Picotls is a TLS library used by many projects, including h2o. The library implements its own ASN.1 decoder for the minicrypto backend. Prior to the fix, the decoder performed a recursive descent into constructed ASN.1 elements without limiting depth. An attacker who supplies a PKCS#8 private-key file containing an arbitrarily deep ASN.1 structure can cause the decoder to exhaust the process stack, leading to a crash of the application that loads the key. The crash results in a denial of service but does not allow remote code execution or data disclosure. The vulnerability is tied to the CWE-835 category, which represents infinite loops or recursion.

The affected software is the picotls library released by h2o before commit c14231d801407640bc42c2dcf92783409ea6a7c7. Any application that links to this library and uses the minicrypto backend to load untrusted private keys, or that calls the public ASN.1 validator directly, is vulnerable. Versions of picotls older than the referenced commit are therefore impacted; no specific version numbers are listed in the CNA data.

The CVSS score of 5.5 indicates a medium severity. The vulnerability requires an attacker to supply a specially crafted ASN.1 file, so the attack vector is local or requires legitimate file upload capability. EPSS information is not available and the flaw is not listed in CISA's KEV catalog, suggesting limited or no catalogued exploitation. Because the impact is a stack exhaustion crash, the risk is moderate; however, for services that rely on continuous availability, the denial of service could be significant. Patch the library promptly to prevent this flaw from being exploitable.