CVE-2026-11856: Successfully using libcurl to do a transfer to a specific HTTP origin (`hostA`) with Digest authentication

This week

CRITICAL 9.8REMOTEUPGRADE

CVE-2026-11856 is a critical-severity flaw (CVSS 9.8) affecting curl. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Successfully using libcurl to do a transfer to a specific HTTP origin (`hostA`) with **Digest** authentication and then changing the origin to a different one (`hostB`) for a second transfer, reusing the same handle, makes libcurl wrongly pass on the `Authorization:` header field meant for `hostA`, to `hostB`.

Why this priority

Critical severity
Public exploit reference
CISA SSVC: automatable with total technical impact
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.7% (top 48%)
CISA exploited list	No
Weakness	CWE-294
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: >= 7.10.6 and < 8.21.0

Fixed in: 8.21.0

Safe to downgrade to: a release before 7.10.6

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 7.10.6 (Debian / Ubuntu)

apt-cache madison curl sudo apt install curl=<VERSION_BEFORE_7.10.6> 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_7.10.6> 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.

Exact fixes by distribution

Fedora 46 · rubygem-mechanize · FIXED · 2.14.2-1.fc46 · FEDORA-2026-d2e3b5fc5d · source

sudo dnf upgrade --advisory=FEDORA-2026-d2e3b5fc5d

Fedora ELN · opkssh · FIXED · 0.19.1-1.eln159 · FEDORA-2026-45a9706969 · source

sudo dnf upgrade --advisory=FEDORA-2026-45a9706969

Ubuntu 20.04 · curl · FIXED · 7.68.0-1ubuntu2.25+esm6

sudo apt update && sudo apt install --only-upgrade curl dpkg --compare-versions "$(dpkg-query -W -f='${Version}' curl)" ge "7.68.0-1ubuntu2.25+esm6" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 22.04 · curl · FIXED · 7.81.0-1ubuntu1.26

sudo apt update && sudo apt install --only-upgrade curl dpkg --compare-versions "$(dpkg-query -W -f='${Version}' curl)" ge "7.81.0-1ubuntu1.26" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 24.04 · curl · FIXED · 8.5.0-2ubuntu10.12

sudo apt update && sudo apt install --only-upgrade curl dpkg --compare-versions "$(dpkg-query -W -f='${Version}' curl)" ge "8.5.0-2ubuntu10.12" && echo PATCHED || echo STILL-VULNERABLE

Debian 12 · curl · NO FIX YET

Debian 13 · curl · NO FIX YET

RHEL 10 · curl · FIXED · curl-8.12.1-4.el10_2.6

sudo dnf upgrade --advisory=RHSA-2026:69125 rpm -q curl

RHEL 10.0 Extended Update Support · curl · FIXED · curl-8.12.1-1.el10_0.10

sudo dnf upgrade --advisory=RHSA-2026:72506 rpm -q curl

RHEL 9 · curl · FIXED · curl-7.76.1-40.el9_8.8

sudo dnf upgrade --advisory=RHSA-2026:80511 rpm -q curl

RHEL 6 · curl · DEFERRED

RHEL 7 · curl · DEFERRED

Alpine 3.23 · curl · FIXED · curl-8.22.0-r0

apk add --upgrade curl apk info -v curl

Alpine 3.24 · curl · FIXED · curl-8.21.0-r0

apk add --upgrade curl apk info -v curl

Amazon Linux 2023 · see advisory · FIXED · ALAS2023-2026-2132 · source

sudo dnf update --advisory=ALAS2023-2026-2132

Advisories and coverage

Official advisories

CISA Alerts and Advisories: Hitachi Energy RTU500 Product 2026-10-20

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-2026-11856?

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

Is CVE-2026-11856 being exploited?

CVE-2026-11856 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-2026-11856 affect?

CVE-2026-11856 affects curl. Severity is critical (CVSS 9.8). Successfully using libcurl to do a transfer to a specific HTTP origin (`hostA`) with **Digest** authentication and then changing the origin to a different one (`hostB`) for a second transfer, reusing the same handle, makes libcurl wrongly p

How urgent is CVE-2026-11856?

Priority 60/100. Critical severity; Public exploit reference; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action

Software pages

curl vulnerabilities

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References

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CVE-2026-11856: Successfully using libcurl to do a transfer to a specific HTTP origin (`hostA`) with **Digest** authentication