CVE-2026-33129

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:

1. Packet A: Contains a password that is known to be incorrect starting at the first character (e.g., AAAA...).
2. 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.

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).

CVE-2026-33129 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.1-rc.9); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33129? CVE-2026-33129 is a medium-severity security vulnerability in h3 (npm), affecting versions >= 2.0.0-beta.0, <= 2.0.0-rc.8. It is fixed in 2.0.1-rc.9.
2. How severe is CVE-2026-33129? CVE-2026-33129 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of h3 are affected by CVE-2026-33129? h3 (npm) versions >= 2.0.0-beta.0, <= 2.0.0-rc.8 is affected.
4. Is there a fix for CVE-2026-33129? Yes. CVE-2026-33129 is fixed in 2.0.1-rc.9. Upgrade to this version or later.
5. Is CVE-2026-33129 exploitable, and should I be worried? Whether CVE-2026-33129 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33129 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33129? Upgrade h3 to 2.0.1-rc.9 or later.