CVE-2019-12526: An issue was discovered in Squid before 4.9

This week

CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2019-12526 is a critical-severity flaw (CVSS 9.8) affecting squid, ubuntu linux, fedora. 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 4.9. URN response handling in Squid suffers from a heap-based buffer overflow. When receiving data from a remote server in response to an URN request, Squid fails to ensure that the response can fit within the buffer. This leads to attacker controlled data overflowing in the heap.

Why this priority

Critical severity
EPSS top 3%: 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	20.3% (top 3%)
CISA exploited list	No
Weakness	CWE-787
Published	2019-11-26
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

2019-11-26 17:15 UTCPublished CVE record published
2026-06-17 02:14 UTCUpdated Record last modified

squid-cache squid UPGRADE

Affected: >= 3.0 and <= 3.5.28 · >= 4.0 and <= 4.8

Fixed in: newer than 4.8

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 newer than 4.8 (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.

canonical ubuntu linux MITIGATE

Affected: 16.04 · 18.04 · 19.04 · 19.10

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.

fedoraproject fedora MITIGATE

Affected: 30 · 31

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.

opensuse leap MITIGATE

Affected: 15.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 15.0

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.

debian debian linux MITIGATE

Affected: 8.0 · 9.0 · 10.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

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.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora ELN · dropbear · FIXED · 2026.94-2.eln159 · FEDORA-2026-1f4bf22735 · source

sudo dnf upgrade --advisory=FEDORA-2026-1f4bf22735

Fedora 46 · python-odata-query · FIXED · 0.10.1-1.fc46 · FEDORA-2026-eab44dc513 · source

sudo dnf upgrade --advisory=FEDORA-2026-eab44dc513

Ubuntu 20.04 · squid · FIXED · 4.9-2ubuntu1

sudo apt update && sudo apt install --only-upgrade squid dpkg --compare-versions "$(dpkg-query -W -f='${Version}' squid)" ge "4.9-2ubuntu1" && echo PATCHED || echo STILL-VULNERABLE

Debian 12 · squid · FIXED · 4.9-1

sudo apt update && sudo apt install --only-upgrade squid dpkg --compare-versions "$(dpkg-query -W -f='${Version}' squid)" ge "4.9-1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · squid · FIXED · 4.9-1

sudo apt update && sudo apt install --only-upgrade squid dpkg --compare-versions "$(dpkg-query -W -f='${Version}' squid)" ge "4.9-1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 5 · squid · NOT AFFECTED

RHEL 6 · squid · DEFERRED

RHEL 6 · squid34 · DEFERRED

RHEL 7 · squid · DEFERRED

Amazon Linux 2 · see advisory · FIXED · ALAS2-2023-2081 · source

sudo yum update --advisory=ALAS2-2023-2081

Amazon Linux 1 · see advisory · FIXED · ALAS-2023-1774 · source

sudo yum update --advisory=ALAS-2023-1774

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-2019-12526?

squid: upgrade to newer than 4.8; ubuntu linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2019-12526 being exploited?

CVE-2019-12526 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-2019-12526 affect?

CVE-2019-12526 affects squid, ubuntu linux, fedora. Severity is critical (CVSS 9.8). An issue was discovered in Squid before 4.9. URN response handling in Squid suffers from a heap-based buffer overflow. When receiving data from a remote server in response to an URN request, Squid fails to ensure that the response can fit w

How urgent is CVE-2019-12526?

Priority 69/100. Critical severity; EPSS top 3%: 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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CVE-2023-46847 HIGH 8.6 UPGRADE MITIGATE Squid is vulnerable to a Denial of Service, where a remote attacker can perform buffer overflow attack by wri
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References

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