CVE-2026-9546: A vulnerability in libcurl caused the HTTP `Referer:` header to persist even when explicitly cleared

This week

HIGH 7.5REMOTEUPGRADE

CVE-2026-9546 is a high-severity flaw (CVSS 7.5) affecting curl. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A vulnerability in libcurl caused the HTTP `Referer:` header to persist even when explicitly cleared. While the documentation states that passing NULL to `CURLOPT_REFERER` suppresses the header, the option failed to clear the internal state. As a result, the previous referrer string was erroneously reused and sent in subsequent requests, potentially leaking sensitive information to unintended servers.

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.7% (top 49%)
CISA exploited list	No
Weakness	CWE-200
Published	2026-07-03
Last modified	2026-09-15

Publisher (CNA): curl · Vendor: haxx

Last updated 2026-09-15 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-03 07:16 UTCPublished CVE record published
2026-09-15 07:16 UTCUpdated Record last modified

haxx curl UPGRADE

Affected: >= 8.18.0 and < 8.21.0

Fixed in: 8.21.0

Safe to downgrade to: a release before 8.18.0

1. Check installed version

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

2. UPGRADE to 8.21.0 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade curl

Note

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

3. If no update is available yet, DOWNGRADE to a release before 8.18.0 (Debian / Ubuntu)

apt-cache madison curl sudo apt install curl=<VERSION_BEFORE_8.18.0> sudo apt-mark hold curl

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list curl sudo dnf downgrade curl-<VERSION_BEFORE_8.18.0> sudo dnf versionlock add curl

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-2026-9546?

curl: upgrade to 8.21.0. Exact commands for each OS are on this page.

Is CVE-2026-9546 being exploited?

CVE-2026-9546 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-2026-9546 affect?

CVE-2026-9546 affects curl. Severity is high (CVSS 7.5). A vulnerability in libcurl caused the HTTP `Referer:` header to persist even when explicitly cleared. While the documentation states that passing NULL to `CURLOPT_REFERER` suppresses the header, the option failed to clear the internal state

How urgent is CVE-2026-9546?

Priority 49/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

curl vulnerabilities

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References

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