CVE-2026-45271: CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') in h2o picotls

Picotls implements its own ASN.1 validation helper used by the minicrypto backend to parse local PKCS#8 private keys. Prior to commit c14231d801407640bc42c2dcf92783409ea6a7c7, the validator recursively descended into constructed ASN.1 elements without limiting nesting depth. This allowed crafted deeply nested ASN.1 structures to exhaust the process stack, causing application crashes. The vulnerability affects applications that load attacker-supplied private keys via ptls_minicrypto_load_private_key() or call the ASN.1 validation API on untrusted DER data. The OpenSSL backend does not use this helper and is not vulnerable. The vulnerability is tracked as CVE-2026-45271 with a CVSS 3.1 score of 5.5 (medium severity).

An attacker can cause a denial of service by supplying a deeply nested ASN.1 structure that triggers an infinite loop and stack exhaustion in the picotls minicrypto backend. This leads to application crashes. There is no impact on confidentiality or integrity. The OpenSSL backend is not affected.

A fix is available and has been implemented in commit c14231d801407640bc42c2dcf92783409ea6a7c7. Users of picotls should update to a version including this commit to prevent denial of service. Applications using the OpenSSL backend are not affected and require no action.