Vulnerability · Published Oct 30, 2026 · Assigned by GitHub_M

CVE-2026-59993Picotls has NULL pointer dereference during handshake, when running as a client

Medium 6.5

In plain English

A medium-severity null pointer dereference in picotls from h2o. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 16% of all known vulnerabilities).

Severity (CVSS 3.1)6.5Medium

Exploit odds (30 days)0.2%top 83.9% of all CVEs

Exploited in the wildNo reportsnot in CISA KEV

Attack toolingNoneno public tooling known

Official description

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.

Affected products (as reported)

Vendor	Product	Versions
h2o	picotls	< a06ca417a16f9126808bf5cabe6c5e6e0f16ecae

Weakness: CWE-476 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

References

https://github.com/h2o/picotls/security/advisories/GHSA-2xcv-h2pg-m99c x_refsource_CONFIRM
https://github.com/h2o/picotls/pull/608 x_refsource_MISC
https://github.com/h2o/picotls/commit/a06ca417a16f9126808bf5cabe6c5e6e0f16ecae x_refsource_MISC