{"id":"CVE-2020-6820","published":"2020-04-24T16:15:13Z","modified":"2026-06-17T03:23:51Z","title":"Mozilla Firefox And Thunderbird Use-After-Free Vulnerability","vendor":"Mozilla","product":"Firefox and Thunderbird","tier":"core","eol":null,"description":{"text":"Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.","provenance":"source","from":"NVD"},"summary":{"text":"A race condition in the handling of ReadableStream objects in Mozilla Firefox and Thunderbird leads to a use-after-free flaw. An attacker with network access can trigger it without authentication or user interaction, though exploitation requires specific conditions that make it harder to reliably achieve.","provenance":"ai","model":"claude-haiku-5-5","generatedAt":"2026-10-23T07:13:35Z"},"cvss":{"score":8.1,"severity":"HIGH","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H","provenance":"source","from":"NVD"},"epss":{"score":0.07063,"percentile":0.9419,"date":"2026-10-29","provenance":"source"},"kev":{"dateAdded":"2021-11-03","dueDate":"2022-05-03","ransomware":false,"requiredAction":"Apply updates per vendor instructions.","notes":"https://nvd.nist.gov/vuln/detail/CVE-2020-6820","provenance":"source"},"vulncheckKev":{"dateAdded":"2020-04-01","exploitedFirstSeen":"2020-04-01","ransomware":false,"evidence":[{"url":"https://docs.google.com/spreadsheets/d/1lkNJ0uQwbeC1ZTRrxdtuPLCIl7mlUreoKfSIgajnSyY/edit","date":"2020-04-01"},{"url":"https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json","date":"2021-11-03"}],"evidenceTotal":2,"provenance":"source"},"exploited":true,"otx":{"pulseCount":50,"pulses":[{"name":"Actively_Exploited_Kev | Oct 29, 2026","url":"https://otx.alienvault.com/pulse/6ae20c1db17e7057ae7d9ad1","created":"2026-10-28"},{"name":"Actively_Exploited_Kev | Oct 28, 2026","url":"https://otx.alienvault.com/pulse/6ae0ba7e0e2a97ff7b218819","created":"2026-10-27"},{"name":"Actively_Exploited_Kev | Oct 27, 2026","url":"https://otx.alienvault.com/pulse/6adf68b85c3e852f3c111d18","created":"2026-10-26"},{"name":"Actively_Exploited_Kev | Oct 24, 2026","url":"https://otx.alienvault.com/pulse/6adb740e397c3ad4799aa4cd","created":"2026-10-23"},{"name":"Actively_Exploited_Kev | Oct 23, 2026","url":"https://otx.alienvault.com/pulse/6ada236fcfba2dfa730ea482","created":"2026-10-22"},{"name":"Actively_Exploited_Kev | Oct 22, 2026","url":"https://otx.alienvault.com/pulse/6ad8d33a077cbb5cab524d80","created":"2026-10-21"},{"name":"Actively_Exploited_Kev | Oct 21, 2026","url":"https://otx.alienvault.com/pulse/6ad78130c4d1ac2be83a8b6d","created":"2026-10-20"},{"name":"Actively_Exploited_Kev | Oct 20, 2026","url":"https://otx.alienvault.com/pulse/6ad62f63c24a405a3423cc13","created":"2026-10-19"},{"name":"Actively_Exploited_Kev | Oct 19, 2026","url":"https://otx.alienvault.com/pulse/6ad4dda174e5797d0bd238b7","created":"2026-10-18"},{"name":"Actively_Exploited_Kev | Oct 18, 2026","url":"https://otx.alienvault.com/pulse/6ad38c22e1848f8189d1c121","created":"2026-10-17"}],"provenance":"machine"},"cwes":[{"id":"CWE-362","name":"Concurrent Execution using Shared Resource with Improper Synchronization"}],"references":[{"url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1626728","tags":["Issue Tracking","Permissions Required"]},{"url":"https://usn.ubuntu.com/4335-1/","tags":["Third Party Advisory"]},{"url":"https://www.mozilla.org/security/advisories/mfsa2020-11/","tags":["Vendor Advisory"]},{"url":"https://www.mozilla.org/security/advisories/mfsa2020-14/","tags":["Vendor Advisory"]},{"url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1626728","tags":["Issue Tracking","Permissions