NULL pointer dereference in H2o Picotls
CVE-2026-59993
Picotls is a TLS protocol library that allows users select different crypto backends based on their use case. In affected revisions through 44bc503, when picotls is used as a `client-side TLS stack`, a peer can send a `HelloRetryRequest` followed by a `spoofed Hello message` that causes a null-pointer dereference. This can terminate the consuming process, including a process performing other work, resulting in denial of service. This issue is fixed in commit a06ca41.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
- H2o Picotls — versions < a06ca417a16f9126808bf5cabe6c5e6e0f16ecae
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2026-59993?
- CVE-2026-59993 is a medium-severity vulnerability in H2o Picotls, classified under NULL Pointer Dereference. CVSS score: 6.5/10. Published 2026-10-30.
- How severe is CVE-2026-59993?
- Medium severity. CVSS v3 base score is 6.5 out of 10.