What is CVE-2026-73548?

CVE-2026-73548 is Envoy: Cross-user response poisoning via a generic (non-WebSocket) HTTP upgrade on Envoy's shared backend pool. The supplied enrichment data classifies it as Unclassified mapped to CWE-444 CWE-444.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Active exploitation is not confirmed from current sources for CVE-2026-73548. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-73548 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

A public PoC is not confirmed from current sources for CVE-2026-73548. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-73548 using CVSS 7.5, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

high

CVE-2026-73548

CVE-2026-73548 — Envoy: Cross-user response poisoning via a generic (non-WebSocket) HTTP upgrade on Envoy's shared backend pool

Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to 1.36.10, 1.37.6, 1.38.4, and 1.39.1, Envoy forwards data for a configured non-WebSocket HTTP upgrade before the upstream accepts the upgrade. An unauthenticated HTTP/2 client can place a complete HTTP/1.1 request in extended CONNECT data; Envoy downgrades the request, writes the data unframed to a keep-alive HTTP/1.1 upstream, and returns the socket to the shared pool while the smuggled response remains queued. A different downstream client can then receive the attacker's response. The relevant scope boundary is that webSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, and max_requests_per_connection set to 1 are not affected by the demonstrated path. This issue is fixed in versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1.

Published Sep 21, 2026Updated Sep 21, 2026Sources: NVD, envoyproxy (CNA), GitHub, Red Hat (VEX), SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Unreported

no source claims exploitation

EPSS

ahead of 0% of scored CVEs

Affects

envoyproxy

4 products listed

CVSS base

7.5

high

Remediation

The vendor's own words where we have them.

Upgrade envoy to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

Red Hat VEX · CVE-2026-73548

fixed20affected0not affected105

vendor_fix: See Red Hat OpenShift Service Mesh 3.0.15 documentation at https://docs.redhat.com/en/documentation/red_hat_openshift_service_mesh/3.0

vendor_fix: See Red Hat OpenShift Service Mesh 3.1.12 documentation at https://docs.redhat.com/en/documentation/red_hat_openshift_service_mesh/3.1

vendor_fix: See Red Hat OpenShift Service Mesh 3.2.9 documentation at https://docs.redhat.com/en/documentation/red_hat_openshift_service_mesh/3.2

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
envoyproxy	envoy	< 1.36.10	Vulnerable
envoyproxy	envoy	>= 1.37.0, < 1.37.6	Vulnerable
envoyproxy	envoy	>= 1.38.0, < 1.38.4	Vulnerable
envoyproxy	envoy	>= 1.39.0, < 1.39.1	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

None

Confidentiality

High

Integrity

None

Scope

Unchanged

Attack Complexity

Low

Attack Vector

Network

Privileges Req

None

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
7.5	CVSS 3.1	high	3.9	3.6	GitHub (CNA), envoyproxy (CNA), Red Hat (VEX)

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

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-444 · Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling')

Unexpected State
Hide Activities
Bypass Protection Mechanism

Mitigation: Use a web server that employs a strict HTTP parsing procedure, such as Apache [REF-433].

Weakness & attack patterns

CWE-444

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-33 · HTTP Request SmugglingCAPEC-105 · HTTP Request Splitting

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Search application, proxy, and WAF logs for requests touching /2, /1.1.
Prioritize edge telemetry for network-reachable envoy.

web/proxy/WAF logsapplication logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 7.5 (AV:N)
Sep 21, 2026 · NVD
Initial · CVE published
Sep 21, 2026 · NVD
CVE published by MITRE.
Sep 21, 2026 · SOCRadar CTI

References

18 on the record

github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361
Exploit, Third Party Advisory, Mitre
github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b
Exploit, Third Party Advisory, Mitre
github.com/envoyproxy/envoy/commit/4633b8cce3d15b8734eefe232e30c12b0140b91c
Exploit, Third Party Advisory, Mitre
github.com/envoyproxy/envoy/commit/bd6711f2617658e28dfa3df3dace6bfe3cfc0766
Exploit, Third Party Advisory, Mitre
github.com/envoyproxy/envoy/releases/tag/v1.36.10
Exploit, Third Party Advisory, Mitre
github.com/envoyproxy/envoy/releases/tag/v1.37.6
Exploit, Third Party Advisory, Mitre
github.com/envoyproxy/envoy/releases/tag/v1.38.4
Exploit, Third Party Advisory, Mitre
github.com/envoyproxy/envoy/releases/tag/v1.39.1
Exploit, Third Party Advisory, Mitre

Elsewhere on this site

envoyproxyevery CVE for this vendor
Unclassifiedsame class
CWE-444other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-73548 is Envoy: Cross-user response poisoning via a generic (non-WebSocket) HTTP upgrade on Envoy's shared backend pool, a high vulnerability affecting envoy from envoyproxy. The current evidence does not list it in CISA KEV, and the exploit status is: Active exploitation is not confirmed from current sources for CVE-2026-73548. Public exploit evidence is: A public PoC is not confirmed from current sources for CVE-2026-73548. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 7.5 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.