSE buffering in the MCP PHP SDK, causing denial of service.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 25 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** GHSA-7m52-jw36-44r3

## What is CVE-2026-53965?

CVE-2026-53965 is a medium-severity denial of service vulnerability in modelcontextprotocol php-sdk, published on 25 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-53965?

2 open source packages ship an affected version, including mcp/sdk.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mcp/sdk` | Packagist | From 0.5.0 | 0.7.1 |
| `mcp/sdk` | Packagist | From 0.5.0 | 0.7.1 |

## How do I fix CVE-2026-53965?

Upgrade mcp/sdk to 0.7.1 or later.

## Is CVE-2026-53965 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-53965 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-53965 and fix it if it is affected.

What it is: CVE-2026-53965 is a medium-severity denial of service vulnerability in modelcontextprotocol php-sdk, published on 25 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-53965 exists.

Affected packages and fixed versions:
- mcp/sdk (Packagist): affected from 0.5.0; fixed in 0.7.1
- mcp/sdk (Packagist): affected from 0.5.0; fixed in 0.7.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-53965/ (Markdown: https://insights.sbomify.com/CVE-2026-53965/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53965
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53965
- OSV: https://osv.dev/vulnerability/CVE-2026-53965

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-53965/
Summaries are generated and may contain mistakes; the official description above is the record.