CVE
Denial of service in streaming RPC handling
- Exploited
- No known exploitation
- Fix
- Fixed in grpc_server 1.1.0 · grpc 1.0.0
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: grpc_server 1.1.0 · grpc 1.0.0.
- Published
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- 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? →
- 35/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- 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 elixir-grpc grpc_server and grpc packages let an unauthenticated attacker over the network tie up a BEAM scheduler indefinitely by sending a client-streaming or bidirectional-streaming request whose body ends partway through a gRPC message. Repeating such requests can exhaust all schedulers.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original description
Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in elixir-grpc grpc_server and grpc allows unauthenticated attackers to occupy a BEAM scheduler indefinitely via a client-streaming or bidirectional-streaming request whose body ends inside a gRPC message.
GRPC.Server.Adapters.Cowboy.read_stream/1 (grpc_server/lib/grpc/server/adapters/cowboy.ex, and the Cowboy adapter in grpc before the package split) asks for more data whenever the buffer does not hold a complete message. Once the request body has ended, every further read returns an empty chunk, so the buffer never changes and the function retries without end. A single 5-byte length prefix sent with END_STREAM keeps one scheduler busy for as long as the connection stays open, and repeated requests exhaust all schedulers.
This issue affects grpc_server: from 1.0.0 before 1.1.0; grpc: from 0.3.1 before 1.0.0.
Who should check
Teams running elixir-grpc grpc_server 1.0.0 before 1.1.0, or grpc 0.3.1 before 1.0.0, that expose a client-streaming or bidirectional-streaming RPC.
Read the source excerpt
“This issue affects grpc_server: from 1.0.0 before 1.1.0; grpc: from 0.3.1 before 1.0.0.”
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- Reach10/20Affects Hex packages
- Severity8.2/10CVSS 8.2
- No patch0/5No signal
What would change the score
- Would score about 80 if added to CISA KEV
- Would score about 75 if CISA's SSVC rates exploitation active
- Would score about 40 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: grpc_server 1.1.0 · grpc 1.0.0.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-121809 (CPU exhaustion via a truncated gRPC message on streaming RPCs in elixir-grpc grpc_server), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-121809 (entry last updated 2026-11-13):
Affected:
- Hex package grpc_server: from 1.0.0, fixed in 1.1.0
- elixir-grpc grpc_server: version range not reported
- Hex package grpc: from 0.3.1, fixed in 1.0.0
- elixir-grpc grpc: version range not reported
- Hex package grpc_server: from 1.0.0, fixed in 1.1.0
- Hex package grpc: from 0.3.1, fixed in 1.0.0
Fixed versions: 1.1.0, 1.0.0.
Conditions (all must be true for the bug to be hit):
- The server must expose at least one client-streaming or bidirectional-streaming RPC method. (source: https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-121809 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-121809 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Packages (Hex): find every lockfile in each project and run the stack.check operation of the cve.now MCP server (execute) on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh
- https://github.com/elixir-grpc/grpc/commit/38955f1392cbf08eef231a347348445410455c85
- https://cna.erlef.org/cves/CVE-2026-121809.html
- https://osv.dev/vulnerability/EEF-CVE-2026-121809
- https://github.com/elixir-grpc/grpc/commit/0332faae5f5e132314794dbfdae5edecbccea292
- https://github.com/elixir-grpc/grpc/commit/6d8a09ad919e208fb600538ac8b19bd733a04310
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-121809
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-121809"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- grpc_serverHex
From 1.0.0 · fixed in 1.1.0
- elixir-grpc/grpc
Version range not reported
- grpcHex
From 0.3.1 · fixed in 1.0.0
- elixir-grpc/grpc
Version range not reported
- grpc_serverHex
From 1.0.0 · fixed in 1.1.0
- grpcHex
From 0.3.1 · fixed in 1.0.0
Evidence 7
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-121809
Is CVE-2026-121809 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
What does this mean?
A truncated gRPC message sent on a streaming RPC can keep one BEAM scheduler busy per open connection. The sources say an unauthenticated client can do this with a 5-byte request, and enough connections can deny service to every application on the node.Source: github.com
Which versions are affected by CVE-2026-121809?
grpc_server (Hex): from 1.0.0 before 1.1.0. grpc (Hex): from 0.3.1 before 1.0.0. grpc_server (Hex): from 1.0.0 before 1.1.0. grpc (Hex): from 0.3.1 before 1.0.0.Source: cve.now feed data
Am I affected, and how do I check?
The checked facts list elixir-grpc grpc_server from 1.0.0 before 1.1.0, and the grpc package from 0.3.1 before 1.0.0. The issue applies only to servers that expose at least one client-streaming or bidirectional-streaming RPC method. To check a project, look at the grpc_server or grpc version in mix.lock and whether its .proto files define stream methods.Source: github.com
How do I fix CVE-2026-121809?
The checked facts do not name a fixed version or vendor patch identifiers, so no upgrade target can be confirmed from them. Check the elixir-grpc advisory for the fixed release before changing dependencies.Source: github.com