- Exploited
- No known exploitation
- Fix
- Fixed in 3.9.0
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 3.9.0.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 16/100 · provisional
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
The built-in Streamable HTTP clients in rmcp (the reqwest and Unix socket clients) read HTTP response bodies into memory with no size limit in releases before the fix. A malicious or compromised MCP server can send a never-ending response, such as a chunked stream of whitespace, during `initialize` before any authentication, which exhausts memory and terminates the host application.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original 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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure?/25Unknown. No advisory context yet
- Reach16/20rmcp: 2.1M downloads a week
- Severity?/10Unknown. No CVSS score or vendor rating
- No patch0/5No signal
What would change the score
- Would score about 71 if added to CISA KEV
- Would score about 66 if CISA's SSVC rates exploitation active
- Would score about 38 if advisories confirm default setups are exposed
Impact
- Widely used
rmcp has 2.1m weekly downloads.Source: crates.io
What to do
Upgrade to a fixed version: 3.9.0.
Timeline
- Advisory published4 days
- Record last updated
Affected versions
- rmcpcrates.io
From 0.2.0 · fixed in 3.9.0
Fixed in 3.9.0 (released Nov 5, 2026, the day of disclosure).
The latest release, 3.9.0, is not affected.
Evidence 3
- Advisory
- Repository
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about RUSTSEC-2026-0462
Is RUSTSEC-2026-0462 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
Which versions are affected by RUSTSEC-2026-0462?
rmcp (crates.io): from 0.2.0 before 3.9.0.Source: cve.now feed data
How do I fix RUSTSEC-2026-0462?
Upgrade to a fixed version: 3.9.0.Source: cve.now feed data