CVE-2020-12390

Security check bypass via unbracketed IPv6 origin serialization

Published May 26, 2020 · Updated Aug 4, 2024

Origin serialization errors in Firefox before 76 allow attackers to cause incorrect security checks via crafted IPv6 URLs. nsPrincipal::GetOriginForURI concatenates an IPv6 host without restoring its square brackets, yielding invalid or potentially colliding origin strings. A security consequence requires a consumer to rely on the malformed origin; Mozilla documented no known caller that produced an exploitable result.

CVSS severity9.8
Critical
EPSS probability1.58%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Origin serialization errors in Firefox before 76 allow attackers to cause incorrect security checks via crafted IPv6 URLs. nsPrincipal::GetOriginForURI concatenates an IPv6 host without restoring its square brackets, yielding invalid or potentially colliding origin strings. A security consequence requires a consumer to rely on the malformed origin; Mozilla documented no known caller that produced an exploitable result.

The record

CVE
CVE-2020-12390
Published
May 26, 2020
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-502
Attack vector
Unavailable
Privileges
Unavailable

Timeline

How it unfolded

  1. May 26, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <76

What conditions does exploitation require?

Attack vectorUnavailable in this record.
Required privilegesUnavailable in this record.

What is affected?

Mozilla · Firefoxversion=unspecified <76

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
VectorUnavailable
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-502Deserialization of Untrusted Data

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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