CVE-2021-38173: Btrbk before 0.31.2 allows command execution because of the mishandling of remote hosts filtering SSH commands

Q: How do I fix CVE-2021-38173?

btrbk: upgrade to 0.31.2; debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-38173 being exploited?

CVE-2021-38173 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-2021-38173 affect?

CVE-2021-38173 affects btrbk, debian linux, fedora. Severity is critical (CVSS 9.8). Btrbk before 0.31.2 allows command execution because of the mishandling of remote hosts filtering SSH commands using ssh_filter_btrbk.sh in authorized_keys.

Q: How urgent is CVE-2021-38173?

Priority 66/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

This week

CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-38173 is a critical-severity flaw (CVSS 9.8) affecting btrbk, debian linux, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Btrbk before 0.31.2 allows command execution because of the mishandling of remote hosts filtering SSH commands using ssh_filter_btrbk.sh in authorized_keys.

Why this priority

Critical severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	3.2% (top 12%)
CISA exploited list	No
Weakness	CWE-77
Published	2021-08-07
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · digint · fedoraproject

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

Am I affected?

Timeline

2021-08-07 19:15 UTCPublished CVE record published
2026-06-17 04:01 UTCUpdated Record last modified

digint btrbk UPGRADE

Affected: < 0.31.2

Fixed in: 0.31.2

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 0.31.2 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade btrbk

Note

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

debian debian linux MITIGATE

Affected: 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

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: 34 · 35

Fixed in: no fixed release known

Safe to downgrade to: a release before 34

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-2021-38173?

btrbk: upgrade to 0.31.2; debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-38173 being exploited?

CVE-2021-38173 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-2021-38173 affect?

CVE-2021-38173 affects btrbk, debian linux, fedora. Severity is critical (CVSS 9.8). Btrbk before 0.31.2 allows command execution because of the mishandling of remote hosts filtering SSH commands using ssh_filter_btrbk.sh in authorized_keys.

How urgent is CVE-2021-38173?

Priority 66/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

References

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