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Fencer Vulns
medium 6.1

CVE-2020-6816 json

Bleach vulnerable to mutation XSS via whitelisted math or svg and raw tag

published 2020-03-24 6 y ago modified 2026-07-08 4 mo ago also GHSA-m6xf-fq7q-8743 , PYSEC-2020-28 , EUVD-2020-0055

cvss
6.1
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 71%
public exploits
0
none known

Summary

In Mozilla Bleach before 3.12, a mutation XSS in bleach.clean when RCDATA and either svg or math tags are whitelisted and the keyword argument strip=False.

Patch / fix

  • pypi bleach 3.1.2
  • mozilla bleach 3.1.2
  • python-bleach Debian 14, Debian 13, Debian 12 3.1.3-1

debian advisories DEBIAN-CVE-2020-6816

  • python-bleach Ubuntu Pro 16.04 LTS 1.4.2-1ubuntu0.1~esm1
  • python-bleach Ubuntu Pro 18.04 LTS 2.1.2-1ubuntu0.1~esm1
  • python-bleach Ubuntu Pro 20.04 LTS 3.1.1-1ubuntu0.1~esm1

ubuntu advisories UBUNTU-CVE-2020-6816

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • python-bleach openSUSE Tumbleweed 6.4.0-1.1
  • python-bleach openSUSE Tumbleweed 6.1.0-1.5
  • python-bleach Package Hub 15 SP2 3.1.5-bp152.2.4.1
  • python-bleach openSUSE Leap 15.2 3.1.5-lp152.2.3.1

opensuse advisories openSUSE-SU-2026:11323-1openSUSE-SU-2024:14134-1openSUSE-SU-2021:0571-1openSUSE-SU-2021:0552-1

  • py3-bleach Alpine 3.24, Alpine 3.23, Alpine 3.22 +11 more 3.1.2-r0

alpine advisories ALPINE-CVE-2020-6816

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

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-2020-6816 updated cvss 6.8

python-bleach: Mutation cross-site scripting in bleach.clean

  • 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

1.34% chance of exploitation in the next 30 days, higher than 71% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • checkmarx.com https://www.checkmarx.com/blog/vulnerabilities-discovered-in-mozilla-bleach referenced PoC, tagged "exploit" by a source, unverified
  • advisory.checkmarx.net https://advisory.checkmarx.net/advisory/CX-2020-4277 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N
GitHub 3.1 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
pypi bleach
<3.1.2
3.1.2

Affected products

  • n/a Mozilla Bleach

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation

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.