CVE-2026-33129 is a medium-severity (CVSS 5.9) CWE-208 vulnerability in h3. A fix is available for h3 — see the affected versions and patch details below.
h3 has an observable timing discrepancy in basic auth utils
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33129.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
h3npmDescription
Summary
A Timing Side-Channel vulnerability exists 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.
Details
The vulnerability is located in the requireBasicAuth function. The code performs a standard string comparison between the user-provided password and the expected password:
if (opts.password && password !== opts.password) {
throw autheFailed(event, opts?.realm);
}
In V8 (and most runtime environments), the !== operator is optimized to "fail fast." It stops execution and returns false as soon as it encounters the first mismatched byte.
- If the first character is wrong, it returns immediately.
- If the first character is correct but the second is wrong, it takes slightly longer.
By statistically analyzing these minute timing differences over many requests, an attacker can determine the correct password one character at a time.
PoC
This vulnerability is exploitable in real-world scenarios without direct access to the server machine.
To reproduce this, an attacker can send two packets (or bursts of packets) at the exact same time:
- Packet A: Contains a password that is known to be incorrect starting at the first character (e.g.,
AAAA...). - Packet B: Contains a password where the first character is a guess (e.g.,
B...).
By measuring the time-to-first-byte (TTFB) or total response time of these concurrent requests, the attacker can filter out network jitter. If Packet B takes consistently longer to return than Packet A, the first character is confirmed as correct. This process is repeated for the second character, and so on. Tests confirm this timing difference is statistically consistent enough to recover credentials remotely.
Impact
This vulnerability allows remote attackers to recover passwords. While network jitter makes this difficult over the internet, it is highly effective in local networks or cloud environments where the attacker is co-located. It reduces the complexity of cracking a password from exponential (guessing the whole string) to linear (guessing one char at a time).
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | h3 | ≥ 2.0.0-beta.0&&< 2.0.1-rc.9 | 2.0.1-rc.9npm install h3@2.0.1-rc.9 |
Affected Products
h3h3Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for h3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update h3 to 2.0.1-rc.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33129 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-33129 in your dependencies?
Find it across npm, including transitive dependencies.