# CVE-2026-59993: Picotls null pointer dereference in TLS client handshake

Source: https://junglewise.ai/threats/cve-2026-59993-picotls-is-a-tls-protocol-library-that-allows-users-select  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-30. Updated: 2026-10-30  
License: CC BY 4.0 with a link back

CVE-2026-59993 · Severity: medium · CVSS 6.5 · Published 2026-10-30

Picotls is a TLS protocol library used to secure network communications. A flaw in how the library handles TLS handshakes on the client side allows a malicious server to send specially crafted messages that crash the client application, causing a denial of service. This could affect any service or application relying on Picotls for secure connections.

Vendors: [H2o](https://junglewise.ai/threats/vendors/h2o).

## Quick answers

**Is CVE-2026-59993 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59993 being exploited in the wild.

**How severe is CVE-2026-59993?** Medium severity, with a CVSS score of 6.5, published 30 October 2026.

**Which products and versions are affected?** H2o Picotls through commit 44bc503.

**Is there a fix for CVE-2026-59993?** Yes: fixed in commit a06ca41. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-59993 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-59993?** Check whether you run H2o Picotls at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in commit a06ca41

## Technical details

A null pointer dereference occurs when a TLS client receives a HelloRetryRequest followed by a spoofed Hello message due to insufficient validation of the second Hello message. The vulnerability is triggered during client-side TLS handshake processing when a peer sends a second HelloRetryRequest or a Hello message with a cipher suite that contradicts the previously established choice. The fix (commit a06ca41) adds proper rejection of duplicate HelloRetryRequest messages and validation of cipher suite consistency.

## Affected products

- h2o Picotls through commit 44bc503

## Timeline

- 2026-10-30: disclosed: CVE-2026-59993 published
- 2026-06-29: patched: Fix merged in commit a06ca41

## References

- [https://github.com/h2o/picotls/commit/a06ca417a16f9126808bf5cabe6c5e6e0f16ecae](https://github.com/h2o/picotls/commit/a06ca417a16f9126808bf5cabe6c5e6e0f16ecae)
- [https://github.com/h2o/picotls/pull/608](https://github.com/h2o/picotls/pull/608)

## Related threats

- [Picotls ASN.1 decoder stack exhaustion](https://junglewise.ai/threats/cve-2026-45271-picotls-asn-1-decoder-stack-exhaustion)