CVE-2021-36386: report_vbuild in report.c in Fetchmail before 6.4.20 sometimes omits initialization of the vsnprintf va_list a

Q: How do I fix CVE-2021-36386?

fetchmail: upgrade to 6.4.20; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-36386 being exploited?

CVE-2021-36386 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-2021-36386 affect?

CVE-2021-36386 affects fetchmail, fedora. Severity is high (CVSS 7.5). report_vbuild in report.c in Fetchmail before 6.4.20 sometimes omits initialization of the vsnprintf va_list argument, which might allow mail servers to cause a denial of service or possibly have unspecified other impact via long error mess

Q: How urgent is CVE-2021-36386?

Priority 53/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 7.5REMOTEUPGRADEMITIGATE

CVE-2021-36386 is a high-severity flaw (CVSS 7.5) affecting fetchmail, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

report_vbuild in report.c in Fetchmail before 6.4.20 sometimes omits initialization of the vsnprintf va_list argument, which might allow mail servers to cause a denial of service or possibly have unspecified other impact via long error messages. NOTE: it is unclear whether use of Fetchmail on any realistic platform results in an impact beyond an inconvenience to the client user.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	2.6% (top 15%)
CISA exploited list	No
Weakness	CWE-909
Published	2021-07-30
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · fetchmail

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

Am I affected?

Timeline

2021-07-30 14:15 UTCPublished CVE record published
2026-06-17 03:58 UTCUpdated Record last modified

fetchmail fetchmail UPGRADE

Affected: < 6.4.20

Fixed in: 6.4.20

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 6.4.20 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade fetchmail

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade fetchmail

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

fedoraproject fedora MITIGATE

Affected: 33 · 34

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

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.

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-2021-36386?

fetchmail: upgrade to 6.4.20; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-36386 being exploited?

CVE-2021-36386 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-2021-36386 affect?

CVE-2021-36386 affects fetchmail, fedora. Severity is high (CVSS 7.5). report_vbuild in report.c in Fetchmail before 6.4.20 sometimes omits initialization of the vsnprintf va_list argument, which might allow mail servers to cause a denial of service or possibly have unspecified other impact via long error mess

How urgent is CVE-2021-36386?

Priority 53/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2021-39272 MEDIUM 5.9 UPGRADE MITIGATE Fetchmail before 6.4.22 fails to enforce STARTTLS session encryption in some circumstances, such as a certain

References

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