EUVD-2026-64653
GHSA-q3jf-9wv7-p29w
Severity by source
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/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
Network-delivered payload requires victim to render Markdown (UI:R); scope changes to browser context (S:C); XSS yields limited confidentiality and integrity impact only.
Primary rating from Vendor (VulnCheck).
CVSS VectorVendor: VulnCheck
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/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
Lifecycle Timeline
3Blast Radius
ecosystem impact- 1 pypi packages depend on justhtml (1 direct, 0 indirect)
Ecosystem-wide dependent count for version 1.13.0.
DescriptionCVE.org
justhtml versions before 1.13.0 contain a cross-site scripting vulnerability in the to_markdown() function when serializing attacker-controlled pre content. Attackers can place backticks inside sanitized pre elements to break out of fixed-length code fences, allowing raw HTML to execute when the generated Markdown is rendered by CommonMark or GFM-style renderers.
AnalysisAI
Cross-site scripting in justhtml versions before 1.13.0 allows attackers who control <pre> element content to break out of fixed-length Markdown code fences and inject raw HTML into the serialized output. When an application passes attacker-supplied HTML through JustHTML(..., sanitize=True).to_markdown() and a downstream CommonMark or GFM renderer processes the result with raw-HTML enabled, the injected script executes in the victim's browser. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires three chained conditions: (1) the application passes attacker-controlled HTML that includes `<pre>` elements to `JustHTML(..., sanitize=True).to_markdown()` - applications that only call `to_html()` are unaffected; (2) the attacker can embed a backtick run of three or more characters and HTML-entity-encoded tags inside the `<pre>` element; and (3) the resulting Markdown output is rendered by a CommonMark- or GFM-compatible renderer that permits raw HTML passthrough (e.g., GitHub Flavored Markdown in permissive mode, marked.js with `sanitize: false`). … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N, score 5.1) accurately reflects the multi-hop nature of this vulnerability: impact is limited to the subsequent system (browser/renderer) and requires active user interaction to render the crafted Markdown. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker submits a comment or document containing `<pre>``` <img src=x onerror=alert(document.cookie)></pre>` to an application that processes user HTML through `JustHTML(..., sanitize=True).to_markdown()` and then displays the result in a web page via a CommonMark renderer. The fixed ``` delimiter is broken by the backtick run, placing the decoded `<img>` tag outside the code fence; when a victim views the page, the `onerror` handler fires and the attacker's script executes. … |
| Remediation | Upgrade justhtml to version 1.13.0 or later via `pip install --upgrade justhtml`; the fix is confirmed in patch commit f35f8f723c713bd8f912d86e9ec6881275ff5af9 (https://github.com/EmilStenstrom/justhtml/commit/f35f8f723c713bd8f912d86e9ec6881275ff5af9). … Detailed patch versions, workarounds, and compensating controls in full report. |
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Same weakness CWE-80 – Basic XSS
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External POC / Exploit Code
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