CVE-2021-36740

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.

Published: 2021-07-14 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-36740 is rated Moderate Risk (53.1/100): CVSS Medium severity, with medium exploitation likelihood (EPSS 0.96%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-36740

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-05-29 0.71% 0.96% +0.25%
2 2025-11-21 0.81% 0.71% -0.10%
3 2025-11-18 — 0.81% —

Full EPSS history (19 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-36740

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
6.5 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 2.5 [email protected]
6.4 2.0 MEDIUM
AV:N/AC:L/Au:N/C:P/I:P/A:N Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:N)
No availability impact.
 10.0 4.9 [email protected]

Weakness enumeration for CVE-2021-36740

OS Trackers for CVE-2021-36740

vendor priority summary link
alpine medium CVE-2021-36740: 1 source package rows (varnish); 9 state rows across 9 repos (3.11-main, 3.12-main, 3.17-main, 3.18-main, 3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 9, open 0. https://security.alpinelinux.org/vuln/CVE-2021-36740
debian not yet assigned CVE-2021-36740 not yet assigned priority: Debian including 1 source packages (varnish), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-36740
redhat high — https://access.redhat.com/security/cve/CVE-2021-36740
suse high CVE-2021-36740 severity important: SUSE including 3 source package names (libvarnishapi3-7.1.0-bp153.2.3.1, varnish-7.1.0-bp153.2.3.1, varnish-devel-7.1.0-bp153.2.3.1), 6 product×package rows across 2 product lines (SUSE Package Hub 15 SP3, openSUSE Leap 15.3): Fixed 6. https://www.suse.com/security/cve/CVE-2021-36740/
ubuntu medium CVE-2021-36740 medium priority: Ubuntu including 1 source packages (varnish), 9 status rows across 9 suites (bionic, focal, groovy, hirsute, impish, jammy, trusty, upstream, xenial): not-affected 4, ignored 2, released 2, needs-triage 1. https://ubuntu.com/security/CVE-2021-36740

Affected software / configurations for CVE-2021-36740

Vendor Product Version Raw CPE
varnish-cache varnish_cache >= 6.0.0, < 6.0.8 cpe:2.3:a:varnish-cache:varnish_cache:*:*:*:*:plus:*:*:*
varnish-cache varnish_cache 6.0.8 cpe:2.3:a:varnish-cache:varnish_cache:6.0.8:r1:*:*:plus:*:*:*
varnish-cache varnish_cache 6.0.8 cpe:2.3:a:varnish-cache:varnish_cache:6.0.8:r2:*:*:plus:*:*:*
varnish-software varnish_cache >= 6.0.0, <= 6.0.5 cpe:2.3:a:varnish-software:varnish_cache:*:*:*:*:*:*:*:*
varnish-software varnish_cache >= 6.0.0, <= 6.0.7 cpe:2.3:a:varnish-software:varnish_cache:*:*:*:*:lts:*:*:*
varnish_cache_project varnish_cache >= 5.0.0, <= 5.2.1 cpe:2.3:a:varnish_cache_project:varnish_cache:*:*:*:*:*:*:*:*
varnish_cache_project varnish_cache >= 6.1.0, <= 6.6.0 cpe:2.3:a:varnish_cache_project:varnish_cache:*:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
debian debian_linux 11.0 cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:*

References for CVE-2021-36740

cvelogic Threat Intelligence