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CVE-2026-59926

Mistune: XSS via unescaped class option in Admonition directive

5.3 MediumFix available

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
Passive

What an attacker can affect

Confidentiality
Low
Integrity
Low
Availability
None

Exploit likelihood

0.33%Unchanged since Jul 9, 2026
90%50%10%1%

What each source reports

CVE List Score used 5.3

Mistune: XSS via unescaped class option in Admonition directive

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
CVE-2026-59926
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
Weakness
CWE-79
Updated
2026-11-02
EUVDEUVD-2026-42334 5.3

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
EUVD-2026-42334
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-g97x-gvcm-x72h 5.3

Mistune: XSS via unescaped class option in Admonition directive

In src/mistune/directives/admonition.py, the render_admonition() function concatenates the :class: option directly into the HTML class attribute without escaping (lines 63-68).

This allows attribute injection and XSS even when HTMLRenderer(escape=True) is used.

The directive name parameter is safe (validated against whitelist), but the class option comes from raw user input.

Record
GHSA-g97x-gvcm-x72h
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
Weakness
CWE-79
Updated
2026-11-02
NVD 5.3

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
CVE-2026-59926
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Weakness
CWE-79
Updated
2026-11-02
OSV 5.3

Mistune: XSS via unescaped class option in Admonition directive

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
CVE-2026-59926
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
Weakness
CWE-79
Updated
2026-11-02
OSVDEBIAN-CVE-2026-59926 6.1

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
DEBIAN-CVE-2026-59926
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-02
Show 6 moreShow less
OSVGHSA-g97x-gvcm-x72h 5.3

Mistune: XSS via unescaped class option in Admonition directive

In src/mistune/directives/admonition.py, the render_admonition() function concatenates the :class: option directly into the HTML class attribute without escaping (lines 63-68).

This allows attribute injection and XSS even when HTMLRenderer(escape=True) is used.

The directive name parameter is safe (validated against whitelist), but the class option comes from raw user input.

Record
GHSA-g97x-gvcm-x72h
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
Weakness
CWE-79
Updated
2026-11-02
OSVPYSEC-2026-2214 6.1

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
PYSEC-2026-2214
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-02
OSVUBUNTU-CVE-2026-59926 5.3

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
UBUNTU-CVE-2026-59926
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-02
OSVAZL-92214 No score

CVE-2026-59926 affecting package python-mistune for versions less than 3.3.0-1

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.

Record
AZL-92214
Updated
2026-11-05
OSVopenSUSE-SU-2026:21339-1 No score

Security update for python-mistune

This update for python-mistune fixes the following issues

  • CVE-2026-59922: quadratic-time parsing on long runs of some markers in formatting.py can lead to DoS (bsc#1271117).
  • CVE-2026-59923: HTMLRenderer.safe_url() does not block percent-encoded javascript URIs and allows for XSS (bsc#1271119).
  • CVE-2026-59924: improper processing of user-supplied include paths in Include.parse() can lead to path traversal and arbitrary file reads (bsc#1271121).
  • CVE-2026-59925: quadratic-time parsing on long runs of some emphasis pairs in inline_parser can lead to DoS (bsc#1271125).
  • CVE-2026-59926: improper escaping in render_admonition() can lead to atribute injection and XSS (bsc#1271127).
  • CVE-2026-59927: uncontrolled recursion when processing two markdown files that include each other can lead to a DoS (bsc#1271128).
  • CVE-2026-59928: quadratic-time parsing on long lists of repeated reference-link definitions in block_parser can lead to DoS (bsc#1271131).
  • CVE-2026-59929: HARMFUL_PROTOCOLS list misses legacy and chained schemes and allow arbitrary script execution in user agents (bsc#1271132).
  • CVE-2026-59930: the toc plugin and TableOfContents directive generate heading IDs with predictable values and allow for collisions with attacker-controlled id="toc_N" content (bsc#1271082).
Record
openSUSE-SU-2026:21339-1
Updated
2026-11-02
OSVSUSE-SU-2026:22655-1 No score

