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Merge pull request #81 from lukecartey/csharp/zipslip-reformat
C#: ZipSlip - Rearrange query, add help and update doc
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‎change-notes/1.18/analysis-csharp.md

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| **Query** | **Tags** | **Purpose** |
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|-----------------------------|-----------|--------------------------------------------------------------------|
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| Arbitrary file write during zip extraction ("Zip Slip") (cs/zipslip) | security external/cwe/cwe-022 | Identifies zip extraction routines which allow arbitrary file overwrite vulnerabilities.
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| Constant condition (cs/constant-condition) | More results | The query has been generalized to cover both `Null-coalescing left operand is constant (cs/constant-null-coalescing)` and `Switch selector is constant (cs/constant-switch-selector)`. |
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| Exposing internal representation (cs/expose-implementation) | Different results | The query has been rewritten, based on the equivalent Java query. |
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| Local scope variable shadows member (cs/local-shadows-member) | maintainability, readability | Replaces the existing queries [Local variable shadows class member (cs/local-shadows-class-member)](https://help.semmle.com/wiki/display/CSHARP/Local+variable+shadows+class+member), [Local variable shadows struct member (cs/local-shadows-struct-member)](https://help.semmle.com/wiki/display/CSHARP/Local+variable+shadows+struct+member), [Parameter shadows class member (cs/parameter-shadows-class-member)](https://help.semmle.com/wiki/display/CSHARP/Parameter+shadows+class+member), and [Parameter shadows struct member (cs/parameter-shadows-struct-member)](https://help.semmle.com/wiki/display/CSHARP/Parameter+shadows+struct+member). |

‎csharp/ql/src/Security Features/CWE-022/ZipSlip.qhelp

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<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>Extracting files from a malicious zip archive without validating that the destination file path
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is within the destination directory can cause files outside the destination directory to be
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overwritten, due to the possible presence of directory traversal elements (<code>..</code>) in
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archive paths.</p>
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<p>Zip archives contain archive entries representing each file in the archive. These entries
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include a file path for the entry, but these file paths are not restricted and may contain
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unexpected special elements such as the directory traversal element (<code>..</code>). If these
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file paths are used to determine an output file to write the contents of the archive item to, then
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the file may be written to an unexpected location. This can result in sensitive information being
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revealed or deleted, or an attacker being able to influence behavior by modifying unexpected
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files.</p>
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<p>For example, if a zip file contains a file entry <code>..\sneaky-file</code>, and the zip file
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is extracted to the directory <code>c:\output</code>, then naively combining the paths would result
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in an output file path of <code>c:\output\..\sneaky-file</code>, which would cause the file to be
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written to <code>c:\sneaky-file</code>.</p>
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</overview>
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<recommendation>
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<p>Ensure that output paths constructed from zip archive entries are validated to prevent writing
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files to unexpected locations.</p>
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<p>The recommended way of writing an output file from a zip archive entry is to:</p>
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<ol>
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<li>Use <code>Path.Combine(destinationDirectory, archiveEntry.FullName)</code> to determine the raw
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output path.</li>
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<li>Use <code>Path.GetFullPath(..)</code> on the raw output path to resolve any directory traversal
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elements.</li>
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<li>Use <code>Path.GetFullPath(destinationDirectory + Path.DirectorySeparatorChar)</code> to
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determine the fully resolved path of the destination directory.</li>
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<li>Validate that the resolved output path <code>StartsWith</code> the resolved destination
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directory, aborting if this is not true.</li>
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</ol>
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<p>Another alternative is to validate archive entries against a whitelist of expected files.</p>
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</recommendation>
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<example>
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<p>In this example, a file path taken from a zip archive item entry is combined with a
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destination directory. The result is used as the destination file path without verifying that
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the result is within the destination directory. If provided with a zip file containing an archive
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path like <code>..\sneaky-file</code>, then this file would be written outside the destination
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directory.</p>
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<sample src="ZipSlipBad.cs" />
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<p>To fix this vulnerability, we need to make three changes. Firstly, we need to resolve any
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directory traversal or other special characters in the path by using <code>Path.GetFullPath</code>.
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Secondly, we need to identify the destination output directory, again using
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<code>Path.GetFullPath</code>, this time on the output directory. Finally, we need to ensure that
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the resolved output starts with the resolved destination directory, and throw an exception if this
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is not the case.</p>
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<sample src="ZipSlipGood.cs" />
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</example>
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<references>
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<li>
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Snyk:
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<a href="https://snyk.io/research/zip-slip-vulnerability">Zip Slip Vulnerability</a>.
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</li>
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<li>
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OWASP:
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<a href="https://www.owasp.org/index.php/Path_traversal">Path Traversal</a>.
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</li>
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</references>
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</qhelp>

