# CVE-2026-45271: Picotls ASN.1 decoder stack exhaustion

Source: https://junglewise.ai/threats/cve-2026-45271-picotls-asn-1-decoder-stack-exhaustion  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-21. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-45271 · Severity: medium · CVSS 5.5 · Published 2026-08-21

Picotls is a cryptographic TLS library that includes its own ASN.1 parser for handling private keys in PKCS#8 format. A flaw in the parser's recursion handling allows an attacker to craft a maliciously nested certificate or key file that, when loaded by an application, causes a stack overflow and crashes the process. This affects applications using picotls's minicrypto backend to parse untrusted key files.

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

## Quick answers

**Is CVE-2026-45271 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-45271 being exploited in the wild.

**How severe is CVE-2026-45271?** Medium severity, with a CVSS score of 5.5, published 21 August 2026.

**Which products and versions are affected?** H2o Picotls commits prior to c14231d.

**Is there a fix for CVE-2026-45271?** A patch was published on 2026-05-29: Fix merged in commit c14231d Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-45271 attacked?** It is attacked with local access.

**Am I affected by CVE-2026-45271?** 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.

## Technical details

The vulnerability is a stack exhaustion flaw in picotls's custom ASN.1 validation helper used by the minicrypto cryptographic backend. The validator recursively descends into constructed ASN.1 elements without enforcing a maximum nesting depth, allowing a deeply nested DER-encoded structure to exhaust the call stack. An attacker can exploit this via ptls_minicrypto_load_private_key() or the public ASN.1 validation API when parsing untrusted DER-encoded data. The impact is denial of service through application crash. The OpenSSL (libcrypto) backend is unaffected as it does not use picotls's custom ASN.1 validator. The fix adds depth limiting to ASN.1 recursion.

## Affected products

- h2o picotls commits prior to c14231d

## Timeline

- 2026-05-29: disclosed: GitHub Security Advisory GHSA-84f5-m5x2-82q4 published
- 2026-05-29: patched: Fix merged in commit c14231d
- 2026-08-21: advisory: CVE-2026-45271 published

## References

- [https://github.com/h2o/picotls/commit/c14231d801407640bc42c2dcf92783409ea6a7c7](https://github.com/h2o/picotls/commit/c14231d801407640bc42c2dcf92783409ea6a7c7)
- [https://github.com/h2o/picotls/security/advisories/GHSA-84f5-m5x2-82q4](https://github.com/h2o/picotls/security/advisories/GHSA-84f5-m5x2-82q4)

## Related threats

- [Picotls null pointer dereference in TLS client handshake](https://junglewise.ai/threats/cve-2026-59993-picotls-is-a-tls-protocol-library-that-allows-users-select)