CVE-2019-20637: An issue was discovered in Varnish Cache before 6.0.5 LTS, 6.1.x and 6.2.x before 6.2.2, and 6.3.x before 6.3.

This month

HIGH 7.5REMOTEUPGRADEUPGRADEDOWNGRADE

CVE-2019-20637 is a high-severity flaw (CVSS 7.5) affecting varnish cache, varnish cache, backports sle. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An issue was discovered in Varnish Cache before 6.0.5 LTS, 6.1.x and 6.2.x before 6.2.2, and 6.3.x before 6.3.1. It does not clear a pointer between the handling of one client request and the next request within the same connection. This sometimes causes information to be disclosed from the connection workspace, such as data structures associated with previous requests within this connection or VCL-related temporary headers.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.8% (top 22%)
CISA exploited list	No
Weakness	CWE-212
Published	2020-04-08
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: opensuse · 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

2020-04-08 23:15 UTCPublished CVE record published
2026-06-17 02:30 UTCUpdated Record last modified

varnish-cache varnish cache UPGRADE

Affected: >= 6.1.0 and < 6.2.2 · >= 6.3.0 and < 6.3.1

Fixed in: 6.2.2, 6.3.1

Safe to downgrade to: a release before 6.1.0

1. Check installed version

apt-cache policy varnish-cache || rpm -q varnish-cache || varnish-cache --version

2. UPGRADE to 6.2.2 (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 6.1.0 (Debian / Ubuntu)

apt-cache madison varnish-cache sudo apt install varnish-cache=<VERSION_BEFORE_6.1.0> 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_6.1.0> 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

Fixed in: 6.0.5

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 6.0.5 (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.

opensuse backports sle DOWNGRADE

Affected: 15.0 sp1

Fixed in: no fixed release known

Safe to downgrade to: a release before 15.0

1. Check installed version

apt-cache policy backports-sle || rpm -q backports-sle || backports-sle --version

2. No fixed release: DOWNGRADE to a release before 15.0 (Debian / Ubuntu)

apt-cache madison backports-sle sudo apt install backports-sle=<VERSION_BEFORE_15.0> sudo apt-mark hold backports-sle

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list backports-sle sudo dnf downgrade backports-sle-<VERSION_BEFORE_15.0> sudo dnf versionlock add backports-sle

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.

opensuse leap MITIGATE

Affected: 15.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 15.1

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.

Quick answers

How do I fix CVE-2019-20637?

varnish cache: upgrade to 6.2.2, 6.3.1; varnish cache: upgrade to 6.0.5; backports sle: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2019-20637 being exploited?

CVE-2019-20637 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-2019-20637 affect?

CVE-2019-20637 affects varnish cache, varnish cache, backports sle. Severity is high (CVSS 7.5). An issue was discovered in Varnish Cache before 6.0.5 LTS, 6.1.x and 6.2.x before 6.2.2, and 6.3.x before 6.3.1. It does not clear a pointer between the handling of one client request and the next request within the same connection. This so

How urgent is CVE-2019-20637?

Priority 51/100. High severity; Remotely exploitable, no login, no user action

Software pages

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References

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