CVE-2021-36740Public exploitNetwork attack vector

Authorization bypass via HTTP/2 length mismatch

Published Jul 14, 2021 · Updated Aug 4, 2024

HTTP request smuggling in affected Varnish Cache and Enterprise releases allows remote attackers to bypass VCL authorization over HTTP/2. The HTTP/2 request-body handler fails to compare the advertised Content-Length with the combined DATA-frame length before rewriting the request for an HTTP/1 backend, leaving trailing body bytes to be parsed as another request. Exploitation requires HTTP/2 and backend connection reuse; smuggled requests bypass VCL and can expose responses or poison cached content.

CVSS severity6.5
Medium
EPSS probability1.60%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in affected Varnish Cache and Enterprise releases allows remote attackers to bypass VCL authorization over HTTP/2. The HTTP/2 request-body handler fails to compare the advertised Content-Length with the combined DATA-frame length before rewriting the request for an HTTP/1 backend, leaving trailing body bytes to be parsed as another request. Exploitation requires HTTP/2 and backend connection reuse; smuggled requests bypass VCL and can expose responses or poison cached content.

The record

CVE
CVE-2021-36740
Published
Jul 14, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
6.4NVDAV:N/AC:L/Au:N/C:P/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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