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CAPEC-201

Serialized Data External Linking

High severityHigh likelihoodDetailed pattern

An adversary creates a serialized data file (e.g. XML, YAML, etc...) that contains an external data reference. Because serialized data parsers may not validate documents with external references, there may be no checks on the nature of the reference in the external data. This can allow an adversary to open arbitrary files or connections, which may further lead to the adversary gaining access to information on the system that they would normally be unable to obtain.

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Show the promptExplain the attack pattern CAPEC-201: Serialized Data External Linking: how it unfolds, what it needs and how to prevent and detect it in my systems. Use https://capec.mitre.org/data/definitions/201.html and https://cve.bz/cve/capec/201?via=ask-ai-copy, which lists recent CVEs with the weaknesses it exploits. Cite the sources.

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How the attack unfolds

  1. Explore

    The attacker looks for a way in.

    1. [Survey the target] Using a browser or an automated tool, an adversary records all instances of web services that process requests with serialized data.

      2 techniques
      • Use an automated tool to record all instances of URLs that process requests with serialized data.
      • Use a browser to manually explore the website and analyze how the application processes serialized data requests.

  2. Exploit

    They carry out the attack.

    1. [Craft malicious payload] The adversary crafts malicious data message that contains references to sensitive files.

    2. [Launch an External Linking attack] Send the malicious crafted message containing the reference to a sensitive file to the target URL.

Prerequisites

  • The target must follow external data references without validating the validity of the reference target.

Skills required

  • Low skill

    To send serialized data messages with maliciously crafted schema.

Resources required

  • None: No specialized resources are required to execute this type of attack.

Consequences

What a successful attack can lead to
ImpactAffectsNote
Read DataConfidentiality

Mitigations

  • Configure the serialized data processor to only retrieve external entities from trusted sources.

Weaknesses it exploits

  • CWE-829 Inclusion of Functionality from Untrusted Control Sphere

Recent CVEs with these weaknesses

  1. 4.4CVE-2026-123858

    In JetBrains PhpStorm before 2026.3 project files could be automatically uploaded to a deployment server from untrusted projects

  2. 5.5CVE-2026-123854

    In JetBrains PyCharm before 2026.3 code execution was possible in Safe Mode via a crafted Jupyter notebook widget module

  3. 7.8CVE-2026-123853

    In JetBrains IntelliJ IDEA before 2026.3, 2026.2.2 code execution was possible in Safe Mode via project-controlled JVM options of the build process

  4. 7.0CVE-2026-123849

    In JetBrains IntelliJ IDEA before 2026.3 code execution was possible in untrusted projects via the Terraform File Watchers

  5. 3.3CVE-2026-123843

    In JetBrains IntelliJ IDEA before 2026.3 untrusted projects could trigger changes to GPG agent configuration

  6. 7.8CVE-2026-123838

    In JetBrains IntelliJ IDEA before 2026.3 code execution was possible in Safe Mode via UI Designer form file

  7. 3.6CVE-2026-123833

    In JetBrains IntelliJ IDEA before 2026.3 database connections could be established in untrusted projects

  8. 9.6CVE-2026-123832

    In JetBrains CLion before 2026.2 code execution was possible in Safe Mode via JSON compilation database

The newest records naming one of them. Each weakness's page lists them all.

Part of

Examples

The following DTD would attempt to open the /dev/tty device:

The following DTD would attempt to open the /dev/tty device:

<!DOCTYPE doc [ <!ENTITY ent SYSTEM "file:///dev/tty"> ]>

A malicious actor could use this crafted DTD to reveal sensitive information.

The following XML snippet would attempt to open the /etc/passwd file:

The following XML snippet would attempt to open the /etc/passwd file:

<foo xmlns:xi="http://www.w3.org/2001/XInclude"> <xi:include parse="text" href="file:///etc/passwd"/></foo>

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