{"id":"CVE-2026-59993","kind":"vulnerability","aliases":["CVE-2026-59993"],"related":[],"title":"Picotls TLS library: malicious server crashes client with null dereference","description":"Picotls is a TLS protocol library that allows users select different crypto backends based on their use case. In affected revisions through 44bc503, when picotls is used as a `client-side TLS stack`, a peer can send a `HelloRetryRequest` followed by a `spoofed Hello message` that causes a null-pointer dereference. This can terminate the consuming process, including a process performing other work, resulting in denial of service. This issue is fixed in commit a06ca41.","summary":"Picotls is a TLS library that can be used as a client-side TLS stack. A peer that sends a HelloRetryRequest followed by a spoofed Hello message triggers a null-pointer dereference, which can terminate the consuming process and cause denial of service.","severity":"medium","cvss":6.5,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["c","client-side","package","picotls","tls"],"not_affected_if":["not-using-picotls","server-only-use"],"how_to_check":"- Check whether your application links picotls as its client-side TLS stack (unknown if the sources do not say how to confirm this further).\n- Check the picotls revision in use against `44bc503`; affected revisions are through `44bc503`.","fix":"- picotls: move to a revision that includes commit `a06ca41`.","status":"open","published_at":"2026-10-30T21:17:13+00:00","first_seen_at":"2026-10-30T21:25:53+00:00","triaged_at":"2026-10-30T21:26:51+00:00","updated_at":"2026-10-30T21:26:51+00:00","category":"package","confidence":0.6,"triage":{"at":"2026-10-30T21:26:51Z","model":"claude-haiku-5-5","output":{"fix":"- picotls: move to a revision that includes commit `a06ca41`.","reach":"network","title":"Picotls TLS library: malicious server crashes client with null dereference","impact":"low","summary":"Picotls is a TLS library that can be used as a client-side TLS stack. A peer that sends a HelloRetryRequest followed by a spoofed Hello message triggers a null-pointer dereference, which can terminate the consuming process and cause denial of service.","audience":"developers","category":"package","nicknames":[],"who_cares":["picotls","c","tls","client-side"],"confidence":0.6,"fix_status":"available","needs_auth":"no","worst_case":"Client process terminates after a malicious server sends crafted handshake messages.","analyst_note":"","how_to_check":"- Check whether your application links picotls as its client-side TLS stack (unknown if the sources do not say how to confirm this further).\n- Check the picotls revision in use against `44bc503`; affected revisions are through `44bc503`.","plain_action":"","needs_analyst":false,"not_affected_if":["server-only-use","not-using-picotls"],"who_is_affected":"Applications that build picotls into a client and use it as the client-side TLS stack are exposed, because the crash is triggered by a peer the client connects to. Servers that only use picotls as a server-side stack, or programs that do not use picotls at all, are not described as affected by the sources. Teams using a different TLS stack can skip it.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000684,"input_tokens":4167,"output_tokens":535,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-2026':12A,15A '-59993':13A,16A '44bc503':91C 'a06ca41':145C 'affect':88C 'allow':76C 'backend':81C 'base':82C 'c':17B 'case':86C 'caus':65B,116C 'client':7A,19B,36B,99C 'client-sid':18B,35B,98C 'commit':144C 'consum':62B,126C 'crash':6A 'crypto':80C 'cve':11A,14A 'denial':66B,136C 'derefer':10A,57B,121C 'differ':79C 'fix':142C 'follow':46B,109C 'hello':50B,113C 'helloretryrequest':45B,108C 'includ':128C 'issu':140C 'librari':3A,28B,74C 'malici':4A 'messag':51B,114C 'null':9A,55B,119C 'null-point':54B,118C 'packag':21B 'peer':41B,104C 'perform':131C 'picotl':1A,22B,24B,69C,93C 'pointer':56B,120C 'process':63B,127C,130C 'protocol':73C 'result':134C 'revis':89C 'select':78C 'send':43B,106C 'server':5A 'servic':68B,138C 'side':20B,37B,100C 'spoof':49B,112C 'stack':39B,102C 'termin':60B,124C 'tls':2A,23B,27B,38B,72C,101C 'trigger':52B 'use':32B,85C,95C 'user':77C 'work':133C","nicknames":[],"mitigations":[],"who_affected":"Applications that build picotls into a client and use it as the client-side TLS stack are exposed, because the crash is triggered by a peer the client connects to. Servers that only use picotls as a server-side stack, or programs that do not use picotls at all, are not described as affected by the sources. Teams using a different TLS stack can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"fix_status":"available","audience":"developers","plain_action":null,"first_public_at":"2026-10-30T21:17:13+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","cvss_by":"security-advisories@github.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-59993","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-59993","severity":null,"published_at":"2026-10-30T21:17:13+00:00","modified_at":"2026-10-30T21:17:13+00:00","first_seen_at":"2026-10-30T21:25:53+00:00","fetched_at":"2026-10-30T21:25:53+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}