Affected versions of the lxml package are vulnerable to XML External Entity Injection due to insecure default parser configuration that resolves external entities. The iterparse() function and the ETCompatXMLParser() class both default to resolve_entities=True, so untrusted XML input processed through either parser will expand external entity references and read referenced local files from the host. An attacker who supplies a crafted XML document to an application using these parsers in their default configuration can read sensitive local files and exfiltrate their contents through the parsed output.

Lxml 4.9.1 includes a fix for CVE-2022-2309: NULL Pointer Dereference allows attackers to cause a denial of service (or application crash). This only applies when lxml is used together with libxml2 2.9.10 through 2.9.14. libxml2 2.9.9 and earlier are not affected. It allows triggering crashes through forged input data, given a vulnerable code sequence in the application. The vulnerability is caused by the iterwalk function (also used by the canonicalize function). Such code shouldn't be in wide-spread use, given that parsing + iterwalk would usually be replaced with the more efficient iterparse function. However, an XML converter that serialises to C14N would also be vulnerable, for example, and there are legitimate use cases for this code sequence. If untrusted input is received (also remotely) and processed via iterwalk function, a crash can be triggered.

Lxml 4.6.2 includes a fix for CVE-2020-27783: A XSS vulnerability was discovered in python-lxml's clean module. The module's parser didn't properly imitate browsers, which caused different behaviors between the sanitizer and the user's page. A remote attacker could exploit this flaw to run arbitrary HTML/JS code.

Lxml version 4.6.3 includes a fix for CVE-2021-28957: An XSS vulnerability was discovered in python-lxml's clean module versions before 4.6.3. When disabling the safe_attrs_only and forms arguments, the Cleaner class does not remove the formation attribute allowing for JS to bypass the sanitizer. A remote attacker could exploit this flaw to run arbitrary JS code on users who interact with incorrectly sanitized HTML.
https://bugs.launchpad.net/lxml/+bug/1888153

Lxml 4.6.5 includes a fix for CVE-2021-43818: Prior to version 4.6.5, the HTML Cleaner in lxml.html lets certain crafted script content pass through, as well as script content in SVG files embedded using data URIs. Users that employ the HTML cleaner in a security relevant context should upgrade to lxml 4.6.5 to receive a patch.