GHSA-c9f9-cp8h-jcj5
Unbounded response body buffering in Streamable HTTP clients
GHSA-c9f9-cp8h-jcj5 is a medium vulnerability (CVSS 6.5) in rmcp, published November 5, 2026. No source lists public exploit code. Fixed in rmcp 3.9.0.
- Severity
- 6.5 MediumCVSS 3.1, OSV
- Exploit chance
- UnscoredNo EPSS score yet
- Exploitation
- None knownNot on CISA’s or ENISA’s lists
- Fixed in
3.9.0rmcp
Description
Before 3.9.0, the built-in Streamable HTTP clients in rmcp read non-streaming
HTTP response bodies into memory with no size limit. Both the reqwest client
(transport-streamable-http-client-reqwest) and the Unix socket client
(transport-streamable-http-client-unix-socket) were affected:
application/jsonresponses to POST requests were read in full before parsing.- Non-2xx responses were read in full for error reporting.
A malicious or compromised MCP server can answer with a body that never ends,
for example a chunked application/json response that streams whitespace. The
client buffers it until the process runs out of memory. This works during
initialize, before any authentication. On desktop or agent hosts, this
terminates the whole host application.
Server-sent event streams are not affected. Each event is already capped by
max_sse_event_size, and events are not accumulated.
The flaw was corrected in 3.9.0. Successful JSON bodies are limited to 16 MiB
and non-success bodies to 64 KiB. Bytes are counted while streaming, after
decompression. An oversized Content-Length is rejected before the body is
read, and an oversized body is rejected with
StreamableHttpError::ResponseBodyTooLarge instead of being truncated.
If you cannot upgrade, use a custom StreamableHttpClient that enforces a
response body limit, or connect only to trusted MCP servers.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- Required
What an attacker can affect
- Confidentiality
- None
- Integrity
- None
- Availability
- High
Affected packages
| Package | Affected | Fixed |
|---|---|---|
rmcpcrates.io | 0.2.0 up to 3.9.0 | 3.9.0 |
What each source reports
OSV Score usedRUSTSEC-2026-0462
Unbounded response body buffering in Streamable HTTP clients
Before 3.9.0, the built-in Streamable HTTP clients in rmcp read non-streaming
HTTP response bodies into memory with no size limit. Both the reqwest client
(transport-streamable-http-client-reqwest) and the Unix socket client
(transport-streamable-http-client-unix-socket) were affected:
application/jsonresponses to POST requests were read in full before parsing.- Non-2xx responses were read in full for error reporting.
A malicious or compromised MCP server can answer with a body that never ends,
for example a chunked application/json response that streams whitespace. The
client buffers it until the process runs out of memory. This works during
initialize, before any authentication. On desktop or agent hosts, this
terminates the whole host application.
Server-sent event streams are not affected. Each event is already capped by
max_sse_event_size, and events are not accumulated.
The flaw was corrected in 3.9.0. Successful JSON bodies are limited to 16 MiB
and non-success bodies to 64 KiB. Bytes are counted while streaming, after
decompression. An oversized Content-Length is rejected before the body is
read, and an oversized body is rejected with
StreamableHttpError::ResponseBodyTooLarge instead of being truncated.
If you cannot upgrade, use a custom StreamableHttpClient that enforces a
response body limit, or connect only to trusted MCP servers.
- Record
RUSTSEC-2026-0462- CVSS 3.1
- 6.5 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H - Updated
- 2026-11-09
History
| Recorded | Changed | Sources |
|---|---|---|
| first indexed | OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.