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EnvoyApplication · Envoyproxy

CVE-2021-32778

HIGH · 7.5 CVSS v3.1 Published 2021-08-24
Fix available
A fix is available. Upgrade to 1.16.5 / 1.17.4 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Envoy's HTTP/2 stream reset procedure exhibits O(N²) complexity, causing exponential CPU consumption when many streams are rapidly opened and closed. Attackers can trigger this DoVulnerability by having a client rapidly open and close HTTP/2 streams, exhausting CPU resources on the Envoy proxy.

MitigationUpgrade Envoy to versions 1.19.1, 1.18.4, 1.17.4, or 1.16.5 which contain the optimized stream reset logic, or temporarily apply the workaround of limiting concurrent HTTP/2 streams to 100 via upstream/downstream configuration.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
EnvoyApplication
Affected:>= 1.16.0, < 1.16.5>= 1.17.0, < 1.17.4>= 1.18.0, < 1.18.4= 1.19.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine Envoy version
    Run 'envoy --version' or check the container/service metadata to identify the installed Envoy version
    Affected if The version falls in these ranges: >=1.16.0,<1.16.5 OR >=1.17.0,<1.17.4 OR >=1.18.0,<1.18.4 OR =1.19.0
  2. Identify HTTP/2 listener configuration
    Review Envoy configuration files for 'http2_protocol_options' in listener or route definitions, or check if HTTP/2 is negotiated via ALPN or h2 upgrade
    Affected if HTTP/2 protocol is enabled on any upstream or downstream listener handling external traffic
  3. Check concurrent stream limits
    Inspect Envoy config for 'max_concurrent_streams' setting under http2_protocol_options or http_protocol_options in listener and cluster definitions
    Affected if No explicit max_concurrent_streams limit is set, or the limit is set higher than 100, allowing unlimited stream creation
  4. Verify exposed HTTP/2 endpoints
    Review Envoy listener configurations to identify exposed services accepting HTTP/2 connections from untrusted clients
    Affected if Public-facing or untrusted clients can establish HTTP/2 connections to Envoy listeners without rate limiting

You are affected if you run an affected Envoy version (1.16.0-1.16.4, 1.17.0-1.17.3, 1.18.0-1.18.3, or 1.19.0) with HTTP/2 enabled and no effective stream concurrency limits, allowing remote clients to trigger excessive CPU consumption via rapid stream open/close cycles.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 1.16.5 / 1.17.4 / 1.18.4 or later
Fixed in 1.16.51.17.41.18.4
Interim mitigation

Upgrade Envoy to versions 1.19.1, 1.18.4, 1.17.4, or 1.16.5 which contain the optimized stream reset logic, or temporarily apply the workaround of limiting concurrent HTTP/2 streams to 100 via upstream/downstream configuration.

Recommended fix High confidence

1.19.1 (if on 1.19.x), 1.18.4 (if on 1.18.x), 1.17.4 (if on 1.17.x), or 1.16.5 (if on 1.16.x)

  1. 1. Identify the current Envoy version in use (e.g., via envoy --version or admin /server_info endpoint)
  2. 2. Determine which version branch you are on (1.16.x, 1.17.x, 1.18.x, or 1.19.0)
  3. 3. Upgrade Envoy to the fixed patch version for your branch: 1.16.5, 1.17.4, 1.18.4, or 1.19.1
  4. 4. Verify the upgrade by checking envoy --version shows the patched release
  5. 5. Test the new version in a staging environment before deploying to production
Caveat Standard upgrade testing recommended; no specific breaking changes noted for this patch

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Envoy Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing8.0 h
  • Review / QA4.0 h
24.0 hours of engineering $4,160
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-32778 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data