CVE-2021-36773: uBlock Origin before 1.36.2 and nMatrix before 4.4.9 support an arbitrary depth of parameter nesting for stric

Q: How do I fix CVE-2021-36773?

nmatrix: upgrade to 4.4.9; ublock origin: upgrade to 1.36.2; umatrix: upgrade to 1.4.2. Exact commands for each OS are on this page.

Q: Is CVE-2021-36773 being exploited?

CVE-2021-36773 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-36773 affect?

CVE-2021-36773 affects nmatrix, ublock origin, umatrix. Severity is high (CVSS 7.5). uBlock Origin before 1.36.2 and nMatrix before 4.4.9 support an arbitrary depth of parameter nesting for strict blocking, which allows crafted web sites to cause a denial of service (unbounded recursion that can trigger memory consumption a

Q: How urgent is CVE-2021-36773?

Priority 59/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

This week

HIGH 7.5REMOTEWATCHLISTUPGRADEUPGRADEUPGRADE

CVE-2021-36773 is a high-severity flaw (CVSS 7.5) affecting nmatrix, ublock origin, umatrix. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

uBlock Origin before 1.36.2 and nMatrix before 4.4.9 support an arbitrary depth of parameter nesting for strict blocking, which allows crafted web sites to cause a denial of service (unbounded recursion that can trigger memory consumption and a loss of all blocking functionality).

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.3% (top 30%)
CISA exploited list	No
Weakness	CWE-674
Published	2021-07-18
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · sciruby · ublockorigin · umatrix project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-07-18 04:15 UTCPublished CVE record published
2026-06-17 03:59 UTCUpdated Record last modified

sciruby nmatrix UPGRADE

Affected: < 4.4.9

Fixed in: 4.4.9

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy nmatrix || rpm -q nmatrix || nmatrix --version

2. UPGRADE to 4.4.9 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade nmatrix

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade nmatrix

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

ublockorigin ublock origin UPGRADE

Affected: < 1.36.2

Fixed in: 1.36.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy ublock-origin || rpm -q ublock-origin || ublock-origin --version

2. UPGRADE to 1.36.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade ublock-origin

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade ublock-origin

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

umatrix project umatrix UPGRADE

Affected: < 1.4.2

Fixed in: 1.4.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy umatrix || rpm -q umatrix || umatrix --version

2. UPGRADE to 1.4.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade umatrix

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade umatrix

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

debian debian linux MITIGATE

Affected: 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-36773?

nmatrix: upgrade to 4.4.9; ublock origin: upgrade to 1.36.2; umatrix: upgrade to 1.4.2. Exact commands for each OS are on this page.

Is CVE-2021-36773 being exploited?

CVE-2021-36773 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-36773 affect?

CVE-2021-36773 affects nmatrix, ublock origin, umatrix. Severity is high (CVSS 7.5). uBlock Origin before 1.36.2 and nMatrix before 4.4.9 support an arbitrary depth of parameter nesting for strict blocking, which allows crafted web sites to cause a denial of service (unbounded recursion that can trigger memory consumption a

How urgent is CVE-2021-36773?

Priority 59/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

References

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