CVE-2009-0478: Squid 2.7 to 2.7.STABLE5, 3.0 to 3.0.STABLE12, and 3.1 to 3.1.0.4 allows remote attackers to cause a denial of

Q: How do I fix CVE-2009-0478?

squid: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2009-0478 being exploited?

CVE-2009-0478 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2009-0478 affect?

CVE-2009-0478 affects squid. Severity is medium (CVSS 5.0). Squid 2.7 to 2.7.STABLE5, 3.0 to 3.0.STABLE12, and 3.1 to 3.1.0.4 allows remote attackers to cause a denial of service via an HTTP request with an invalid version number, which triggers a reachable assertion in (1) HttpMsg.c and (2) HttpSta

Q: How urgent is CVE-2009-0478?

Priority 47/100. EPSS top 1%: very likely to be exploited; Public exploit reference

This week

MEDIUM 5.0DOWNGRADE

CVE-2009-0478 is a medium-severity flaw (CVSS 5.0) affecting squid. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Squid 2.7 to 2.7.STABLE5, 3.0 to 3.0.STABLE12, and 3.1 to 3.1.0.4 allows remote attackers to cause a denial of service via an HTTP request with an invalid version number, which triggers a reachable assertion in (1) HttpMsg.c and (2) HttpStatusLine.c.

Why this priority

EPSS top 1%: very likely to be exploited
Public exploit reference

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.0 (Medium)
EPSS exploit chance	72.0% (top 1%)
CISA exploited list	No
Weakness	CWE-20
Published	2009-02-08
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: squid

Last updated 2026-06-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2009-02-08 22:30 UTCPublished CVE record published
2026-06-16 23:05 UTCUpdated Record last modified

squid squid DOWNGRADE

Affected: 2.7.stable1 · 2.7.stable2 · 2.7.stable3 · 2.7.stable4 · 2.7.stable5 · 3.0.stable1 · 3.0.stable2 · 3.0.stable3

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.7.stable1

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 2.7.stable1 (Debian / Ubuntu)

apt-cache madison squid sudo apt install squid=<VERSION_BEFORE_2.7.stable1> sudo apt-mark hold squid

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

dnf --showduplicates list squid sudo dnf downgrade squid-<VERSION_BEFORE_2.7.stable1> 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.

Public exploits and detection

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

Exploit-DB: Squid < 3.1 5 - HTTP Version Number Parsing Denial of Service

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-2009-0478?

squid: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2009-0478 being exploited?

CVE-2009-0478 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-2009-0478 affect?

CVE-2009-0478 affects squid. Severity is medium (CVSS 5.0). Squid 2.7 to 2.7.STABLE5, 3.0 to 3.0.STABLE12, and 3.1 to 3.1.0.4 allows remote attackers to cause a denial of service via an HTTP request with an invalid version number, which triggers a reachable assertion in (1) HttpMsg.c and (2) HttpSta

How urgent is CVE-2009-0478?

Priority 47/100. EPSS top 1%: very likely to be exploited; Public exploit reference

Software pages

Squid vulnerabilities

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References

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