{"id":"CVE-2026-126135","kind":"vulnerability","aliases":["CVE-2026-126135"],"related":[],"title":"Thunderbird: STARTTLS command injection lets attackers spoof mail send and fetch results","description":"If an attacker was able to perform a man-in-the-middle attack on an SMTP or POP3 server connection using STARTTLS, the attacker could inject server responses before the TLS handshake completed. Thunderbird would process these responses after the handshake, as if they had been received from the server over the encrypted connection. This could be used to make Thunderbird report a different outcome for sending or fetching mail, and to show attacker-controlled text in error messages. The encrypted connection itself was not compromised, and credentials were not exposed. Connections using SSL/TLS instead of STARTTLS were not affected. This vulnerability was fixed in Thunderbird 160.","summary":"Thunderbird processed server responses that an attacker injected before the TLS handshake finished on SMTP or POP3 STARTTLS connections, treating them as encrypted server replies. An attacker performing a man-in-the-middle attack can make Thunderbird report false send or fetch outcomes and show attacker-chosen text in error messages, but the encryption and credentials are not exposed.","severity":"medium","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["desktop","email","other","thunderbird"],"not_affected_if":["ssl-tls-accounts-only","thunderbird-160-or-later"],"how_to_check":"- Check the Thunderbird version under Help > About Thunderbird; versions below `160` are affected.\n- Check whether mail accounts use STARTTLS rather than SSL/TLS in account server settings; only STARTTLS connections were affected.","fix":"- Thunderbird: move to `160` or later.","status":"open","published_at":"2026-11-10T19:21:14+00:00","first_seen_at":"2026-11-10T19:25:15+00:00","triaged_at":"2026-11-10T20:04:50+00:00","updated_at":"2026-11-10T20:04:50+00:00","category":"other","confidence":0.7,"triage":{"at":"2026-11-10T20:04:50Z","model":"claude-haiku-5-5","output":{"fix":"- Thunderbird: move to `160` or later.","reach":"network","title":"Thunderbird: STARTTLS command injection lets attackers spoof mail send and fetch results","impact":"low","summary":"Thunderbird processed server responses that an attacker injected before the TLS handshake finished on SMTP or POP3 STARTTLS connections, treating them as encrypted server replies. An attacker performing a man-in-the-middle attack can make Thunderbird report false send or fetch outcomes and show attacker-chosen text in error messages, but the encryption and credentials are not exposed.","audience":"everyone","category":"other","nicknames":[],"who_cares":["thunderbird","email","desktop"],"confidence":0.7,"fix_status":"available","needs_auth":"no","worst_case":"Attacker-chosen text shown in errors and false send or fetch results","analyst_note":"","how_to_check":"- Check the Thunderbird version under Help > About Thunderbird; versions below `160` are affected.\n- Check whether mail accounts use STARTTLS rather than SSL/TLS in account server settings; only STARTTLS connections were affected.","plain_action":"Update Thunderbird to version 160 or later: Help > About Thunderbird.","product_name":"","needs_analyst":false,"not_affected_if":["thunderbird-160-or-later","ssl-tls-accounts-only"],"who_is_affected":"Thunderbird installations below version 160 that send or fetch mail over SMTP or POP3 using STARTTLS. Accounts using SSL/TLS ports are not affected. Exploitation requires a man-in-the-middle position on the mail server connection.","user_interaction":"no","product_ecosystem":"","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000775,"input_tokens":4665,"output_tokens":617,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-126135':15A,18A '-2026':14A,17A '160':192C 'abl':88C 'affect':185C 'attack':6A,29B,49B,57B,70B,86C,97C,108C,159C 'attacker-chosen':69B 'attacker-control':158C 'chosen':71B 'command':3A 'complet':117C 'compromis':171C 'connect':41B,104C,138C,167C,177C 'control':160C 'could':109C,140C 'credenti':80B,173C 'cve':13A,16A 'desktop':19B 'differ':148C 'email':20B 'encrypt':45B,78B,137C,166C 'error':74B,163C 'expos':83B,176C 'fals':62B 'fetch':11A,65B,153C 'finish':35B 'fix':189C 'handshak':34B,116C,125C 'inject':4A,30B,110C 'instead':180C 'let':5A 'mail':8A,154C 'make':59B,144C 'man':53B,93C 'man-in-the-middl':52B,92C 'messag':75B,164C 'middl':56B,96C 'outcom':66B,149C 'perform':50B,90C 'pop3':39B,102C 'process':24B,120C 'receiv':131C 'repli':47B 'report':61B,146C 'respons':26B,112C,122C 'result':12A 'send':9A,63B,151C 'server':25B,46B,103C,111C,134C 'show':68B,157C 'smtp':37B,100C 'spoof':7A 'ssl/tls':179C 'starttl':2A,40B,106C,182C 'text':72B,161C 'thunderbird':1A,22B,23B,60B,118C,145C,191C 'tls':33B,115C 'treat':42B 'use':105C,142C,178C 'vulner':187C 'would':119C","nicknames":[],"mitigations":[],"who_affected":"Thunderbird installations below version 160 that send or fetch mail over SMTP or POP3 using STARTTLS. Accounts using SSL/TLS ports are not affected. Exploitation requires a man-in-the-middle position on the mail server connection.","tier":"C","verified":{},"analyst_revisit_at":null,"fix_status":"available","audience":"everyone","plain_action":"Update Thunderbird to version 160 or later: Help > About Thunderbird.","pkeys":[],"ptokens":[],"labels":["desktop","email","medium","other","thunderbird"],"campaign_type":null,"search_a":"'-126135':15A,18A '-2026':14A,17A 'attack':6A 'command':3A 'cve':13A,16A 'fetch':11A 'inject':4A 'let':5A 'mail':8A 'result':12A 'send':9A 'spoof':7A 'starttl':2A 'thunderbird':1A","search_b":"'attack':29B,49B,57B,70B 'attacker-chosen':69B 'chosen':71B 'connect':41B 'credenti':80B 'desktop':19B 'email':20B 'encrypt':45B,78B 'error':74B 'expos':83B 'fals':62B 'fetch':65B 'finish':35B 'handshak':34B 'inject':30B 'make':59B 'man':53B 'man-in-the-middl':52B 'messag':75B 'middl':56B 'outcom':66B 'perform':50B 'pop3':39B 'process':24B 'repli':47B 'report':61B 'respons':26B 'send':63B 'server':25B,46B 'show':68B 'smtp':37B 'starttl':40B 'text':72B 'thunderbird':22B,23B,60B 'tls':33B 'treat':42B","search_c":"'160':192C 'abl':88C 'affect':185C 'attack':86C,97C,108C,159C 'attacker-control':158C 'complet':117C 'compromis':171C 'connect':104C,138C,167C,177C 'control':160C 'could':109C,140C 'credenti':173C 'differ':148C 'encrypt':137C,166C 'error':163C 'expos':176C 'fetch':153C 'fix':189C 'handshak':116C,125C 'inject':110C 'instead':180C 'mail':154C 'make':144C 'man':93C 'man-in-the-middl':92C 'messag':164C 'middl':96C 'outcom':149C 'perform':90C 'pop3':102C 'process':120C 'receiv':131C 'report':146C 'respons':112C,122C 'send':151C 'server':103C,111C,134C 'show':157C 'smtp':100C 'ssl/tls':179C 'starttl':106C,182C 'text':161C 'thunderbird':118C,145C,191C 'tls':115C 'use':105C,142C,178C 'vulner':187C 'would':119C","cause_entry":null,"cause_path":null,"disclosures_checked_at":null,"first_public_at":"2026-11-10T19:21:14+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-126135","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-126135","severity":null,"published_at":"2026-11-10T19:21:14+00:00","modified_at":"2026-11-10T19:21:14+00:00","first_seen_at":"2026-11-10T19:25:15+00:00","fetched_at":"2026-11-10T19:25:15+00:00","note":null}],"documents":[],"disclosures":[],"exploit_sources":[],"notice":"Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai"}