CVE-2026-45271: H2O — MEDIUM

Picotls is a TLS protocol library that allows users select different crypto backends based on their use case. Picotls implements its own ASN.1 validation helper, which is used by the minicrypto backend while parsing local PKCS#8 private keys. Prior to commit...

Full CISO analysis pending enrichment.

What systems are affected?

Package	Ecosystem	Vulnerable Range	Patched
H2O	pip	—	No patch
7.5K OpenSSF 5.5 27 dependents Pushed 6d ago 6% patched ~0d to patch Full package profile →

Do you use H2O? You're affected.

How severe is it?

CVSS 3.1

5.5 / 10

EPSS

N/A

Exploitation Status

No known exploitation

Sophistication

N/A

What is the attack surface?

AV Local

AC Low

PR None

UI Required

S Unchanged

C None

I None

A High

What should I do?

No patch available

Monitor for updates. Consider compensating controls or temporary mitigations.

Which compliance frameworks are affected?

Compliance analysis pending. Sign in for full compliance mapping when available.

Frequently Asked Questions

What is CVE-2026-45271?

Picotls is a TLS protocol library that allows users select different crypto backends based on their use case. Picotls implements its own ASN.1 validation helper, which is used by the minicrypto backend while parsing local PKCS#8 private keys. Prior to commit c14231d801407640bc42c2dcf92783409ea6a7c7, the validator recursively descends into constructed ASN.1 elements without enforcing a maximum nesting depth. If an application loads an attacker-supplied private-key file through ptls_minicrypto_load_private_key(), or otherwise calls the public ASN.1 validation API on untrusted DER, a crafted deeply nested ASN.1 structure can exhaust the process stack and crash the application. Note that the libcrypto (OpenSSL) backend does not use the ASN.1 validation helper of picotls, and therefore is immune to this vulnerability. The vulnerability has been addressed in commit c14231d801407640bc42c2dcf92783409ea6a7c7.

Is CVE-2026-45271 actively exploited?

No confirmed active exploitation of CVE-2026-45271 has been reported, but organizations should still patch proactively.

How to fix CVE-2026-45271?

No patch is currently available. Monitor vendor advisories for updates.

What is the CVSS score for CVE-2026-45271?

CVE-2026-45271 has a CVSS v3.1 base score of 5.5 (MEDIUM).

What are the technical details?

Original Advisory

Picotls is a TLS protocol library that allows users select different crypto backends based on their use case. Picotls implements its own ASN.1 validation helper, which is used by the minicrypto backend while parsing local PKCS#8 private keys. Prior to commit c14231d801407640bc42c2dcf92783409ea6a7c7, the validator recursively descends into constructed ASN.1 elements without enforcing a maximum nesting depth. If an application loads an attacker-supplied private-key file through ptls_minicrypto_load_private_key(), or otherwise calls the public ASN.1 validation API on untrusted DER, a crafted deeply nested ASN.1 structure can exhaust the process stack and crash the application. Note that the libcrypto (OpenSSL) backend does not use the ASN.1 validation helper of picotls, and therefore is immune to this vulnerability. The vulnerability has been addressed in commit c14231d801407640bc42c2dcf92783409ea6a7c7.

Weaknesses (CWE)

CWE-835 Loop with Unreachable Exit Condition ('Infinite Loop')

CWE-835 — Loop with Unreachable Exit Condition ('Infinite Loop'): The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.

Source: MITRE CWE corpus.