CVE-2026-33129: H3 is a minimal H(TTP) framework

Q: How do I fix CVE-2026-33129?

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

Q: Is CVE-2026-33129 being exploited?

CVE-2026-33129 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-2026-33129 affect?

CVE-2026-33129 affects h3. Severity is medium (CVSS 5.9). H3 is a minimal H(TTP) framework. Versions 2.0.1-beta.0 through 2.0.0-rc.8 contain a Timing Side-Channel vulnerability in the requireBasicAuth function due to the use of unsafe string comparison (!==). This allows an attacker to deduce the

Q: How urgent is CVE-2026-33129?

Priority 37/100. Public exploit reference; Remotely exploitable, no login, no user action

This week

MEDIUM 5.9REMOTEDOWNGRADE

CVE-2026-33129 is a medium-severity flaw (CVSS 5.9) 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 2.0.1-beta.0 through 2.0.0-rc.8 contain a Timing Side-Channel vulnerability in the requireBasicAuth function due to the use of unsafe string comparison (!==). This allows an attacker to deduce the valid password character-by-character by measuring the server's response time, effectively bypassing password complexity protections. This issue is fixed in version 2.0.1-rc.9.

Why this priority

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	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 75%)
CISA exploited list	No
Weakness	CWE-208
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 10:16 UTCPublished CVE record published
2026-06-17 10:36 UTCUpdated Record last modified

h3 h3 DOWNGRADE

Affected: 2.0.0 · 2.0.1 rc1 · 2.0.1 rc2 · 2.0.1 rc3 · 2.0.1 rc4 · 2.0.1 rc5 · 2.0.1 rc6 · 2.0.1 rc7

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-2026-33129?

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

Is CVE-2026-33129 being exploited?

CVE-2026-33129 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-2026-33129 affect?

CVE-2026-33129 affects h3. Severity is medium (CVSS 5.9). H3 is a minimal H(TTP) framework. Versions 2.0.1-beta.0 through 2.0.0-rc.8 contain a Timing Side-Channel vulnerability in the requireBasicAuth function due to the use of unsafe string comparison (!==). This allows an attacker to deduce the

How urgent is CVE-2026-33129?

Priority 37/100. 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-2026-33131 HIGH 7.4 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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