‎csharp/ql/src/Security Features/CWE-022/ZipSlip.ql

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/**
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* @name Potential ZipSlip vulnerability
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* @description When extracting files from an archive, don't add archive item's path to the target file system path. Archive path can be relative and can lead to
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* file system access outside of the expected file system target path, leading to malicious config changes and remote code execution via lay-and-wait technique
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* @name Arbitrary file write during zip extraction ("Zip Slip")
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* @description Extracting files from a malicious zip archive without validating that the
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* destination file path is within the destination directory can cause files outside
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* the destination directory to be overwritten.
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* @kind problem
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* @id cs/zipslip
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* @problem.severity error
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* @precision high
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* @tags security
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* external/cwe/cwe-022
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*/
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import csharp
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import semmle.code.csharp.security.dataflow.ZipSlip::ZipSlip
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// access to full name of the archive item
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Expr archiveFullName(PropertyAccess pa) {
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pa.getTarget().getDeclaringType().hasQualifiedName("System.IO.Compression.ZipArchiveEntry")
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and pa.getTarget().getName() = "FullName"
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and result = pa
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}
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// argument to extract to file extension method
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Expr compressionExtractToFileArgument(MethodCall mc) {
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mc.getTarget().hasQualifiedName("System.IO.Compression.ZipFileExtensions", "ExtractToFile")
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and result = mc.getArgumentForName("destinationFileName")
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}
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// File Stream created from tainted file name through File.Open/File.Create
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Expr fileOpenArgument(MethodCall mc) {
29-
(mc.getTarget().hasQualifiedName("System.IO.File", "Open") or
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mc.getTarget().hasQualifiedName("System.IO.File", "OpenWrite") or
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mc.getTarget().hasQualifiedName("System.IO.File", "Create"))
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and result = mc.getArgumentForName("path")
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}
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// File Stream created from tainted file name passed directly to the constructor
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Expr streamConstructorArgument(ObjectCreation oc) {
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oc.getTarget().getDeclaringType().hasQualifiedName("System.IO.FileStream")
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and result = oc.getArgumentForName("path")
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}
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// constructor to FileInfo can take tainted file name and subsequently be used to open file stream
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Expr fileInfoConstructorArgument(ObjectCreation oc) {
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oc.getTarget().getDeclaringType().hasQualifiedName("System.IO.FileInfo")
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and result = oc.getArgumentForName("fileName")
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}
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// extracting just file name, not the full path
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Expr fileNameExtraction(MethodCall mc) {
48-
mc.getTarget().hasQualifiedName("System.IO.Path", "GetFileName")
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and result = mc.getAnArgument()
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}
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// Checks the string for relative path, or checks the destination folder for whitelisted/target path, etc.
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Expr stringCheck(MethodCall mc) {
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(mc.getTarget().hasQualifiedName("System.String", "StartsWith") or
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mc.getTarget().hasQualifiedName("System.String", "Substring"))
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and result = mc.getQualifier()
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}
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// Taint tracking configuration for ZipSlip
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class ZipSlipTaintTrackingConfiguration extends TaintTracking::Configuration {
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ZipSlipTaintTrackingConfiguration() {
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this = "ZipSlipTaintTracking"
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}
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override predicate isSource(DataFlow::Node source) {
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exists(PropertyAccess pa |
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source.asExpr() = archiveFullName(pa))
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}
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override predicate isSink(DataFlow::Node sink) {
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exists(MethodCall mc |
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sink.asExpr() = compressionExtractToFileArgument(mc) or
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sink.asExpr() = fileOpenArgument(mc))
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or
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exists(ObjectCreation oc |
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sink.asExpr() = streamConstructorArgument(oc) or
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sink.asExpr() = fileInfoConstructorArgument(oc))
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}
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override predicate isSanitizer(DataFlow::Node node) {
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exists(MethodCall mc |
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node.asExpr() = fileNameExtraction(mc) or
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node.asExpr() = stringCheck(mc))
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}
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}
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from ZipSlipTaintTrackingConfiguration zipTaintTracking, DataFlow::Node source, DataFlow::Node sink
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from TaintTrackingConfiguration zipTaintTracking, DataFlow::Node source, DataFlow::Node sink
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where zipTaintTracking.hasFlow(source, sink)
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select sink, "Make sure to sanitize relative archive item path before creating path for file extraction if the source of $@ is untrusted", source, "zip archive"
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select sink, "Unsanitized zip archive $@, which may contain '..', is used in a file system operation.", source, "item path"

‎csharp/ql/src/Security Features/CWE-022/ZipSlipBad.cs

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using System.IO;
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using System.IO.Compression;
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class Bad
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{
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public static void WriteToDirectory(ZipArchiveEntry entry,
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string destDirectory)
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{
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string destFileName = Path.Combine(destDirectory, entry.FullName);
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entry.ExtractToFile(destFileName);
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}
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}