Required"]},{"url":"https://usn.ubuntu.com/4335-1/","tags":["Third Party Advisory"]},{"url":"https://www.mozilla.org/security/advisories/mfsa2020-11/","tags":["Vendor Advisory"]},{"url":"https://www.mozilla.org/security/advisories/mfsa2020-14/","tags":["Vendor Advisory"]},{"url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-6820","tags":["US Government Resource"]}],"techniques":[{"id":"T1055","name":"Process Injection","tactics":["privilege-escalation","stealth"],"provenance":"source","basis":"ctid-cve","bases":["ctid-cve"],"basisDetail":"CTID attack_to_cve (Phase 1)","mappingType":"uncategorized","evidence":[],"confidence":"low","confidenceNote":"CTID attack_to_cve Phase 1 \"Uncategorized\" mapping (not staged by impact)"},{"id":"T1189","name":"Drive-by Compromise","tactics":["initial-access"],"provenance":"source","basis":"ctid-cve","bases":["ctid-cve","ai-mapping"],"basisDetail":"CTID attack_to_cve (Phase 1); claude-haiku-5-5: Drive-by via crafted content; targeted in-the-wild abuse","mappingType":"uncategorized","evidence":[{"from":"description","quote":"We are aware of targeted attacks in the wild abusing this flaw","start":103,"end":165}],"confidence":"low","confidenceNote":"server-side bug exploitable without user interaction; client-execution technique does not fit"},{"id":"T1203","name":"Exploitation for Client Execution","tactics":["execution"],"provenance":"ai","basis":"ai-mapping","bases":["ai-mapping"],"basisDetail":"claude-haiku-5-5: Memory corruption yields arbitrary code execution in client","mappingType":"primary_impact","evidence":[{"from":"description","quote":"a race condition can cause a use-after-free","start":58,"end":101}],"model":"claude-haiku-5-5","confidence":"low","confidenceNote":"server-side bug exploitable without user interaction; client-execution technique does not fit"}],"reports":[],"affected":[],"advisories":[],"euvd":{"id":"EUVD-2020-27964","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2020-27964","exploited":true,"exploitedSince":"2021-11-03","provenance":"source"},"exploits":[],"exploitSources":null,"exploitRefs":[],"flow":[{"stage":"related","techniques":["T1189","T1203","T1055"],"provenance":"ai","lowConfidence":true}],"timeline":[{"date":"2020-04-01","kind":"exploited-reported","label":"Exploitation first reported (VulnCheck KEV evidence)","url":"https://docs.google.com/spreadsheets/d/1lkNJ0uQwbeC1ZTRrxdtuPLCIl7mlUreoKfSIgajnSyY/edit","provenance":"source"},{"date":"2020-04-24","kind":"published","label":"Published (NVD)","url":"https://nvd.nist.gov/vuln/detail/CVE-2020-6820","provenance":"source"},{"date":"2021-11-03","kind":"kev","label":"Added to CISA KEV","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","provenance":"source"},{"date":"2021-11-03","kind":"euvd-exploited","label":"ENISA EUVD: exploited","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2020-27964","provenance":"source"}],"speed":{"daysToPublicExploit":null,"firstExploitFrom":null,"daysToKev":558},"priority":{"score":62.5,"factors":[{"name":"In a KEV catalog","points":35.0,"detail":"listed in CISA KEV and VulnCheck KEV"},{"name":"Recent KEV listing","points":0.0,"detail":"added 2402 days ago"},{"name":"Known ransomware use","points":0.0,"detail":"not known"},{"name":"EPSS percentile","points":18.84,"detail":"percentile 0.9419"},{"name":"Named in threat reports","points":0.0,"detail":"0 reports"},{"name":"Public exploit code","points":0.0,"detail":"none found"},{"name":"Mentioned in OTX pulses","points":3.0,"detail":"50 pulses"},{"name":"CVSS severity","points":5.67,"detail":"base score 8.1"},{"name":"End-of-life product","points":0.0,"detail":"supported or unknown"}]}}