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/**
* Provides classes modeling security-relevant aspects of the `lxml` PyPI package.
*
* See
* - https://pypi.org/project/lxml/
* - https://lxml.de/tutorial.html
*/
private import python
private import semmle.python.dataflow.new.DataFlow
private import semmle.python.Concepts
private import semmle.python.ApiGraphs
/**
* Provides classes modeling security-relevant aspects of the `lxml` PyPI package
*
* See
* - https://pypi.org/project/lxml/
* - https://lxml.de/tutorial.html
*/
private module Lxml {
// ---------------------------------------------------------------------------
// XPath
// ---------------------------------------------------------------------------
/**
* A class constructor compiling an XPath expression.
*
* from lxml import etree
* find_text = etree.XPath("`sink`")
* find_text = etree.ETXPath("`sink`")
*
* See
* - https://lxml.de/apidoc/lxml.etree.html#lxml.etree.XPath
* - https://lxml.de/apidoc/lxml.etree.html#lxml.etree.ETXPath
*/
private class XPathClassCall extends XML::XPathConstruction::Range, DataFlow::CallCfgNode {
XPathClassCall() {
this = API::moduleImport("lxml").getMember("etree").getMember(["XPath", "ETXPath"]).getACall()
}
override DataFlow::Node getXPath() { result in [this.getArg(0), this.getArgByName("path")] }
override string getName() { result = "lxml.etree" }
}
/**
* A call to the `xpath` method of a parsed document.
*
* from lxml import etree
* root = etree.fromstring(file(XML_DB).read(), XMLParser())
* find_text = root.xpath("`sink`")
*
* See https://lxml.de/apidoc/lxml.etree.html#lxml.etree._ElementTree.xpath
* as well as
* - https://lxml.de/apidoc/lxml.etree.html#lxml.etree.parse
* - https://lxml.de/apidoc/lxml.etree.html#lxml.etree.fromstring
* - https://lxml.de/apidoc/lxml.etree.html#lxml.etree.fromstringlist
* - https://lxml.de/apidoc/lxml.etree.html#lxml.etree.HTML
* - https://lxml.de/apidoc/lxml.etree.html#lxml.etree.XML
*/
class XPathCall extends XML::XPathExecution::Range, DataFlow::CallCfgNode {
XPathCall() {
exists(API::Node parseResult |
parseResult =
API::moduleImport("lxml")
.getMember("etree")
.getMember(["parse", "fromstring", "fromstringlist", "HTML", "XML"])
.getReturn()
or
// TODO: lxml.etree.parseid(<text>)[0] will contain the root element from parsing <text>
// but we don't really have a way to model that nicely.
parseResult =
API::moduleImport("lxml")
.getMember("etree")
.getMember("XMLParser")
.getReturn()
.getMember("close")
.getReturn()
|
this = parseResult.getMember("xpath").getACall()
)
}
override DataFlow::Node getXPath() { result in [this.getArg(0), this.getArgByName("_path")] }
override string getName() { result = "lxml.etree" }
}
class XPathEvaluatorCall extends XML::XPathExecution::Range, DataFlow::CallCfgNode {
XPathEvaluatorCall() {
this =
API::moduleImport("lxml")
.getMember("etree")
.getMember("XPathEvaluator")
.getReturn()
.getACall()
}
override DataFlow::Node getXPath() { result = this.getArg(0) }
override string getName() { result = "lxml.etree" }
}
// ---------------------------------------------------------------------------
// Parsing
// ---------------------------------------------------------------------------
/**
* Provides models for `lxml.etree` parsers.
*
* See https://lxml.de/apidoc/lxml.etree.html?highlight=xmlparser#lxml.etree.XMLParser
*/
module XmlParser {
/**
* A source of instances of `lxml.etree` parsers, extend this class to model new instances.
*
* This can include instantiations of the class, return values from function
* calls, or a special parameter that will be set when functions are called by an external
* library.
