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CVE-2021-36740: Varnish Cache, with HTTP/2 enabled, allows request smuggling and VCL authorization bypass via a large Conte...

Varnish Cache, with HTTP/2 enabled, allows request smuggling and VCL authorization bypass via a large Content-Length header for a POST request. This affects Varnish Enterprise 6.0.x before 6.0.8r3, and Varnish Cache 5.x and 6.x before 6.5.2, 6.6.x before 6.6.1, and 6.0 LTS before 6.0.8.

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 affects Varnish deployments that accept HTTP/2. A specially malformed POST request with a large Content-Length value can trigger request smuggling and bypass authorization checks implemented in VCL. The business risk is unauthorized access through an edge cache or reverse proxy when vulnerable versions are exposed.

Executive priority

Treat this as a high-priority edge infrastructure fix if Varnish handles public HTTP/2 traffic or enforces access decisions. If HTTP/2 is not enabled, priority can be lower after validation.

Technical view

CVE-2021-36740 is an HTTP/2-specific request smuggling issue in Varnish. The CVE description ties exploitation to large Content-Length handling in POST requests and states VCL authorization bypass impact. Fixed versions are Varnish Enterprise 6.0.8r3, Varnish Cache 6.5.2, 6.6.1, and 6.0 LTS 6.0.8.

Likely exposure

Exposure is likely limited to Varnish Cache or Varnish Enterprise instances running affected versions with HTTP/2 enabled and reachable by untrusted clients. Deployments without HTTP/2 enabled are not described as affected in the provided source bundle.

Exploitation context

The source bundle does not show CISA KEV listing or cited evidence of active exploitation. Public advisories and distribution updates exist, but no exploit-in-the-wild claim is provided here.

Researcher notes

Key uncertainty is severity scoring: the provided CVE bundle has no CVSS, CWE, or exploit telemetry. The most important validation points are exact Varnish version, HTTP/2 enablement, and whether VCL performs authorization decisions.

Mitigation direction

  • Upgrade Varnish Cache to 6.5.2, 6.6.1, 6.0.8 LTS, or later.
  • Upgrade Varnish Enterprise 6.0.x to 6.0.8r3 or later.
  • If immediate upgrade is blocked, review vendor advisory guidance for temporary controls.
  • Prioritize internet-facing Varnish systems that terminate or accept HTTP/2 traffic.

Validation and detection

  • Inventory Varnish Cache and Varnish Enterprise versions across edge and proxy tiers.
  • Confirm whether HTTP/2 is enabled on each Varnish listener or frontend path.
  • Review VCL authorization rules for paths relying on proxy-side access control.
  • Verify package updates from vendor, Fedora, or Debian channels where applicable.
Prepared
Confidence
high
Sources
6

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

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.

description · low confidence lookup

Privilege behavior lookup

The CVE wording references privilege impact, so privilege escalation and authorization behavior review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2021-36740 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
8Source 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.