Security readout for executives and security teams
Plain-English summary
This is an Envoy denial-of-service flaw. When Envoy is configured to tunnel TCP over HTTP, a client disconnect at the wrong time can crash the proxy. The business impact is service availability disruption for applications relying on affected Envoy gateways or sidecars.
Executive priority
Treat as high priority for any Envoy deployment on critical traffic paths with TCP-over-HTTP tunneling. It affects availability, not data confidentiality or integrity, but proxy crashes can create customer-facing outages.
Technical view
Affected Envoy versions can hit a CWE-416 use-after-free condition in upstream tunneling when the downstream connection closes while the upstream connection or HTTP/2 stream is still being established. CVSS is 7.5 with network access, low complexity, no privileges, and availability-only impact.
Likely exposure
Exposure is most likely where affected Envoy versions run with tcp_proxy upstream tunneling_config enabled, especially internet-facing gateways, ingress paths, or service-mesh traffic handling TCP tunneling over HTTP.
Exploitation context
The source bundle does not show CISA KEV listing or any cited evidence of active exploitation. The issue is remotely reachable under the vulnerable configuration and can crash Envoy, but sources do not provide exploit prevalence.
Researcher notes
The key condition is downstream disconnect during upstream connection or HTTP/2 stream establishment with tunneling enabled. Public sources identify the bug class as CWE-416 and point to an Envoy commit, but do not document a workaround.
Mitigation direction
- Upgrade Envoy to a non-affected release for the deployed branch.
- Prioritize externally reachable Envoy deployments using upstream TCP tunneling.
- Check managed proxy or service-mesh vendor guidance for packaged Envoy updates.
- Monitor Envoy crash loops and availability alerts during remediation.
Validation and detection
- Inventory Envoy versions across gateways, sidecars, and proxy appliances.
- Identify configurations using tcp_proxy upstream tunneling_config.
- Confirm affected ranges: before 1.18.6, 1.19.0-1.19.2, 1.20.0-1.20.1, 1.21.0.
- After upgrade, verify Envoy remains stable under normal connection churn.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-416: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-43826 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
7.5HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/envoyproxy/envoy/security/advisories/GHSA-cmx3-fvgf-83mfCVE reference · x_refsource_CONFIRM
- https://github.com/envoyproxy/envoy/commit/ce0ae309057a216aba031aff81c445c90c6ef145CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Use After Free
Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.