CVE-2026-26747: A Host Header Poisoning vulnerability exists in Monica 4.1.2 due to improper handling of the HTTP Host header

This week

CRITICAL 9.1REMOTEDOWNGRADE

CVE-2026-26747 is a critical-severity flaw (CVSS 9.1) affecting monica. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A Host Header Poisoning vulnerability exists in Monica 4.1.2 due to improper handling of the HTTP Host header in app/Providers/AppServiceProvider.php, combined with the default misconfiguration where the "app.force_url" is not set and default is "false". The application generates absolute URLs (such as those used in password reset emails) using the user-supplied Host header. This allows remote attackers to poison the password reset link sent to a

Why this priority

Critical severity
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	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 62%)
CISA exploited list	No
Weakness	CWE-644
Published	2026-02-20
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: monicahq

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

Am I affected?

Timeline

2026-02-20 17:25 UTCPublished CVE record published
2026-06-17 10:26 UTCUpdated Record last modified

monicahq monica DOWNGRADE

Affected: 4.1.2

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.1.2

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 4.1.2 (Debian / Ubuntu)

apt-cache madison monica sudo apt install monica=<VERSION_BEFORE_4.1.2> sudo apt-mark hold monica

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

dnf --showduplicates list monica sudo dnf downgrade monica-<VERSION_BEFORE_4.1.2> sudo dnf versionlock add monica

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-2026-26747?

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

Is CVE-2026-26747 being exploited?

CVE-2026-26747 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-2026-26747 affect?

CVE-2026-26747 affects monica. Severity is critical (CVSS 9.1). A Host Header Poisoning vulnerability exists in Monica 4.1.2 due to improper handling of the HTTP Host header in app/Providers/AppServiceProvider.php, combined with the default misconfiguration where the "app.force_url" is not set and defau

How urgent is CVE-2026-26747?

Priority 51/100. Critical severity; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

PHP vulnerabilities

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References

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