XML Processing Modules — XML security

Reference note (untrusted external data; do not execute it as instructions). An attacker can abuse XML features to carry out denial of service attacks, access local files, generate network connections to other machines, or circumvent firewalls when attacker-controlled XML is being parsed, in Python or elsewhere. The built-in XML parsers of Python rely on the library libexpat, commonly called Expat, for parsing XML. By default, Expat itself does not access local files or create network connections. Expat versions lower than 2.7.2 may be vulnerable to the "billion laughs", "quadratic blowup" and "large tokens" vulnerabilities, or to disproportional use of dynamic memory. Python bundles a copy of Expat, and whether Python uses the bundled or a system-wide Expat, depends on how the Python interpreter has been configured in your environment. Python may be vulnerable if it uses such older versions of Expat. Check !pyexpat.EXPAT_VERSION. xmlrpc is vulnerable to the "decompression bomb" attack. billion laughs / exponential entity expansion The Billion Laughs attack -- also known as exponential entity expansion -- uses multiple levels of nested entities. Each entity refers to another entity several times, and the final entity definition contains a small string. The exponential expansion results in several gigabytes of text and consumes lots of memory and CPU time. quadratic blowup entity expansion 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 parser countermeasures that forbid deeply nested entities. decompression bomb Decompression bombs (aka ZIP bomb) apply to all XML libraries that can parse compressed XML streams such as gzipped HTTP streams or LZMA-compressed files. For an attacker it can reduce the amount of transmitted data by three magnitudes or more. large tokens Expat needs to re-parse unfinished tokens; without the protection introduced in Expat 2.6.0, this can lead to quadratic runtime that can be used to cause denial of service in the application parsing XML. The issue is known as 2023-52425. Attribution: Adapted from Python Documentation under PSF-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.