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| Vulnerability ID | VCID-72kf-cb91-dkcy |
| Aliases |
GHSA-5vp3-3cg6-2rq3
|
| Summary | JustHTML is vulnerable to XSS via code fence breakout in <pre> content ## Summary `to_markdown()` is vulnerable when serializing attacker-controlled `<pre>` content. The `<pre>` handler emits a fixed three-backtick fenced code block, but writes decoded text content into that fence without choosing a delimiter longer than any backtick run inside the content. An attacker can place backticks and HTML-like text inside a sanitized `<pre>` element so that the generated Markdown closes the fence early and leaves raw HTML outside the code block. When that Markdown is rendered by a CommonMark/GFM-style renderer that allows raw HTML, the HTML executes. This is a bypass of the v1.12.0 Markdown hardening. That fix escaped HTML-significant characters for regular text nodes, but `<pre>` uses a separate serialization path and does not apply the same protection. ## Details The vulnerable `<pre>` Markdown path: - extracts decoded text from the `<pre>` subtree - opens a fenced block with a fixed delimiter of `````` - writes the decoded text directly into the output - closes with another fixed `````` Because the fence length is fixed, attacker-controlled content containing a backtick run of length 3 or more can terminate the code block. If the content also contains decoded HTML-like text such as `<img ...>`, that text appears outside the fence in the resulting Markdown and is treated as raw HTML by downstream Markdown renderers. The issue is not that HTML-like text appears inside code blocks. The issue is that the serializer allows attacker-controlled `<pre>` text to break out of the fixed fence. ## Reproduction ```python from justhtml import JustHTML payload = "<pre>```\n<img src=x onerror=alert(1)></pre>" doc = JustHTML(payload, fragment=True) # default sanitize=True print(doc.to_html(pretty=False)) # <pre>``` # <img src=x onerror=alert(1)></pre> print(doc.to_markdown()) # ``` # ``` # <img src=x onerror=alert(1)> # ``` ``` Rendered as CommonMark/GFM-style Markdown, that output is interpreted as: 1. Line 1 opens a fenced code block 2. Line 2 closes it 3. Line 3 is raw HTML outside the fence 4. Line 4 opens a new fence ## Impact Applications that treat `JustHTML(..., sanitize=True).to_markdown()` output as safe for direct rendering in Markdown contexts may be exposed to XSS, depending on the downstream Markdown renderer's raw-HTML handling. ## Root Cause The `<pre>` Markdown serializer uses a fixed fence instead of selecting a delimiter longer than the longest backtick run in the content. ## Fix When serializing `<pre>` content to Markdown, choose a fence length longer than any backtick run present in the code block content, with a minimum length of 3. |
| Status | Published |
| Exploitability | 0.5 |
| Weighted Severity | 8.0 |
| Risk | 4.0 |
| Affected and Fixed Packages | Package Details |
| System | Score | Found at |
|---|---|---|
| cvssv3.1_qr | HIGH | https://github.com/advisories/GHSA-5vp3-3cg6-2rq3 |
| cvssv4 | 7.1 | https://github.com/EmilStenstrom/justhtml |
| generic_textual | HIGH | https://github.com/EmilStenstrom/justhtml |
| cvssv4 | 7.1 | https://github.com/EmilStenstrom/justhtml/commit/f35f8f723c713bd8f912d86e9ec6881275ff5af9 |
| generic_textual | HIGH | https://github.com/EmilStenstrom/justhtml/commit/f35f8f723c713bd8f912d86e9ec6881275ff5af9 |
| cvssv4 | 7.1 | https://github.com/EmilStenstrom/justhtml/releases/tag/v1.13.0 |
| generic_textual | HIGH | https://github.com/EmilStenstrom/justhtml/releases/tag/v1.13.0 |
| cvssv3.1_qr | HIGH | https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3 |
| cvssv4 | 7.1 | https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3 |
| generic_textual | HIGH | https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3 |
| Attack Vector (AV) | Attack Complexity (AC) | Attack Requirements (AT) | Privileges Required (PR) | User Interaction (UI) | Vulnerable System Impact Confidentiality (VC) | Vulnerable System Impact Integrity (VI) | Vulnerable System Impact Availability (VA) | Subsequent System Impact Confidentiality (SC) | Subsequent System Impact Integrity (SI) | Subsequent System Impact Availability (SA) |
|---|---|---|---|---|---|---|---|---|---|---|
network adjacent local physical |
low high |
none present |
none low high |
none passive active |
high low none |
high low none |
high low none |
high low none |
high low none |
high low none |
| Attack Vector (AV) | Attack Complexity (AC) | Attack Requirements (AT) | Privileges Required (PR) | User Interaction (UI) | Vulnerable System Impact Confidentiality (VC) | Vulnerable System Impact Integrity (VI) | Vulnerable System Impact Availability (VA) | Subsequent System Impact Confidentiality (SC) | Subsequent System Impact Integrity (SI) | Subsequent System Impact Availability (SA) |
|---|---|---|---|---|---|---|---|---|---|---|
network adjacent local physical |
low high |
none present |
none low high |
none passive active |
high low none |
high low none |
high low none |
high low none |
high low none |
high low none |
| Attack Vector (AV) | Attack Complexity (AC) | Attack Requirements (AT) | Privileges Required (PR) | User Interaction (UI) | Vulnerable System Impact Confidentiality (VC) | Vulnerable System Impact Integrity (VI) | Vulnerable System Impact Availability (VA) | Subsequent System Impact Confidentiality (SC) | Subsequent System Impact Integrity (SI) | Subsequent System Impact Availability (SA) |
|---|---|---|---|---|---|---|---|---|---|---|
network adjacent local physical |
low high |
none present |
none low high |
none passive active |
high low none |
high low none |
high low none |
high low none |
high low none |
high low none |
| Attack Vector (AV) | Attack Complexity (AC) | Attack Requirements (AT) | Privileges Required (PR) | User Interaction (UI) | Vulnerable System Impact Confidentiality (VC) | Vulnerable System Impact Integrity (VI) | Vulnerable System Impact Availability (VA) | Subsequent System Impact Confidentiality (SC) | Subsequent System Impact Integrity (SI) | Subsequent System Impact Availability (SA) |
|---|---|---|---|---|---|---|---|---|---|---|
network adjacent local physical |
low high |
none present |
none low high |
none passive active |
high low none |
high low none |
high low none |
high low none |
high low none |
high low none |
No EPSS data available for this vulnerability.
| Date | Actor | Action | Source | VulnerableCode Version |
|---|---|---|---|---|
| 2026-06-12T07:48:50.485641+00:00 | GithubOSV Importer | Import | https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-5vp3-3cg6-2rq3/GHSA-5vp3-3cg6-2rq3.json | 38.6.0 |