{"id":"CVE-2026-128321","kind":"vulnerability","aliases":["CVE-2026-128321"],"related":[],"title":"TightVNC and UltraVNC viewers: malicious server can overflow heap memory","description":"Tight JPEG decoding in TightVNC and UltraVNC viewers contains a heap-based buffer overflow because an attacker-controlled framebuffer rectangle width is not validated against a fixed-size decoding buffer. A malicious VNC server can cause an out-of-bounds heap write when a user connects, resulting in application crash and potentially other memory-corruption impacts.","summary":"TightVNC and UltraVNC viewers mishandle Tight JPEG frames because a rectangle width from the server is not checked against a fixed-size buffer. A malicious VNC server can trigger an out-of-bounds heap write when a user connects, crashing the viewer and possibly corrupting memory further.","severity":"unknown","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["other","tightvnc","ultravnc","vnc","windows"],"not_affected_if":["trusted-servers-only"],"how_to_check":"- Check the installed TightVNC or UltraVNC viewer version against the vendor's release notes, since the sources do not name affected version ranges","fix":null,"status":"open","published_at":"2026-11-13T02:18:56+00:00","first_seen_at":"2026-11-13T02:41:37+00:00","triaged_at":"2026-11-13T02:43:14+00:00","updated_at":"2026-11-13T02:43:14+00:00","category":"other","confidence":0.5,"triage":{"at":"2026-11-13T02:43:14Z","model":"claude-haiku-5-5","output":{"fix":"unknown","reach":"network","title":"TightVNC and UltraVNC viewers: malicious server can overflow heap memory","impact":"medium","summary":"TightVNC and UltraVNC viewers mishandle Tight JPEG frames because a rectangle width from the server is not checked against a fixed-size buffer. A malicious VNC server can trigger an out-of-bounds heap write when a user connects, crashing the viewer and possibly corrupting memory further.","audience":"businesses","category":"other","nicknames":[],"who_cares":["vnc","tightvnc","ultravnc","windows"],"confidence":0.5,"fix_status":"unknown","needs_auth":"no","worst_case":"Viewer crashes; attacker may corrupt memory further","analyst_note":"Find the affected and fixed versions of TightVNC and UltraVNC viewers from the NVD entry or vendor advisories; the sources given do not state them.","how_to_check":"- Check the installed TightVNC or UltraVNC viewer version against the vendor's release notes, since the sources do not name affected version ranges\n- unknown","plain_action":"","product_name":"","needs_analyst":true,"not_affected_if":["trusted-servers-only"],"who_is_affected":"The viewer is the vulnerable side: a team is exposed when it connects to VNC servers it does not fully trust, such as remote-support or third-party hosts. Teams that only run VNC servers, or use viewers exclusively against their own trusted servers, are less exposed. The sources do not list affected versions.","user_interaction":"yes","product_ecosystem":"","needs_special_setup":"unknown","severity_if_unknown":"unknown"},"cost_usd":0.000747,"input_tokens":4576,"output_tokens":579,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-128321':13A,16A '-2026':12A,15A 'applic':122C 'attack':88C 'attacker-control':87C 'base':82C 'bound':56B,113C 'buffer':45B,83C,102C 'caus':108C 'check':39B 'connect':62B,119C 'contain':78C 'control':89C 'corrupt':68B,129C 'crash':63B,123C 'cve':11A,14A 'decod':72C,101C 'fix':43B,99C 'fixed-s':42B,98C 'frame':29B 'framebuff':90C 'heap':9A,57B,81C,114C 'heap-bas':80C 'impact':130C 'jpeg':28B,71C 'malici':5A,47B,104C 'memori':10A,69B,128C 'memory-corrupt':127C 'mishandl':26B 'out-of-bound':53B,110C 'overflow':8A,84C 'possibl':67B 'potenti':125C 'rectangl':32B,91C 'result':120C 'server':6A,36B,49B,106C 'size':44B,100C 'tight':27B,70C 'tightvnc':1A,18B,22B,74C 'trigger':51B 'ultravnc':3A,19B,24B,76C 'user':61B,118C 'valid':95C 'viewer':4A,25B,65B,77C 'vnc':20B,48B,105C 'width':33B,92C 'window':21B 'write':58B,115C","nicknames":[],"mitigations":[],"who_affected":"The viewer is the vulnerable side: a team is exposed when it connects to VNC servers it does not fully trust, such as remote-support or third-party hosts. Teams that only run VNC servers, or use viewers exclusively against their own trusted servers, are less exposed. The sources do not list affected versions.","tier":"C","verified":{},"analyst_revisit_at":null,"fix_status":"none","audience":"businesses","plain_action":null,"pkeys":[],"ptokens":[],"labels":["other","tightvnc","ultravnc","unknown","vnc","windows"],"campaign_type":null,"search_a":"'-128321':13A,16A '-2026':12A,15A 'cve':11A,14A 'heap':9A 'malici':5A 'memori':10A 'overflow':8A 'server':6A 'tightvnc':1A 'ultravnc':3A 'viewer':4A","search_b":"'bound':56B 'buffer':45B 'check':39B 'connect':62B 'corrupt':68B 'crash':63B 'fix':43B 'fixed-s':42B 'frame':29B 'heap':57B 'jpeg':28B 'malici':47B 'memori':69B 'mishandl':26B 'out-of-bound':53B 'possibl':67B 'rectangl':32B 'server':36B,49B 'size':44B 'tight':27B 'tightvnc':18B,22B 'trigger':51B 'ultravnc':19B,24B 'user':61B 'viewer':25B,65B 'vnc':20B,48B 'width':33B 'window':21B 'write':58B","search_c":"'applic':122C 'attack':88C 'attacker-control':87C 'base':82C 'bound':113C 'buffer':83C,102C 'caus':108C 'connect':119C 'contain':78C 'control':89C 'corrupt':129C 'crash':123C 'decod':72C,101C 'fix':99C 'fixed-s':98C 'framebuff':90C 'heap':81C,114C 'heap-bas':80C 'impact':130C 'jpeg':71C 'malici':104C 'memori':128C 'memory-corrupt':127C 'out-of-bound':110C 'overflow':84C 'potenti':125C 'rectangl':91C 'result':120C 'server':106C 'size':100C 'tight':70C 'tightvnc':74C 'ultravnc':76C 'user':118C 'valid':95C 'viewer':77C 'vnc':105C 'width':92C 'write':115C","cause_entry":null,"cause_path":null,"disclosures_checked_at":null,"first_public_at":"2026-11-13T02:18:56+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-128321","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-128321","severity":null,"published_at":"2026-11-13T02:18:56+00:00","modified_at":"2026-11-13T02:18:56+00:00","first_seen_at":"2026-11-13T02:41:37+00:00","fetched_at":"2026-11-13T02:41:37+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"}