CVE-2026-9545: In this scenario, libcurl first uses a proper HTTP/3 server for the initial transfers, and when it makes a sec

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HIGH 7.5REMOTEUPGRADE

CVE-2026-9545 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.

In this scenario, libcurl first uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate. When libcurl returns to the hostname the second time with a cached SSL session (`CURLOPT_SSL_SESSIONID_CACHE` is not disabled) and early data enabled (the `CURLSSLOPT_EARLYDATA` bit is set in `CURLOPT_SSL_OPTIONS`), libcur

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	0.4% (top 65%)
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: >= 8.11.0 and < 8.21.0

Fixed in: 8.21.0

Safe to downgrade to: a release before 8.11.0

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 8.11.0 (Debian / Ubuntu)

apt-cache madison curl sudo apt install curl=<VERSION_BEFORE_8.11.0> 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_8.11.0> 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-2026-9545?

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

Is CVE-2026-9545 being exploited?

CVE-2026-9545 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-2026-9545 affect?

CVE-2026-9545 affects curl. Severity is high (CVSS 7.5). In this scenario, libcurl first uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate. When libcurl

How urgent is CVE-2026-9545?

Priority 45/100. 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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