PatchTheCVECVE-2021-36740: Varnish Cache, with HTTP/2 enabled, allows request smuggling and VCL authorization bypass via a large Content-
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MEDIUM 6.5REMOTEWATCHLISTUPGRADEUPGRADEUPGRADE
CVE-2021-36740 is a medium-severity flaw (CVSS 6.5) affecting varnish cache, varnish cache, varnish cache. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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.
Why this priority
- Remotely exploitable, no login, no user action
- On your watchlist
A fixed release or distro patch exists.
Risk facts
| CVSS score | 6.5 (Medium) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 1.6% (top 24%) |
| CISA exploited list | No |
| Weakness | CWE-444 |
| Published | 2021-07-14 |
| Last modified | 2026-06-17 |
Publisher (CNA): MITRE · Vendor: debian · fedoraproject · varnish cache project · varnish-cache · varnish-software
Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
varnish-cache varnish cache UPGRADE
Affected: >= 6.0.0 and < 6.0.8 · 6.0.8 r1 · 6.0.8 r2
Fixed in: 6.0.8
Safe to downgrade to: a release before 6.0.0
1. Check installed version
apt-cache policy varnish-cache || rpm -q varnish-cache || varnish-cache --version
2. UPGRADE to 6.0.8 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade varnish-cache
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade varnish-cache
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 6.0.0 (Debian / Ubuntu)
apt-cache madison varnish-cache
sudo apt install varnish-cache=<VERSION_BEFORE_6.0.0>
sudo apt-mark hold varnish-cache
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list varnish-cache
sudo dnf downgrade varnish-cache-<VERSION_BEFORE_6.0.0>
sudo dnf versionlock add varnish-cache
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
varnish-software varnish cache UPGRADE
Affected: >= 6.0.0 and <= 6.0.5 · >= 6.0.0 and <= 6.0.7
Fixed in: newer than 6.0.7
Safe to downgrade to: a release before 6.0.0
1. Check installed version
apt-cache policy varnish-cache || rpm -q varnish-cache || varnish-cache --version
2. UPGRADE to newer than 6.0.7 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade varnish-cache
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade varnish-cache
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 6.0.0 (Debian / Ubuntu)
apt-cache madison varnish-cache
sudo apt install varnish-cache=<VERSION_BEFORE_6.0.0>
sudo apt-mark hold varnish-cache
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list varnish-cache
sudo dnf downgrade varnish-cache-<VERSION_BEFORE_6.0.0>
sudo dnf versionlock add varnish-cache
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
varnish cache project varnish cache UPGRADE
Affected: >= 5.0.0 and <= 5.2.1 · >= 6.1.0 and <= 6.6.0
Fixed in: newer than 6.6.0
Safe to downgrade to: a release before 5.0.0
1. Check installed version
apt-cache policy varnish-cache || rpm -q varnish-cache || varnish-cache --version
2. UPGRADE to newer than 6.6.0 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade varnish-cache
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade varnish-cache
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 5.0.0 (Debian / Ubuntu)
apt-cache madison varnish-cache
sudo apt install varnish-cache=<VERSION_BEFORE_5.0.0>
sudo apt-mark hold varnish-cache
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list varnish-cache
sudo dnf downgrade varnish-cache-<VERSION_BEFORE_5.0.0>
sudo dnf versionlock add varnish-cache
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
fedoraproject fedora MITIGATE
Affected: 33 · 34
Fixed in: no fixed release known
Safe to downgrade to: a release before 33
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
debian debian linux MITIGATE
Affected: 10.0 · 11.0
Fixed in: no fixed release known
Safe to downgrade to: a release before 10.0
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2021-36740?
varnish cache: upgrade to 6.0.8; varnish cache: upgrade to newer than 6.0.7; varnish cache: upgrade to newer than 6.6.0. Exact commands for each OS are on this page.
Is CVE-2021-36740 being exploited?
CVE-2021-36740 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2021-36740 affect?
CVE-2021-36740 affects varnish cache, varnish cache, varnish cache. Severity is medium (CVSS 6.5). 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
How urgent is CVE-2021-36740?
Priority 52/100. Remotely exploitable, no login, no user action; On your watchlist
Software pages
Debian vulnerabilities · Varnish vulnerabilities
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