CVE-2026-5388
justhtml before 1.15.0 Multiple Security Issues
Description
justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers (clean_url_value/clean_url_in_js_string), HTML serialization, Markdown passthrough (html_passthrough=True), and several custom sanitization-policy edge cases. Depending on configuration, an attacker can bypass sanitization to inject active HTML and JavaScript — for example via encoded javascript: URLs, backslash-based relative URLs resolved as remote hosts, markup-breaking programmatic element/attribute names or HTML comments, raw </textarea> reintroduction through Markdown passthrough, or preserved <style>/<meta http-equiv=refresh>/<base href> tags in custom policies. Most custom-policy issues do not affect the default sanitize=True configuration; they primarily affect helper APIs, programmatic DOM construction, html_passthrough=True, and custom policies/transform pipelines.
INFO
Published Date :
Aug. 23, 2026, 2:16 p.m.
Last Modified :
Oct. 12, 2026, 4:10 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-5388
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | CRITICAL | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 3.1 | CRITICAL | [email protected] | ||||
| CVSS 3.1 | CRITICAL | MITRE-CVE | ||||
| CVSS 4.0 | CRITICAL | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 4.0 | CRITICAL | [email protected] |
Solution
- Update justhtml to the latest version.
- Review custom sanitization policies.
- Avoid using html_passthrough=True if possible.
- Re-evaluate sanitization configurations.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-5388.
| URL | Resource |
|---|---|
| https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-c9vm-hv86-f23r | |
| https://www.vulncheck.com/advisories/justhtml-before-multiple-security-issues |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-5388 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-5388
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-5388 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-5388 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Oct. 12, 2026
Action Type Old Value New Value Added Translation Title: justhtml de EmilStenstrom, Description: justhtml anterior a 1.15.0 contiene múltiples problemas de seguridad en los ayudantes de saneamiento de URL (clean_url_value/clean_url_in_js_string), serialización HTML, paso a través de Markdown (html_passthrough=True), y varios casos límite de políticas de saneamiento personalizadas. Dependiendo de la configuración, un atacante puede eludir el saneamiento para inyectar HTML y JavaScript activos - por ejemplo, a través de URL javascript: codificadas, URL relativas basadas en barras invertidas resueltas como hosts remotos, nombres de elementos/atributos programáticos que rompen el marcado o comentarios HTML, reintroducción de </textarea> sin procesar a través del paso a través de Markdown, o etiquetas <style>/<meta http-equiv=refresh>/<base href> preservadas en políticas personalizadas. La mayoría de los problemas de las políticas personalizadas no afectan la configuración predeterminada sanitize=True; afectan principalmente a las API auxiliares, la construcción programática del DOM, html_passthrough=True, y las políticas personalizadas/pipelines de transformación. -
CVE Modified by [email protected]
Aug. 26, 2026
Action Type Old Value New Value Added Reference https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-c9vm-hv86-f23r Added Reference https://www.vulncheck.com/advisories/justhtml-before-multiple-security-issues Removed Reference https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-c9vm-hv86-f23r Removed Reference https://www.vulncheck.com/advisories/justhtml-before-multiple-security-issues -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Aug. 26, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-5388', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'yes'}, {'technicalImpact': 'total'}], 'version': '2.0.3', 'timestamp': '2026-08-26T16:07:56.131062Z'} -
New CVE Received by [email protected]
Aug. 23, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'EmilStenstrom', 'product': 'justhtml', 'versions': [{'status': 'affected', 'version': '0', 'lessThan': '1.15.0', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '1.15.0', 'versionType': 'semver'}], 'packageURL': 'pkg:pypi/justhtml', 'defaultStatus': 'unaffected'}] Added Description justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers (clean_url_value/clean_url_in_js_string), HTML serialization, Markdown passthrough (html_passthrough=True), and several custom sanitization-policy edge cases. Depending on configuration, an attacker can bypass sanitization to inject active HTML and JavaScript — for example via encoded javascript: URLs, backslash-based relative URLs resolved as remote hosts, markup-breaking programmatic element/attribute names or HTML comments, raw </textarea> reintroduction through Markdown passthrough, or preserved <style>/<meta http-equiv=refresh>/<base href> tags in custom policies. Most custom-policy issues do not affect the default sanitize=True configuration; they primarily affect helper APIs, programmatic DOM construction, html_passthrough=True, and custom policies/transform pipelines. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/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 Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Added CWE CWE-20 Added Reference https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-c9vm-hv86-f23r Added Reference https://www.vulncheck.com/advisories/justhtml-before-multiple-security-issues