CVE-2021-36440: Unrestricted File Upload in ShowDoc v2.9.5 allows remote attackers to execute arbitrary code via the 'file_url

This week

CRITICAL 9.8REMOTEDOWNGRADE

CVE-2021-36440 is a critical-severity flaw (CVSS 9.8) affecting showdoc. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Unrestricted File Upload in ShowDoc v2.9.5 allows remote attackers to execute arbitrary code via the 'file_url' parameter in the component AdminUpdateController.class.php'.

Why this priority

Critical severity
EPSS top 8%: 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	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	4.8% (top 8%)
CISA exploited list	No
Weakness	CWE-434
Published	2021-09-08
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: showdoc

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

Am I affected?

Timeline

2021-09-08 21:15 UTCPublished CVE record published
2026-06-17 03:58 UTCUpdated Record last modified

showdoc showdoc DOWNGRADE

Affected: 2.9.5

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.9.5

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 2.9.5 (Debian / Ubuntu)

apt-cache madison showdoc sudo apt install showdoc=<VERSION_BEFORE_2.9.5> sudo apt-mark hold showdoc

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

dnf --showduplicates list showdoc sudo dnf downgrade showdoc-<VERSION_BEFORE_2.9.5> sudo dnf versionlock add showdoc

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.

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-2021-36440?

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

Is CVE-2021-36440 being exploited?

CVE-2021-36440 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-2021-36440 affect?

CVE-2021-36440 affects showdoc. Severity is critical (CVSS 9.8). Unrestricted File Upload in ShowDoc v2.9.5 allows remote attackers to execute arbitrary code via the 'file_url' parameter in the component AdminUpdateController.class.php'.

How urgent is CVE-2021-36440?

Priority 67/100. Critical severity; EPSS top 8%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

PHP vulnerabilities

Related CVEs

CVE-2022-1034 HIGH 7.2 UPGRADE There is a Unrestricted Upload of File vulnerability in ShowDoc v2.10.3 in GitHub repository star7th/showdoc p
CVE-2022-0967 MEDIUM 5.4 UPGRADE Stored XSS via File Upload in star7th/showdoc in star7th/showdoc in GitHub repository star7th/showdoc prior to
CVE-2022-0966 MEDIUM 5.4 UPGRADE Stored XSS via File Upload in star7th/showdoc in GitHub repository star7th/showdoc prior to 2.4.10.
CVE-2022-0965 MEDIUM 5.4 UPGRADE Stored XSS viva .ofd file upload in GitHub repository star7th/showdoc prior to 2.10.4.
CVE-2022-0964 MEDIUM 5.4 UPGRADE Stored XSS viva .webmv file upload in GitHub repository star7th/showdoc prior to 2.10.4.

References

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