*
* Use the predicate `XmlParser::instance()` to get references to instances of `lxml.etree` parsers.
*/
abstract class InstanceSource extends DataFlow::LocalSourceNode {
/** Holds if this instance is vulnerable to `kind`. */
abstract predicate vulnerableTo(XML::XmlParsingVulnerabilityKind kind);
}
/**
* A call to `lxml.etree.XMLParser`.
*
* See https://lxml.de/apidoc/lxml.etree.html?highlight=xmlparser#lxml.etree.XMLParser
*/
private class LxmlParser extends InstanceSource, API::CallNode {
LxmlParser() {
this = API::moduleImport("lxml").getMember("etree").getMember("XMLParser").getACall()
}
// NOTE: it's not possible to change settings of a parser after constructing it
override predicate vulnerableTo(XML::XmlParsingVulnerabilityKind kind) {
kind.isXxe() and
(
// resolve_entities has default True
not exists(this.getArgByName("resolve_entities"))
or
this.getKeywordParameter("resolve_entities").getAValueReachingSink().asExpr() =
any(True t)
)
or
kind.isXmlBomb() and
this.getKeywordParameter("huge_tree").getAValueReachingSink().asExpr() = any(True t) and
not this.getKeywordParameter("resolve_entities").getAValueReachingSink().asExpr() =
any(False t)
or
kind.isDtdRetrieval() and
this.getKeywordParameter("load_dtd").getAValueReachingSink().asExpr() = any(True t) and
this.getKeywordParameter("no_network").getAValueReachingSink().asExpr() = any(False t)
}
}
/**
* A call to `lxml.etree.get_default_parser`.
*
* See https://lxml.de/apidoc/lxml.etree.html?highlight=xmlparser#lxml.etree.get_default_parser
*/
private class LxmlDefaultParser extends InstanceSource, DataFlow::CallCfgNode {
LxmlDefaultParser() {
this =
API::moduleImport("lxml").getMember("etree").getMember("get_default_parser").getACall()
}
override predicate vulnerableTo(XML::XmlParsingVulnerabilityKind kind) {
// as highlighted by
// https://lxml.de/apidoc/lxml.etree.html?highlight=xmlparser#lxml.etree.XMLParser
// by default XXE is allow. so as long as the default parser has not been
// overridden, the result is also vuln to XXE.
kind.isXxe()
// TODO: take into account that you can override the default parser with `lxml.etree.set_default_parser`.
}
}
/** Gets a reference to an `lxml.etree` parsers instance, with origin in `origin` */
private DataFlow::TypeTrackingNode instance(DataFlow::TypeTracker t, InstanceSource origin) {
t.start() and
result = origin
or
exists(DataFlow::TypeTracker t2 | result = instance(t2, origin).track(t2, t))
}
/** Gets a reference to an `lxml.etree` parsers instance, with origin in `origin` */
DataFlow::Node instance(InstanceSource origin) {
instance(DataFlow::TypeTracker::end(), origin).flowsTo(result)
}
/** Gets a reference to an `lxml.etree` parser instance, that is vulnerable to `kind`. */
DataFlow::Node instanceVulnerableTo(XML::XmlParsingVulnerabilityKind kind) {
exists(InstanceSource origin | result = instance(origin) and origin.vulnerableTo(kind))
}
/**
* A call to the `feed` method of an `lxml` parser.
