CVE-2009-4491: thttpd 2.25b0 writes data to a log file without sanitizing non-printable characters, which might allow remote

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CRITICAL 9.8REMOTEDOWNGRADE

CVE-2009-4491 is a critical-severity flaw (CVSS 9.8) affecting thttpd. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

thttpd 2.25b0 writes data to a log file without sanitizing non-printable characters, which might allow remote attackers to modify a window's title, or possibly execute arbitrary commands or overwrite files, via an HTTP request containing an escape sequence for a terminal emulator.

Why this priority

Critical 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	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	13.7% (top 3%)
CISA exploited list	No
Weakness	CWE-20, CWE-94
Published	2010-01-13
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: acme

Last updated 2026-06-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2010-01-13 20:30 UTCPublished CVE record published
2026-06-16 23:13 UTCUpdated Record last modified

acme thttpd DOWNGRADE

Affected: 2.25 b

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.25

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 2.25 (Debian / Ubuntu)

apt-cache madison thttpd sudo apt install thttpd=<VERSION_BEFORE_2.25> sudo apt-mark hold thttpd

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

dnf --showduplicates list thttpd sudo dnf downgrade thttpd-<VERSION_BEFORE_2.25> sudo dnf versionlock add thttpd

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.

Exploit-DB: thttpd 2.24 - HTTP Request Escape Sequence Terminal Command Injection

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-2009-4491?

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

Is CVE-2009-4491 being exploited?

CVE-2009-4491 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-2009-4491 affect?

CVE-2009-4491 affects thttpd. Severity is critical (CVSS 9.8). thttpd 2.25b0 writes data to a log file without sanitizing non-printable characters, which might allow remote attackers to modify a window's title, or possibly execute arbitrary commands or overwrite files, via an HTTP request containing an

How urgent is CVE-2009-4491?

Priority 68/100. Critical severity; EPSS top 3%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2000-0900 HIGH 7.5 DOWNGRADE Directory traversal vulnerability in ssi CGI program in thttpd 2.19 and earlier allows remote attackers to rea
CVE-2017-17663 CRITICAL 9.8 UPGRADE UPGRADE The htpasswd implementation of mini_httpd before v1.28 and of thttpd before v2.28 is affected by a buffer over
CVE-2013-0348 LOW 2.1 UPGRADE MITIGATE thttpd.c in sthttpd before 2.26.4-r2 and thttpd 2.25b use world-readable permissions for /var/log/thttpd.log,
CVE-2012-5640 MEDIUM 5.5 MITIGATE thttpd has a local DoS vulnerability via specially-crafted .htpasswd files
CVE-2007-0664 MEDIUM 5.0 UPGRADE thttpd before 2.25b-r6 in Gentoo Linux is started from the system root directory (/) by the Gentoo baselayout

References

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