CVE-2021-33558: Boa 0.94.13 allows remote attackers to obtain sensitive information via a misconfiguration involving backup.ht

This week

HIGH 7.5REMOTEDOWNGRADE

CVE-2021-33558 is a high-severity flaw (CVSS 7.5) affecting boa. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Boa 0.94.13 allows remote attackers to obtain sensitive information via a misconfiguration involving backup.html, preview.html, js/log.js, log.html, email.html, online-users.html, and config.js. NOTE: multiple third parties report that this is a site-specific issue because those files are not part of Boa.

Why this priority

High severity
EPSS top 4%: 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	12.3% (top 4%)
CISA exploited list	No
Published	2021-05-27
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: boa

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

Am I affected?

Timeline

2021-05-27 11:15 UTCPublished CVE record published
2026-06-17 03:54 UTCUpdated Record last modified

boa boa DOWNGRADE

Affected: 0.94.13

Fixed in: no fixed release known

Safe to downgrade to: a release before 0.94.13

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 0.94.13 (Debian / Ubuntu)

apt-cache madison boa sudo apt install boa=<VERSION_BEFORE_0.94.13> sudo apt-mark hold boa

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

dnf --showduplicates list boa sudo dnf downgrade boa-<VERSION_BEFORE_0.94.13> sudo dnf versionlock add boa

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: Boa 0.94.13 - Information Disclosure

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-2021-33558?

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

Is CVE-2021-33558 being exploited?

CVE-2021-33558 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-2021-33558 affect?

CVE-2021-33558 affects boa. Severity is high (CVSS 7.5). Boa 0.94.13 allows remote attackers to obtain sensitive information via a misconfiguration involving backup.html, preview.html, js/log.js, log.html, email.html, online-users.html, and config.js. NOTE: multiple third parties report that this

How urgent is CVE-2021-33558?

Priority 60/100. High severity; EPSS top 4%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2022-45956 MEDIUM 5.3 DOWNGRADE Boa Web Server versions 0.94.13 through 0.94.14 fail to validate the correct security constraint on the HEAD H
CVE-2022-44117 CRITICAL 9.8 DOWNGRADE Boa 0.94.14rc21 is vulnerable to SQL Injection via username

References

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