CVE-2026-33131: H3 is a minimal H(TTP) framework

Q: How do I fix CVE-2026-33131?

h3: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2026-33131 being exploited?

CVE-2026-33131 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-33131 affect?

CVE-2026-33131 affects h3. Severity is high (CVSS 7.4). H3 is a minimal H(TTP) framework. Versions 2.0.0-0 through 2.0.1-rc.14 contain a Host header spoofing vulnerability in the NodeRequestUrl (which extends FastURL) which allows middleware bypass. When event.url, event.url.hostname, or event.u

Q: How urgent is CVE-2026-33131?

Priority 44/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

This week

HIGH 7.4REMOTEDOWNGRADE

CVE-2026-33131 is a high-severity flaw (CVSS 7.4) affecting h3. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

H3 is a minimal H(TTP) framework. Versions 2.0.0-0 through 2.0.1-rc.14 contain a Host header spoofing vulnerability in the NodeRequestUrl (which extends FastURL) which allows middleware bypass. When event.url, event.url.hostname, or event.url._url is accessed, such as in a logging middleware, the _url getter constructs a URL from untrusted data, including the user-controlled Host header. Because H3's router resolves the route handler before middl

Why this priority

High 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	7.4 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 66%)
CISA exploited list	No
Weakness	CWE-290
Published	2026-03-20
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: h3

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

Am I affected?

Timeline

2026-03-20 11:18 UTCPublished CVE record published
2026-06-17 10:36 UTCUpdated Record last modified

h3 h3 DOWNGRADE

Affected: 2.0.0 · 2.0.1 rc10 · 2.0.1 rc11 · 2.0.1 rc12 · 2.0.1 rc13 · 2.0.1 rc14 · 2.0.1 rc2 · 2.0.1 rc3

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.0.0

1. Check installed version

npm ls h3

2. No fixed release: DOWNGRADE to a release before 2.0.0

npm view h3 versions npm install h3@"<2.0.0"

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-2026-33131?

h3: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-33131 being exploited?

CVE-2026-33131 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-2026-33131 affect?

CVE-2026-33131 affects h3. Severity is high (CVSS 7.4). H3 is a minimal H(TTP) framework. Versions 2.0.0-0 through 2.0.1-rc.14 contain a Host header spoofing vulnerability in the NodeRequestUrl (which extends FastURL) which allows middleware bypass. When event.url, event.url.hostname, or event.u

How urgent is CVE-2026-33131?

Priority 44/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-33490 LOW 3.7 DOWNGRADE H3 is a minimal H(TTP) framework
CVE-2026-33129 MEDIUM 5.9 DOWNGRADE H3 is a minimal H(TTP) framework
CVE-2026-33128 HIGH 7.5 UPGRADE H3 is a minimal H(TTP) framework

References

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