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Fencer Vulns
high 7.5

CVE-2021-36773 json

uBlock Origin before 1.36.2 and nMatrix before 4.4.9 support an arbitrary depth of parameter nesting for strict blocking, which allows craf…

published 2021-07-18 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-23361

cvss
7.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.3%
EPSS 1.3% chance of exploitation in 30 days, percentile 69%
public exploits
0
none known

Summary

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).

Patch / fix

  • sciruby nmatrix 4.4.9
  • ublockorigin ublock_origin 1.36.2
  • umatrix_project umatrix 1.4.2
  • ublock-origin Debian 14, Debian 13, Debian 12 1.37.0+dfsg-1

debian advisories DEBIAN-CVE-2021-36773

  • firefox-ublock-origin Arch Linux 1.36.2-1

arch linux advisories AVG-2172

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-36773 updated cvss 7.5

mozilla-ublock-origin: crafted web sites may cause a denial of service

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.26% chance of exploitation in the next 30 days, higher than 69% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/vtriolet/writings/blob/main/posts/2021/ublock_origin_and_umatrix_denial_of_service.adoc referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:N/A:P

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.