CVE-2021-23999Network attack vector

Privilege escalation via incorrect Blob principal

Published Jun 24, 2021 · Updated Aug 3, 2024

Privilege escalation in Mozilla Firefox before 88 allows remote attackers to gain additional browser privileges via a Blob URL loaded after unusual user interaction. ContentParent::TransmitBlobURLsForPrincipal sends the caller's principal instead of the Blob's principal and accepts broader Subsumes matches, so browser-UI loads can assign the System Principal. Exploitation requires a user to copy and paste a crafted Blob URL into the address bar and load it in another process; the result is elevated browser privileges for web content, with potential compromise of Firefox or Thunderbird.

CVSS severity8.8
High
EPSS probability1.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Privilege escalation in Mozilla Firefox before 88 allows remote attackers to gain additional browser privileges via a Blob URL loaded after unusual user interaction. ContentParent::TransmitBlobURLsForPrincipal sends the caller's principal instead of the Blob's principal and accepts broader Subsumes matches, so browser-UI loads can assign the System Principal. Exploitation requires a user to copy and paste a crafted Blob URL into the address bar and load it in another process; the result is elevated browser privileges for web content, with potential compromise of Firefox or Thunderbird.

The record

CVE
CVE-2021-23999
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-697, CWE-269, T1068
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=unspecified <88

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <88
Mozilla · Thunderbirdversion=unspecified <78.10
Mozilla · Firefox ESRversion=unspecified <78.10

Published CVSS scores

8.8NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-697Incorrect Comparison
CWE-269Improper Privilege Management
T1068Exploitation for Privilege Escalation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo