# CVE-2026-121809: CPU exhaustion via a truncated gRPC message on streaming RPCs in elixir-grpc grpc_server

Status: published. Published: 2026-11-13. Updated: 2026-11-13.

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.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/elixir-grpc/grpc/commit/38955f1392cbf08eef231a347348445410455c85

## Affected products and versions

- elixir-grpc / grpc_server (grpc_server): ≥ 1.0.0; < 1.1.0 (affected, semver). Source: CNA: EEF
- elixir-grpc / grpc_server (elixir-grpc/grpc): ≥ 0332faae5f5e132314794dbfdae5edecbccea292; < 38955f1392cbf08eef231a347348445410455c85 (affected, git). Source: CNA: EEF
- elixir-grpc / grpc (grpc): ≥ 0.3.1; < 1.0.0 (affected, semver). Source: CNA: EEF
- elixir-grpc / grpc (elixir-grpc/grpc): ≥ 6d8a09ad919e208fb600538ac8b19bd733a04310; < 0332faae5f5e132314794dbfdae5edecbccea292 (affected, git). Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 8.2 (high); CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N; source: CNA: EEF

- CVSS 4.0: 8.2 (high); CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-400, CWE-835

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh (CNA: EEF; vendor-advisory, related)

- https://cna.erlef.org/cves/CVE-2026-121809.html (CNA: EEF; related)

- https://osv.dev/vulnerability/EEF-CVE-2026-121809 (CNA: EEF; related)

- https://github.com/elixir-grpc/grpc/commit/0332faae5f5e132314794dbfdae5edecbccea292 (CNA: EEF; related)

- https://github.com/elixir-grpc/grpc/commit/38955f1392cbf08eef231a347348445410455c85 (CNA: EEF; patch)

- https://github.com/elixir-grpc/grpc/commit/6d8a09ad919e208fb600538ac8b19bd733a04310 (CNA: EEF; related)

- https://cna.erlef.org/cves/CVE-2026-121809.html (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/elixir-grpc/grpc/commit/0332faae5f5e132314794dbfdae5edecbccea292 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/elixir-grpc/grpc/commit/38955f1392cbf08eef231a347348445410455c85 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/elixir-grpc/grpc/commit/6d8a09ad919e208fb600538ac8b19bd733a04310 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://osv.dev/vulnerability/EEF-CVE-2026-121809 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-121809; retrieved 2026-11-13; source updated 2026-11-13

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-121809; retrieved 2026-11-13; source updated 2026-11-13

Revision: 47e99b1cb8e598e25c8c0bd2614f6b23d9355c3052847c3976e7d74f9361f5bb

Canonical: https://y2.dev/cve/CVE-2026-121809

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