CVE-2025-12642: lighttpd1.4.80 incorrectly merged trailer fields into headers after http request parsing

This month

CRITICAL 9.1REMOTEDOWNGRADE

CVE-2025-12642 is a critical-severity flaw (CVSS 9.1) affecting lighttpd. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

lighttpd1.4.80 incorrectly merged trailer fields into headers after http request parsing. This behavior can be exploited to conduct HTTP Header Smuggling attacks. Successful exploitation may allow an attacker to: * Bypass access control rules * Inject unsafe input into backend logic that trusts request headers * Execute HTTP Request Smuggling attacks under some conditions This issue affects lighttpd1.4.80

Why this priority

Critical severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 69%)
CISA exploited list	No
Weakness	CWE-444
Published	2025-11-03
Last modified	2026-10-19

Publisher (CNA): Toreon · Vendor: lighttpd

Last updated 2026-10-19 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2025-11-03 20:17 UTCPublished CVE record published
2026-10-19 13:10 UTCUpdated Record last modified

lighttpd lighttpd DOWNGRADE

Affected: 1.4.80

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.4.80

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 1.4.80 (Debian / Ubuntu)

apt-cache madison lighttpd sudo apt install lighttpd=<VERSION_BEFORE_1.4.80> sudo apt-mark hold lighttpd

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

dnf --showduplicates list lighttpd sudo dnf downgrade lighttpd-<VERSION_BEFORE_1.4.80> sudo dnf versionlock add lighttpd

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-2025-12642?

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

Is CVE-2025-12642 being exploited?

CVE-2025-12642 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-2025-12642 affect?

CVE-2025-12642 affects lighttpd. Severity is critical (CVSS 9.1). lighttpd1.4.80 incorrectly merged trailer fields into headers after http request parsing. This behavior can be exploited to conduct HTTP Header Smuggling attacks. Successful exploitation may allow an attacker to: * Bypass access contro

How urgent is CVE-2025-12642?

Priority 44/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

lighttpd vulnerabilities

References

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