CVE-2013-6890: denyhosts 2.6 uses an incorrect regular expression when analyzing authentication logs, which allows remote att

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MEDIUM 5.0WATCHLISTMITIGATEMITIGATEDOWNGRADE

CVE-2013-6890 is a medium-severity flaw (CVSS 5.0) affecting debian linux, fedora, denyhosts. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

denyhosts 2.6 uses an incorrect regular expression when analyzing authentication logs, which allows remote attackers to cause a denial of service (incorrect block of IP addresses) via crafted login names.

Why this priority

EPSS top 5%: very likely to be exploited
Public exploit reference
On your watchlist

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

Risk facts

CVSS score	5.0 (Medium)
EPSS exploit chance	8.9% (top 5%)
CISA exploited list	No
Weakness	CWE-287
Published	2013-12-23
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · fedoraproject · phil schwartz

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

Am I affected?

Timeline

2013-12-23 22:55 UTCPublished CVE record published
2026-06-17 00:01 UTCUpdated Record last modified

debian debian linux MITIGATE

Affected: 6.0 · 7.0 · 7.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 6.0

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.

fedoraproject fedora MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

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.

phil schwartz denyhosts DOWNGRADE

Affected: 2.6

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.6

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 2.6 (Debian / Ubuntu)

apt-cache madison denyhosts sudo apt install denyhosts=<VERSION_BEFORE_2.6> sudo apt-mark hold denyhosts

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

dnf --showduplicates list denyhosts sudo dnf downgrade denyhosts-<VERSION_BEFORE_2.6> sudo dnf versionlock add denyhosts

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: DenyHosts - 'regex.py' Remote 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-2013-6890?

debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation; denyhosts: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2013-6890 being exploited?

CVE-2013-6890 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-2013-6890 affect?

CVE-2013-6890 affects debian linux, fedora, denyhosts. Severity is medium (CVSS 5.0). denyhosts 2.6 uses an incorrect regular expression when analyzing authentication logs, which allows remote attackers to cause a denial of service (incorrect block of IP addresses) via crafted login names.

How urgent is CVE-2013-6890?

Priority 51/100. EPSS top 5%: very likely to be exploited; Public exploit reference; On your watchlist

Software pages

Debian vulnerabilities

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References

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