19.5. XML处理模块

用于处理XML的Python接口分组在 xml 包中。

警告

The XML modules are not secure against erroneous or maliciously constructed data. If you need to parse untrusted or unauthenticated data see XML 漏洞.

值得注意的是 xml 包中的模块要求至少有一个 SAX 兼容的 XML 解析器可用。在 Pythonm中包含 Expat 解析器，因此 xml.parsers.expat 模块将始终可用。

xml.dom 和 xml.sax 包的文档是 DOM 和 SAX 接口的 Python 绑定的定义。

XML 处理子模块包括:

• xml.etree.ElementTree： ElementTree API，一个简单而轻量级的XML处理器

• xml.dom：DOM API 定义

• xml.dom.minidom：最小的 DOM 实现

• xml.dom.pulldom：支持构建部分 DOM 树

• xml.sax：SAX2 基类和便利函数

• xml.parsers.expat：Expat解析器绑定

19.6. XML 漏洞

The XML processing modules are not secure against maliciously constructed data. An attacker can abuse vulnerabilities for e.g. denial of service attacks, to access local files, to generate network connections to other machines, or to or circumvent firewalls. The attacks on XML abuse unfamiliar features like inline DTD (document type definition) with entities.

The following table gives an overview of the known attacks and if the various modules are vulnerable to them.

种类	sax	etree	minidom	pulldom	xmlrpc
billion laughs	易受攻击	易受攻击	易受攻击	易受攻击	易受攻击
quadratic blowup	易受攻击	易受攻击	易受攻击	易受攻击	易受攻击
external entity expansion	易受攻击	安全 (1)	安全 (2)	易受攻击	安全 (3)
DTD retrieval	易受攻击	安全	安全	易受攻击	安全
decompression bomb	安全	安全	安全	安全	易受攻击

1. xml.etree.ElementTree doesn’t expand external entities and raises a ParserError when an entity occurs.

2. xml.dom.minidom 不会扩展外部实体，只是简单地返回未扩展的实体。

3. xmlrpclib 不扩展外部实体并省略它们。

billion laughs / exponential entity expansion （狂笑/递归实体扩展）

The Billion Laughs attack – also known as exponential entity expansion – uses multiple levels of nested entities. Each entity refers to another entity several times, the final entity definition contains a small string. Eventually the small string is expanded to several gigabytes. The exponential expansion consumes lots of CPU time, too.

quadratic blowup entity expansion（二次爆炸实体扩展）

A quadratic blowup attack is similar to a Billion Laughs attack; it abuses entity expansion, too. Instead of nested entities it repeats one large entity with a couple of thousand chars over and over again. The attack isn’t as efficient as the exponential case but it avoids triggering countermeasures of parsers against heavily nested entities.

external entity expansion

Entity declarations can contain more than just text for replacement. They can also point to external resources by public identifiers or system identifiers. System identifiers are standard URIs or can refer to local files. The XML parser retrieves the resource with e.g. HTTP or FTP requests and embeds the content into the XML document.

DTD retrieval

Python 的一些 XML 库 xml.dom.pulldom 从远程或本地位置检索文档类型定义。 该功能与外部实体扩展问题具有相似的含义。

decompression bomb

The issue of decompression bombs (aka ZIP bomb) apply to all XML libraries that can parse compressed XML stream like gzipped HTTP streams or LZMA-ed files. For an attacker it can reduce the amount of transmitted data by three magnitudes or more.

The documentation of defusedxml on PyPI has further information about all known attack vectors with examples and references.

19.6.1. defused packages

These external packages are recommended for any code that parses untrusted XML data.

defusedxml is a pure Python package with modified subclasses of all stdlib XML parsers that prevent any potentially malicious operation. The package also ships with example exploits and extended documentation on more XML exploits like xpath injection.

defusedexpat provides a modified libexpat and patched replacement pyexpat extension module with countermeasures against entity expansion DoS attacks. Defusedexpat still allows a sane and configurable amount of entity expansions. The modifications will be merged into future releases of Python.

The workarounds and modifications are not included in patch releases as they break backward compatibility. After all inline DTD and entity expansion are well-defined XML features.