---
id: "CVE-2026-59993"
kind: "vulnerability"
aliases: ["CVE-2026-59993"]
title: "Picotls TLS library: malicious server crashes client with null dereference"
severity: "medium"
cvss: 6.5
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["c", "client-side", "package", "picotls", "tls"]
not_affected_if: ["not-using-picotls", "server-only-use"]
published_at: "2026-10-30T21:17:13+00:00"
first_seen_at: "2026-10-30T21:25:53+00:00"
updated_at: "2026-10-30T21:26:51+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-59993", "first_seen_at": "2026-10-30T21:25:53+00:00"}]
---

# Picotls TLS library: malicious server crashes client with null dereference

Picotls is a TLS library that can be used as a client-side TLS stack. A peer that sends a HelloRetryRequest followed by a spoofed Hello message triggers a null-pointer dereference, which can terminate the consuming process and cause denial of service.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check whether your application links picotls as its client-side TLS stack (unknown if the sources do not say how to confirm this further).
- Check the picotls revision in use against `44bc503`; affected revisions are through `44bc503`.

## Fix

- picotls: move to a revision that includes commit `a06ca41`.

## Timeline

- 2026-10-30T21:25:53+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-59993

## From the sources

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_