CVE-2021-40978: The mkdocs 1.2.2 built-in dev-server allows directory traversal using the port 8000, enabling remote exploitat

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HIGH 7.5REMOTEDOWNGRADE

CVE-2021-40978 is a high-severity flaw (CVSS 7.5) affecting mkdocs. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The mkdocs 1.2.2 built-in dev-server allows directory traversal using the port 8000, enabling remote exploitation to obtain :sensitive information. NOTE: the vendor has disputed this as described in https://github.com/mkdocs/mkdocs/issues/2601.] and https://github.com/nisdn/CVE-2021-40978/issues/1

Why this priority

High severity
EPSS top 3%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action

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

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	15.1% (top 3%)
CISA exploited list	No
Weakness	CWE-22
Published	2021-10-07
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: mkdocs

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

Am I affected?

Timeline

2021-10-07 14:15 UTCPublished CVE record published
2026-06-17 04:07 UTCUpdated Record last modified

mkdocs mkdocs DOWNGRADE

Affected: 1.2.2

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.2.2

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 1.2.2 (Debian / Ubuntu)

apt-cache madison mkdocs sudo apt install mkdocs=<VERSION_BEFORE_1.2.2> sudo apt-mark hold mkdocs

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

dnf --showduplicates list mkdocs sudo dnf downgrade mkdocs-<VERSION_BEFORE_1.2.2> sudo dnf versionlock add mkdocs

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.

Nuclei scan template: MKdocs 1.2.2 - Directory Traversal

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-2021-40978?

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

Is CVE-2021-40978 being exploited?

CVE-2021-40978 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-2021-40978 affect?

CVE-2021-40978 affects mkdocs. Severity is high (CVSS 7.5). The mkdocs 1.2.2 built-in dev-server allows directory traversal using the port 8000, enabling remote exploitation to obtain :sensitive information. NOTE: the vendor has disputed this as described in https://github.com/mkdocs/mkdocs/issues/2

How urgent is CVE-2021-40978?

Priority 60/100. High severity; EPSS top 3%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

GitHub vulnerabilities

References

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