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CVE Record

CVE-2021-36371: Emissary-Ingress (formerly Ambassador API Gateway) through 1.13.9 allows attackers to bypass client certifi...

Emissary-Ingress (formerly Ambassador API Gateway) through 1.13.9 allows attackers to bypass client certificate requirements (i.e., mTLS cert_required) on backend upstreams when more than one TLSContext is defined and at least one configuration exists that does not require client certificate authentication. The attacker must send an SNI specifying an unprotected backend and an HTTP Host header specifying a protected backend. (2.x versions are unaffected. 1.x versions are unaffected with certain configuration settings involving prune_unreachable_routes and a wildcard Host resource.)

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This flaw can let a requester reach a backend that was intended to require client certificates. It affects Emissary-Ingress/Ambassador 1.x through 1.13.9 only under specific mixed TLS configuration conditions. The business concern is unauthorized access to protected services where mTLS was the control boundary.

Executive priority

Treat as urgent if mTLS protects customer, administrative, or internal service access through affected 1.x gateways. If the product is not deployed or 2.x is confirmed, priority drops substantially.

Technical view

When multiple TLSContext objects exist and at least one route does not require client certificate authentication, a crafted mismatch between SNI and HTTP Host can cause protected upstream client certificate requirements to be bypassed. The CVE states 2.x is unaffected, and some 1.x configurations are unaffected with specific settings.

Likely exposure

Exposure is likely limited to organizations running Emissary-Ingress or Ambassador API Gateway 1.x through 1.13.9 with multiple TLSContexts and mixed mTLS-required and non-mTLS configurations.

Exploitation context

The source bundle does not show CISA KEV listing or active exploitation. The issue is a configuration-dependent authentication bypass at the ingress layer, so internet-facing gateways protecting sensitive backends deserve priority review.

Researcher notes

Evidence is limited to the CVE description and referenced Emissary issue and release page. No CVSS, CWE, exploit status, or detailed patch note is provided in the bundle, so validation should focus on version and configuration exposure.

Mitigation direction

  • Inventory Emissary-Ingress or Ambassador API Gateway 1.x deployments through 1.13.9.
  • Check vendor issue and release guidance before changing production configuration.
  • Plan migration or upgrade to an unaffected 2.x release where feasible.
  • Review whether 1.x deployments use the vendor-described unaffected configuration conditions.
  • Avoid mixed protected and unprotected ingress paths until configuration is verified.

Validation and detection

  • Confirm product name and version across clusters and ingress controllers.
  • Audit TLSContext definitions for multiple contexts and mixed client-certificate requirements.
  • Identify protected backends relying on mTLS at the ingress layer.
  • Review logs for mismatched SNI and Host targeting protected backends.
  • Document whether prune_unreachable_routes and wildcard Host conditions apply.
Prepared
Confidence
medium
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2021-36371 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

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.

0CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.