*/
private class LxmlParserFeedCall extends DataFlow::MethodCallNode, XML::XmlParsing::Range {
LxmlParserFeedCall() { this.calls(instance(_), "feed") }
override DataFlow::Node getAnInput() { result in [this.getArg(0), this.getArgByName("data")] }
override predicate vulnerableTo(XML::XmlParsingVulnerabilityKind kind) {
this.calls(instanceVulnerableTo(kind), "feed")
}
override predicate mayExecuteInput() { none() }
override DataFlow::Node getOutput() {
exists(DataFlow::Node objRef |
DataFlow::localFlow(this.getObject(), objRef) and
result.(DataFlow::MethodCallNode).calls(objRef, "close")
)
}
}
}
/**
* A call to either of:
* - `lxml.etree.fromstring`
* - `lxml.etree.fromstringlist`
* - `lxml.etree.XML`
* - `lxml.etree.XMLID`
* - `lxml.etree.parse`
* - `lxml.etree.parseid`
*
* See
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.fromstring
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.fromstringlist
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.XML
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.XMLID
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.parse
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.parseid
*/
private class LxmlParsing extends DataFlow::CallCfgNode, XML::XmlParsing::Range {
string functionName;
LxmlParsing() {
functionName in ["fromstring", "fromstringlist", "XML", "XMLID", "parse", "parseid"] and
this = API::moduleImport("lxml").getMember("etree").getMember(functionName).getACall()
}
override DataFlow::Node getAnInput() {
result in [
this.getArg(0),
// fromstring / XML / XMLID
this.getArgByName("text"),
// fromstringlist
this.getArgByName("strings"),
// parse / parseid
this.getArgByName("source"),
]
}
DataFlow::Node getParserArg() { result in [this.getArg(1), this.getArgByName("parser")] }
override predicate vulnerableTo(XML::XmlParsingVulnerabilityKind kind) {
this.getParserArg() = XmlParser::instanceVulnerableTo(kind)
or
kind.isXxe() and
not exists(this.getParserArg())
}
override predicate mayExecuteInput() { none() }
override DataFlow::Node getOutput() {
// Note: for `parseid`/XMLID the result of the call is a tuple with `(root, dict)`, so
// maybe we should not just say that the entire tuple is the decoding output... my
// gut feeling is that THIS instance doesn't matter too much, but that it would be
// nice to be able to do this in general. (this is a problem for both `lxml.etree`
// and `xml.etree`)
result = this
}
}
/**
* A call to `lxml.etree.ElementTree.parse` or `lxml.etree.ElementTree.parseid`, which
* takes either a filename or a file-like object as argument. To capture the filename
* for path-injection, we have this subclass.
*
* See
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.parse
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.parseid
*/
private class FileAccessFromLxmlParsing extends LxmlParsing, FileSystemAccess::Range {
FileAccessFromLxmlParsing() {
functionName in ["parse", "parseid"]
// I considered whether we should try to reduce FPs from people passing file-like
// objects, which will not be a file system access (and couldn't cause a
// path-injection).
//
// I suppose that once we have proper flow-summary support for file-like objects,
// we can make the XXE/XML-bomb sinks allow an access-path, while the
// path-injection sink wouldn't, and then we will not end up with such FPs.
}
override DataFlow::Node getAPathArgument() { result = this.getAnInput() }
}
/**
* A call to `lxml.etree.iterparse`
*
* See
* - https://lxml.de/apidoc/lxml.etree.html?highlight=parseids#lxml.etree.iterparse
*/
private class LxmlIterparseCall extends API::CallNode, XML::XmlParsing::Range,
FileSystemAccess::Range {
LxmlIterparseCall() {
this = API::moduleImport("lxml").getMember("etree").getMember("iterparse").getACall()
}
override DataFlow::Node getAnInput() { result in [this.getArg(0), this.getArgByName("source")] }
override predicate vulnerableTo(XML::XmlParsingVulnerabilityKind kind) {
// note that there is no `resolve_entities` argument, so it's not possible to turn off XXE :O
kind.isXxe()
or
kind.isXmlBomb() and
this.getKeywordParameter("huge_tree").getAValueReachingSink().asExpr() = any(True t)
or
kind.isDtdRetrieval() and
this.getKeywordParameter("load_dtd").getAValueReachingSink().asExpr() = any(True t) and
this.getKeywordParameter("no_network").getAValueReachingSink().asExpr() = any(False t)
}
override predicate mayExecuteInput() { none() }
override DataFlow::Node getOutput() { result = this }
override DataFlow::Node getAPathArgument() { result = this.getAnInput() }
}
}