MediumFix available
firefox-esr, thunderbird — security advisory
A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
- Products
- firefox-esr, thunderbird
- Affected versions
- Not specified
- Fixed versions
- 1:78.5.0-1, 78.5.0esr-1
- CVEs
- CVE-2020-26951
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 1 Nov 2026, 13:45 UTC · Revision 1
Source evidence
A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
osv:DEBIAN-CVE-2020-26951
firefox-esr
Fixed versions: 78.5.0esr-1
firefox-esr
Fixed versions: 78.5.0esr-1
firefox-esr
Fixed versions: 78.5.0esr-1
thunderbird
Fixed versions: 1:78.5.0-1
thunderbird
Fixed versions: 1:78.5.0-1
thunderbird
Fixed versions: 1:78.5.0-1
Complete source record
{
"id": "7c5d20acb094656a41ac50244c0d6721885017f65fdbdfc645b8d3f16e594b67",
"url": "https://osv.dev/vulnerability/DEBIAN-CVE-2020-26951",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "DEBIAN-CVE-2020-26951",
"summary": "A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.",
"evidence": "osv:DEBIAN-CVE-2020-26951",
"products": [
{
"name": {
"value": "firefox-esr",
"evidence": "/affected/0/package/name"
},
"prerequisites": [],
"fixed_versions": [
{
"value": "78.5.0esr-1",
"evidence": "/affected/0/ranges"
}
],
"affected_versions": []
},
{
"name": {
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"evidence": "/affected/1/package/name"
},
"prerequisites": [],
"fixed_versions": [
{
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"evidence": "/affected/1/ranges"
}
],
"affected_versions": []
},
{
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},
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"affected_versions": []
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{
"name": {
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{
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"affected_versions": []
},
{
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"affected_versions": []
}
],
"severity": "medium",
"structured": {
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"aliases": [
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],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
}
],
"affected": [
{
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"events": [
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{
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],
"package": {
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"purl": "pkg:deb/debian/firefox-esr?arch=source&distro=bookworm",
"ecosystem": "Debian:12"
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"database_specific": {
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"ecosystem_specific": {
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{
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"package": {
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"ecosystem": "Debian:13"
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{
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"package": {
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"ecosystem": "Debian:14"
},
"database_specific": {
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},
"ecosystem_specific": {
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"package": {
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},
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{
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],
"package": {
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"ecosystem": "Debian:13"
},
"database_specific": {
"source": "https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2020-26951.json"
},
"ecosystem_specific": {
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}
},
{
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],
"package": {
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"purl": "pkg:deb/debian/thunderbird?arch=source&distro=forky",
"ecosystem": "Debian:14"
},
"database_specific": {
"source": "https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2020-26951.json"
},
"ecosystem_specific": {
"urgency": "not yet assigned"
}
}
],
"modified": "2026-10-26T12:02:41.458718529Z",
"provider": "osv",
"upstream": [
"CVE-2020-26951"
],
"references": [
"https://security-tracker.debian.org/tracker/CVE-2020-26951"
]
},
"related_cves": [
{
"value": "CVE-2020-26951",
"evidence": "/upstream"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 452769,
"published_at": "2020-12-09T01:15:12.643+00:00"
}