‎csharp/ql/src/Security Features/CWE-022/ZipSlipGood.cs

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using System.IO;
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using System.IO.Compression;
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class Good
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{
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public static void WriteToDirectory(ZipArchiveEntry entry,
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string destDirectory)
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{
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string destFileName = Path.GetFullPath(Path.Combine(destDirectory, entry.FullName));
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string fullDestDirPath = Path.GetFullPath(destDirectory + Path.DirectorySeparatorChar);
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if (!destFileName.StartsWith(fullDestDirPath)) {
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throw new System.InvalidOperationException("Entry is outside the target dir: " +
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destFileName);
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}
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entry.ExtractToFile(destFileName);
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}
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}

‎csharp/ql/src/semmle/code/csharp/security/dataflow/ZipSlip.qll

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/**
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* Provides a taint tracking configuration for reasoning about unsafe zip extraction.
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*/
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import csharp
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module ZipSlip {
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/**
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* A data flow source for unsafe zip extraction.
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*/
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abstract class Source extends DataFlow::Node { }
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/**
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* A data flow sink for unsafe zip extraction.
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*/
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abstract class Sink extends DataFlow::ExprNode { }
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/**
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* A sanitizer for unsafe zip extraction.
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*/
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abstract class Sanitizer extends DataFlow::ExprNode { }
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/** A taint tracking configuration for Zip Slip */
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class TaintTrackingConfiguration extends TaintTracking::Configuration {
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TaintTrackingConfiguration() {
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this = "ZipSlipTaintTracking"
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}
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override predicate isSource(DataFlow::Node source) {
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source instanceof Source
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}
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override predicate isSink(DataFlow::Node sink) {
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sink instanceof Sink
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}
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override predicate isSanitizer(DataFlow::Node node) {
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node instanceof Sanitizer
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}
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}
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/** An access to the `FullName` property of a `ZipArchiveEntry`. */
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class ArchiveFullNameSource extends Source {
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ArchiveFullNameSource() {
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exists(PropertyAccess pa |
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this.asExpr() = pa |
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pa.getTarget().getDeclaringType().hasQualifiedName("System.IO.Compression.ZipArchiveEntry") and
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pa.getTarget().getName() = "FullName"
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)
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}
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}
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/** An argument to the `ExtractToFile` extension method. */
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class ExtractToFileArgSink extends Sink {
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ExtractToFileArgSink() {
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exists(MethodCall mc |
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mc.getTarget().hasQualifiedName("System.IO.Compression.ZipFileExtensions", "ExtractToFile") and
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this.asExpr() = mc.getArgumentForName("destinationFileName")
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)
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}
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}
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/** A path argument to a `File.Open`, `File.OpenWrite`, or `File.Create` method call. */
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class FileOpenArgSink extends Sink {
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FileOpenArgSink() {
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exists(MethodCall mc |
66+
mc.getTarget().hasQualifiedName("System.IO.File", "Open") or
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mc.getTarget().hasQualifiedName("System.IO.File", "OpenWrite") or
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mc.getTarget().hasQualifiedName("System.IO.File", "Create") |
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this.asExpr() = mc.getArgumentForName("path")
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)
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}
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}
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/** A path argument to a call to the `FileStream` constructor. */
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class FileStreamArgSink extends Sink {
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FileStreamArgSink() {
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exists(ObjectCreation oc |
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oc.getTarget().getDeclaringType().hasQualifiedName("System.IO.FileStream") |
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this.asExpr() = oc.getArgumentForName("path")
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)
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}
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}
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/**
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* A path argument to a call to the `FileStream` constructor.
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*
87+
* This constructor can accept a tainted file name and subsequently be used to open a file stream.
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*/
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class FileInfoArgSink extends Sink {
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FileInfoArgSink() {
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exists(ObjectCreation oc |
92+
oc.getTarget().getDeclaringType().hasQualifiedName("System.IO.FileInfo") |
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this.asExpr() = oc.getArgumentForName("fileName")
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)
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}
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}
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/**
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* An argument to `GetFileName`.
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*
101+
* This is considered a sanitizer because it extracts just the file name, not the full path.
102+
*/
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class GetFileNameSanitizer extends Sanitizer {
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GetFileNameSanitizer() {
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exists(MethodCall mc |
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mc.getTarget().hasQualifiedName("System.IO.Path", "GetFileName") |
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this.asExpr() = mc.getAnArgument()
108+
)
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}
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}
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/**
113+
* A qualifier in a call to `StartsWith` or `Substring` string method.
114+
*
115+
* A call to a String method such as `StartsWith` or `Substring` can indicate a check for a
116+
* relative path, or a check against the destination folder for whitelisted/target path, etc.
117+
*/
118+
class StringCheckSanitizer extends Sanitizer {
119+
StringCheckSanitizer() {
120+
exists(MethodCall mc |
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mc.getTarget().hasQualifiedName("System.String", "StartsWith") or
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mc.getTarget().hasQualifiedName("System.String", "Substring") |
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this.asExpr() = mc.getQualifier()
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)
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}
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}
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}

