{"id":"CVE-2020-6820","kind":"vulnerability","aliases":["CVE-2020-6820"],"related":[],"title":"Mozilla Firefox and Thunderbird: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability","description":"Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts.\n\nCISA required action: Apply updates per vendor instructions.","summary":"Mozilla Firefox and Thunderbird have a race condition in how they handle a ReadableStream, which leads to a use-after-free memory bug. The sources say the impact is unspecified, but the bug is listed as actively exploited in the wild.","severity":"high","cvss":8.1,"epss":0.07063,"exploited":true,"exploited_at":"2021-11-03T00:00:00+00:00","products":[{"name":"Mozilla Firefox and Thunderbird","fixed":[],"affected":"","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["desktop","exploited","firefox","linux","macos","runtime","thunderbird","windows"],"not_affected_if":["unknown"],"how_to_check":"Check the installed Firefox or Thunderbird version under Help > About; unknown for command-line checks.","fix":"Update Firefox and Thunderbird to the current vendor release, as CISA directs (apply updates per vendor instructions); unknown for exact fixed version.","status":"open","published_at":"2020-04-24T16:15:13+00:00","first_seen_at":"2026-10-23T19:11:53+00:00","triaged_at":"2026-10-23T21:37:18+00:00","updated_at":"2026-10-23T21:57:34+00:00","category":"runtime","confidence":0.6,"triage":{"at":"2026-10-23T21:37:18Z","model":"claude-haiku-5-5","output":{"fix":"Update Firefox and Thunderbird to the current vendor release, as CISA directs (apply updates per vendor instructions); unknown for exact fixed version.","reach":"unknown","impact":"high","summary":"Mozilla Firefox and Thunderbird have a race condition in how they handle a ReadableStream, which leads to a use-after-free memory bug. The sources say the impact is unspecified, but the bug is listed as actively exploited in the wild.","category":"runtime","who_cares":["firefox","thunderbird","desktop","windows","macos","linux"],"confidence":0.6,"needs_auth":"unknown","worst_case":"Attacker may corrupt memory in the browser or mail client, possibly running code.","analyst_note":"Find the Mozilla advisory for CVE-2020-6820 and the fixed Firefox and Thunderbird versions, and confirm the trigger conditions and whether user interaction is needed.","how_to_check":"Check the installed Firefox or Thunderbird version under Help > About; unknown for command-line checks.","needs_analyst":true,"not_affected_if":["unknown"],"user_interaction":"unknown","needs_special_setup":"unknown","severity_if_unknown":"high"},"cost_usd":0.000498,"input_tokens":2873,"output_tokens":422,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2020':15A,18A '-6820':16A,19A 'action':105C 'activ':69B 'and':22A 'appli':106C 'bug':55B,65B 'caus':100C 'certain':88C 'cisa':103C 'condit':39B,81C,89C,92C 'contain':78C 'creat':93C 'cve':14A,17A 'desktop':24B 'exploit':25B,70B 'firefox':2A,6A,21A,26B,33B,75C 'free':12A,53B,98C 'handl':43B,84C 'impact':60B,102C 'instruct':110C 'lead':47B 'linux':27B 'list':67B 'maco':28B 'memori':54B 'mozilla':1A,5A,20A,32B,74C 'per':108C 'race':38B,80C,91C 'readablestream':45B,86C 'requir':104C 'runtim':29B 'say':58B 'sourc':57B 'thunderbird':4A,8A,23A,30B,35B,77C 'unspecifi':62B,101C 'updat':107C 'use':10A,51B,96C 'use-after-fre':9A,50B,95C 'vendor':109C 'vulner':13A,82C,99C 'wild':73B 'window':31B","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2020-04-24T16:15:13+00:00","kev":{"dateAdded":"2021-11-03","dueDate":"2022-05-03","requiredAction":"Apply updates per vendor instructions.","knownRansomwareCampaignUse":"Unknown"},"cvss_vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H","cvss_by":"nvd@nist.gov","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2020-6820","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2020-6820","severity":null,"published_at":"2021-11-03T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:53+00:00","fetched_at":"2026-10-25T11:39:18+00:00","note":null},{"source":"epss","external_id":"CVE-2020-6820","url":"https://api.first.org/data/v1/epss?cve=CVE-2020-6820","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-23T19:13:43+00:00","fetched_at":"2026-10-23T19:13:43+00:00","note":null},{"source":"nvd","external_id":"CVE-2020-6820","url":"https://nvd.nist.gov/vuln/detail/CVE-2020-6820","severity":null,"published_at":"2020-04-24T16:15:13+00:00","modified_at":"2026-06-17T03:23:51+00:00","first_seen_at":"2026-10-23T21:57:34+00:00","fetched_at":"2026-10-23T21:57:34+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"}