CVE-2020-11945: An issue was discovered in Squid before 5.0.2

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CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2020-11945 is a critical-severity flaw (CVSS 9.8) affecting squid, debian linux, leap. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An issue was discovered in Squid before 5.0.2. A remote attacker can replay a sniffed Digest Authentication nonce to gain access to resources that are otherwise forbidden. This occurs because the attacker can overflow the nonce reference counter (a short integer). Remote code execution may occur if the pooled token credentials are freed (instead of replayed as valid credentials).

Why this priority

Critical severity
EPSS top 2%: very likely to be exploited
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	27.2% (top 2%)
CISA exploited list	No
Weakness	CWE-190
Published	2020-04-23
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: canonical · debian · fedoraproject · opensuse · squid-cache

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2020-04-23 15:15 UTCPublished CVE record published
2026-06-17 02:51 UTCUpdated Record last modified

squid-cache squid UPGRADE

Affected: >= 3.0 and <= 3.5.28 · >= 4.0 and < 4.11 · >= 5.0 and < 5.0.2

Fixed in: 4.11, 5.0.2

Safe to downgrade to: a release before 3.0

1. Check installed version

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

2. UPGRADE to 4.11 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade squid

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade squid

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 3.0 (Debian / Ubuntu)

apt-cache madison squid sudo apt install squid=<VERSION_BEFORE_3.0> sudo apt-mark hold squid

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list squid sudo dnf downgrade squid-<VERSION_BEFORE_3.0> sudo dnf versionlock add squid

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.

debian debian linux MITIGATE

Affected: 9.0 · 10.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

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.

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.

fedoraproject fedora MITIGATE

Affected: 30 · 31 · 32

Fixed in: no fixed release known

Safe to downgrade to: a release before 30

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.

canonical ubuntu linux MITIGATE

Affected: 16.04 · 18.04 · 19.10 · 20.04

Fixed in: no fixed release known

Safe to downgrade to: a release before 16.04

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.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · lis · FIXED · 2.1.21-2.fc46 · FEDORA-2026-039245acc6 · source

sudo dnf upgrade --advisory=FEDORA-2026-039245acc6

Fedora ELN · bitcoin-core · FIXED · 32.1-2.eln159 · FEDORA-2026-41e13beebe · source

sudo dnf upgrade --advisory=FEDORA-2026-41e13beebe

Debian 12 · squid · FIXED · 4.11-1

sudo apt update && sudo apt install --only-upgrade squid dpkg --compare-versions "$(dpkg-query -W -f='${Version}' squid)" ge "4.11-1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · squid · FIXED · 4.11-1

sudo apt update && sudo apt install --only-upgrade squid dpkg --compare-versions "$(dpkg-query -W -f='${Version}' squid)" ge "4.11-1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 7 · squid · FIXED · squid-3.5.20-15.el7_8.1

sudo dnf upgrade --advisory=RHSA-2020:2040 rpm -q squid

RHEL 5 · squid · DEFERRED

RHEL 6 · squid · DEFERRED

RHEL 6 · squid34 · DEFERRED

Amazon Linux 2 · see advisory · FIXED · ALAS2-2020-1448 · source

sudo yum update --advisory=ALAS2-2020-1448

Amazon Linux 1 · see advisory · FIXED · ALAS-2020-1386 · source

sudo yum update --advisory=ALAS-2020-1386

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Write-up: github.com/squid-cache/squid/pull/585

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-2020-11945?

squid: upgrade to 4.11, 5.0.2; debian linux: mitigate or apply the listed mitigation; leap: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2020-11945 being exploited?

CVE-2020-11945 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-2020-11945 affect?

CVE-2020-11945 affects squid, debian linux, leap. Severity is critical (CVSS 9.8). An issue was discovered in Squid before 5.0.2. A remote attacker can replay a sniffed Digest Authentication nonce to gain access to resources that are otherwise forbidden. This occurs because the attacker can overflow the nonce reference co

How urgent is CVE-2020-11945?

Priority 69/100. Critical severity; EPSS top 2%: very likely to be exploited; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Ubuntu vulnerabilities · Debian vulnerabilities · Squid vulnerabilities

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References

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