---
id: CVE-2026-59993
title: >-
  Picotls has NULL pointer dereference during handshake, when running as a
  client
summary: >-
  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`…
severity: medium
cvss: 6.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H'
cvssSource: cna
cwe:
  - CWE-476
vendor: h2o
product: picotls
affected:
  - picotls < a06ca417a16f9126808bf5cabe6c5e6e0f16ecae
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-06T18:22:20.935065Z'
published: '2026-10-30'
updated: '2026-11-06'
sourceUpdated: '2026-11-06T18:24:13.150Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-59993'
references:
  - url: 'https://github.com/h2o/picotls/security/advisories/GHSA-2xcv-h2pg-m99c'
    label: 'https://github.com/h2o/picotls/security/advisories/GHSA-2xcv-h2pg-m99c'
  - url: 'https://github.com/h2o/picotls/pull/608'
    label: 'https://github.com/h2o/picotls/pull/608'
  - url: >-
      https://github.com/h2o/picotls/commit/a06ca417a16f9126808bf5cabe6c5e6e0f16ecae
    label: >-
      https://github.com/h2o/picotls/commit/a06ca417a16f9126808bf5cabe6c5e6e0f16ecae
tags:
  - cve.org
epss: 0.00247
epssPercentile: 0.16098
ingestedAt: '2026-11-05T16:24:31.466Z'
---

## Overview

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

- `picotls < a06ca417a16f9126808bf5cabe6c5e6e0f16ecae`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.