vulnerability / NVI-00006448 / Revision 2
MCP PHP SDK: client HttpTransport SSE buffer (sseBuffer .= chunk) grows unbounded when server withholds the event delimiter
OSV carries this publisher advisory for mcp/sdk. This is an aggregated source copy; consult the original publisher and compare update dates.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
0.615%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.49472. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
mcp/sdk
source listedECOSYSTEM: introduced 0.5.0; fixed 0.7.1
## Summary The HTTP client transport in `mcp/sdk` reads a Server-Sent-Events (SSE) response stream incrementally and appends each 4 KiB chunk to an in-memory buffer (`$this->sseBuffer .= $chunk;`) with **no upper bound**. The buffer is only ever flushed when an SSE event delimiter (`"\n\n"`) appears. A remote MCP server (the peer the client connects to) that streams response bytes without ever sending the `"\n\n"` delimiter makes `$sseBuffer` grow without limit until the client process exhausts its PHP `memory_limit` (fatal "Allowed memory size … exhausted") or is killed by the OS OOM-killer. This is a denial-of-service against the MCP **client**: any server it talks to — or a network position that controls the server's response body — can crash the client by withholding the event delimiter while streaming data. ## Impact - **Type:** Denial of service (memory exhaustion / process crash) of the MCP client. - **Who can trigger it:** The remote MCP server endpoint the client connects to via `HttpTransport`, or any party that can control/inject into that server's SSE response body (e.g. a man-in-the-middle on a plaintext endpoint, or a malicious or compromised server). The bu
03 / Response
What to do next
- vendor patch
The original advisory lists fixed boundaries 0.7.1 for mcp/sdk. Confirm the publisher’s latest guidance.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database via OSV
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-7m52-jw36-44r3 ↗
GitHub Advisory Database via OSV · vendor advisory
Observed . GitHub-origin advisory, CC-BY-4.0; OSV is an aggregate copy, not independent corroboration.
- CVE-2026-53965 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 2 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.