Security update for python-mistune

This update for python-mistune fixes the following issues

  • CVE-2026-59922: quadratic-time parsing on long runs of some markers in formatting.py can lead to DoS (bsc#1271117).
  • CVE-2026-59923: HTMLRenderer.safe_url() does not block percent-encoded javascript URIs and allows for XSS (bsc#1271119).
  • CVE-2026-59924: improper processing of user-supplied include paths in Include.parse() can lead to path traversal and arbitrary file reads (bsc#1271121).
  • CVE-2026-59925: quadratic-time parsing on long runs of some emphasis pairs in inline_parser can lead to DoS (bsc#1271125).
  • CVE-2026-59926: improper escaping in render_admonition() can lead to atribute injection and XSS (bsc#1271127).
  • CVE-2026-59927: uncontrolled recursion when processing two markdown files that include each other can lead to a DoS (bsc#1271128).
  • CVE-2026-59928: quadratic-time parsing on long lists of repeated reference-link definitions in block_parser can lead to DoS (bsc#1271131).
  • CVE-2026-59929: HARMFUL_PROTOCOLS list misses legacy and chained schemes and allow arbitrary script execution in user agents (bsc#1271132).
  • CVE-2026-59930: the toc plugin and TableOfContents directive generate heading IDs with predictable values and allow for collisions with attacker-controlled id="toc_N" content (bsc#1271082).
Record
SUSE-SU-2026:22655-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
mistune
PyPI
 before 3.3.0
3.3.0
python-mistune
Azure Linux:3
 before 3.3.0-1
3.3.0-1
mistune
Debian:13
 All versions, plus listed versions
3.1.3-13.1.4-13.3.4-13.4.0-1
 None known
mistune
Debian:14
 before 3.1.4-1, plus listed versions
3.1.3-1
3.1.4-1
mistune
PyPI
 before 3.3.0, plus listed versions
0.1.00.2.00.3.00.3.10.4
Show 45 moreShow less
0.4.10.50.5.10.60.70.7.10.7.20.7.30.7.40.80.8.10.8.20.8.30.8.42.0.02.0.0a12.0.0a22.0.0a32.0.0a42.0.0a52.0.0a62.0.0rc12.0.12.0.22.0.32.0.42.0.52.1.03.0.03.0.0a13.0.0a23.0.0a33.0.0rc13.0.0rc23.0.0rc33.0.0rc43.0.0rc53.0.13.0.23.1.03.1.13.1.23.1.33.1.43.2.0
3.3.0
python-mistune
SUSE:Linux Enterprise Server 16.0
From an advisory covering 10 CVEs
 before 3.1.3-160000.5.1
3.1.3-160000.5.1
python-mistune
SUSE:Linux Enterprise Server for SAP applications 16.0
From an advisory covering 10 CVEs
 before 3.1.3-160000.5.1
3.1.3-160000.5.1
mistune
Ubuntu:16.04:LTS
 All versions, plus listed versions
0.6-20.7.1-1
 None known
mistune
Ubuntu:18.04:LTS
 All versions, plus listed versions
0.7.4-10.8-10.8.1-10.8.3-2
 None known
mistune
Ubuntu:20.04:LTS
 All versions, plus listed versions
0.8.4-10.8.4-2
 None known
Show 6 moreShow less
Affected packages
PackageAffectedFixed
mistune
Ubuntu:22.04:LTS
 All versions, plus listed versions
0.8.4-40.8.4-52.0.0-1+really0.8.4-1
 None known
mistune
Ubuntu:24.04:LTS
 All versions, plus listed versions
2.0.4-23.0.2-1
 None known
mistune
Ubuntu:25.10
 All versions, plus listed versions
3.0.2-23.1.3-1
 None known
mistune
Ubuntu:26.10
 All versions, plus listed versions
3.1.4-1
 None known
mistune
Ubuntu:26.04:LTS
 All versions, plus listed versions
3.1.3-13.1.4-1
 None known
python-mistune
openSUSE:Leap 16.0
From an advisory covering 10 CVEs
 before 3.1.3-160000.5.1
3.1.3-160000.5.1

History

History
RecordedChangedSources
affected
factsOSV
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-59926 is a medium vulnerability (CVSS 5.3) in Lepture Mistune, published July 8, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.33%, the 25th percentile. Fixed in mistune 3.3.0.

References

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