{"id":"CVE-2026-121506","kind":"vulnerability","aliases":["CVE-2026-121506"],"related":[],"title":"NTPsec: crafted NTP replies corrupt memory and can bypass NTS time authentication","description":"An out-of-bounds read and write in NTS client extension field parsing (extens_client_recv) in NTPsec before 1.2.5 allows a remote attacker who can send NTP server-mode replies to an NTS client to corrupt heap memory and crash ntpd, and, combined with the NTS-KE client out-of-bounds read fixed in 1.2.4, to bypass NTS authentication and set the client's time, via an NTS Authenticator and Encrypted Extension Fields field whose nonce length exceeds the field.","summary":"NTPsec's NTS client parses extension fields without checking bounds, so a crafted server-mode NTP reply can corrupt heap memory and crash ntpd. Combined with a separate NTS-KE out-of-bounds read fixed in 1.2.4, an attacker can bypass NTS authentication and set the client's clock.","severity":"high","cvss":7.4,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["linux-servers","ntp","ntpsec","nts-client","runtime"],"not_affected_if":["managed-time-service","no-nts-client","other-ntp-implementation"],"how_to_check":"- Run `ntpd --version` or `ntpq -c version` on each host and compare against the fixed release.\n- Check whether NTS is enabled in the ntp configuration (look for `nts` directives in ntp.conf).","fix":"- NTPsec `1.2.5` or later (the sources say fixed before 1.2.5; the 1.2.4 NTS-KE fix is also needed).","status":"open","published_at":"2026-11-03T15:17:37+00:00","first_seen_at":"2026-11-03T15:22:01+00:00","triaged_at":"2026-11-03T15:22:54+00:00","updated_at":"2026-11-03T15:22:54+00:00","category":"runtime","confidence":0.7,"triage":{"at":"2026-11-03T15:22:54Z","model":"claude-haiku-5-5","output":{"fix":"- NTPsec `1.2.5` or later (the sources say fixed before 1.2.5; the 1.2.4 NTS-KE fix is also needed).","reach":"network","title":"NTPsec: crafted NTP replies corrupt memory and can bypass NTS time authentication","impact":"high","summary":"NTPsec's NTS client parses extension fields without checking bounds, so a crafted server-mode NTP reply can corrupt heap memory and crash ntpd. Combined with a separate NTS-KE out-of-bounds read fixed in 1.2.4, an attacker can bypass NTS authentication and set the client's clock.","audience":"businesses","category":"runtime","nicknames":[],"who_cares":["ntpsec","ntp","nts-client","linux-servers"],"confidence":0.7,"fix_status":"available","needs_auth":"no","worst_case":"remote crash of ntpd, and with a second bug, attacker-set system clock","analyst_note":"Confirm the exact fixed version range and whether 1.2.4 is required alongside 1.2.5 using the NVD entry and the NTPsec release notes.","how_to_check":"- Run `ntpd --version` or `ntpq -c version` on each host and compare against the fixed release.\n- Check whether NTS is enabled in the ntp configuration (look for `nts` directives in ntp.conf).","plain_action":"","needs_analyst":true,"not_affected_if":["other-ntp-implementation","no-nts-client","managed-time-service"],"who_is_affected":"Systems running NTPsec ntpd as an NTS client that accept NTP server-mode replies. Hosts that only use plain NTP without NTS, or that run other NTP implementations, are not covered by these sources.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"high"},"cost_usd":0.000738,"input_tokens":4205,"output_tokens":635,"severity_before":"high"},"analyst_status":null,"analyst_note":null,"search":"'-121506':15A,18A '-2026':14A,17A '1.2.4':67B,139C '1.2.5':100C 'allow':101C 'attack':69B,104C 'authent':12A,73B,143C,153C 'bound':37B,63B,84C,135C 'bypass':9A,71B,141C 'check':36B 'client':26B,31B,77B,90C,95C,116C,131C,147C 'clock':79B 'combin':53B,125C 'corrupt':5A,47B,118C 'craft':2A,40B 'crash':51B,122C 'cve':13A,16A 'encrypt':155C 'exceed':162C 'exten':94C 'extens':33B,91C,156C 'field':34B,92C,157C,158C,164C 'fix':65B,137C 'heap':48B,119C 'ke':59B,130C 'length':161C 'linux':20B 'linux-serv':19B 'memori':6A,49B,120C 'mode':43B,111C 'nonc':160C 'ntp':3A,22B,44B,108C 'ntpd':52B,123C 'ntpsec':1A,23B,28B,98C 'nts':10A,25B,30B,58B,72B,89C,115C,129C,142C,152C 'nts-client':24B 'nts-ke':57B,128C 'out-of-bound':60B,81C,132C 'pars':32B,93C 'read':64B,85C,136C 'recv':96C 'remot':103C 'repli':4A,45B,112C 'runtim':27B 'send':107C 'separ':56B 'server':21B,42B,110C 'server-mod':41B,109C 'set':75B,145C 'time':11A,149C 'via':150C 'whose':159C 'without':35B 'write':87C","nicknames":[],"mitigations":[],"who_affected":"Systems running NTPsec ntpd as an NTS client that accept NTP server-mode replies. Hosts that only use plain NTP without NTS, or that run other NTP implementations, are not covered by these sources.","tier":"B","verified":{},"analyst_revisit_at":null,"fix_status":"available","audience":"businesses","plain_action":null,"pkeys":[],"ptokens":[],"labels":["high","linuxservers","ntp","ntpsec","ntsclient","runtime"],"first_public_at":"2026-11-03T15:17:37+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H","cvss_by":"cve@gitlab.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-121506","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-121506","severity":null,"published_at":"2026-11-03T15:17:37+00:00","modified_at":"2026-11-03T15:17:37+00:00","first_seen_at":"2026-11-03T15:22:01+00:00","fetched_at":"2026-11-03T15:22:01+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"}