‎csharp/ql/test/query-tests/Security Features/CWE-022/ZipSlip/ZipSlip.expected

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1-
| ZipSlip.cs:24:41:24:52 | access to local variable destFileName | Make sure to sanitize relative archive item path before creating path for file extraction if the source of $@ is untrusted | ZipSlip.cs:19:31:19:44 | access to property FullName | zip archive |
2-
| ZipSlip.cs:32:41:32:52 | access to local variable destFilePath | Make sure to sanitize relative archive item path before creating path for file extraction if the source of $@ is untrusted | ZipSlip.cs:16:52:16:65 | access to property FullName | zip archive |
3-
| ZipSlip.cs:61:74:61:85 | access to local variable destFilePath | Make sure to sanitize relative archive item path before creating path for file extraction if the source of $@ is untrusted | ZipSlip.cs:54:72:54:85 | access to property FullName | zip archive |
4-
| ZipSlip.cs:68:71:68:82 | access to local variable destFilePath | Make sure to sanitize relative archive item path before creating path for file extraction if the source of $@ is untrusted | ZipSlip.cs:54:72:54:85 | access to property FullName | zip archive |
5-
| ZipSlip.cs:75:57:75:68 | access to local variable destFilePath | Make sure to sanitize relative archive item path before creating path for file extraction if the source of $@ is untrusted | ZipSlip.cs:54:72:54:85 | access to property FullName | zip archive |
6-
| ZipSlip.cs:83:58:83:69 | access to local variable destFilePath | Make sure to sanitize relative archive item path before creating path for file extraction if the source of $@ is untrusted | ZipSlip.cs:54:72:54:85 | access to property FullName | zip archive |
1+
| ZipSlip.cs:24:41:24:52 | access to local variable destFileName | Unsanitized zip archive $@, which may contain '..', is used in a file system operation. | ZipSlip.cs:19:31:19:44 | access to property FullName | item path |
2+
| ZipSlip.cs:32:41:32:52 | access to local variable destFilePath | Unsanitized zip archive $@, which may contain '..', is used in a file system operation. | ZipSlip.cs:16:52:16:65 | access to property FullName | item path |
3+
| ZipSlip.cs:61:74:61:85 | access to local variable destFilePath | Unsanitized zip archive $@, which may contain '..', is used in a file system operation. | ZipSlip.cs:54:72:54:85 | access to property FullName | item path |
4+
| ZipSlip.cs:68:71:68:82 | access to local variable destFilePath | Unsanitized zip archive $@, which may contain '..', is used in a file system operation. | ZipSlip.cs:54:72:54:85 | access to property FullName | item path |
5+
| ZipSlip.cs:75:57:75:68 | access to local variable destFilePath | Unsanitized zip archive $@, which may contain '..', is used in a file system operation. | ZipSlip.cs:54:72:54:85 | access to property FullName | item path |
6+
| ZipSlip.cs:83:58:83:69 | access to local variable destFilePath | Unsanitized zip archive $@, which may contain '..', is used in a file system operation. | ZipSlip.cs:54:72:54:85 | access to property FullName | item path |
7+
| ZipSlipBad.cs:10:29:10:40 | access to local variable destFileName | Unsanitized zip archive $@, which may contain '..', is used in a file system operation. | ZipSlipBad.cs:9:59:9:72 | access to property FullName | item path |

‎csharp/ql/test/query-tests/Security Features/CWE-022/ZipSlip/ZipSlipBad.cs

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using System.IO;
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using System.IO.Compression;
3+
4+
class Bad
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{
6+
public static void WriteToDirectory(ZipArchiveEntry entry,
7+
string destDirectory)
8+
{
9+
string destFileName = Path.Combine(destDirectory, entry.FullName);
10+
entry.ExtractToFile(destFileName);
11+
}
12+
}

‎csharp/ql/test/query-tests/Security Features/CWE-022/ZipSlip/ZipSlipGood.cs

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1+
using System.IO;
2+
using System.IO.Compression;
3+
4+
class Good
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{
6+
public static void WriteToDirectory(ZipArchiveEntry entry,
7+
string destDirectory)
8+
{
9+
string destFileName = Path.GetFullPath(Path.Combine(destDirectory, entry.FullName));
10+
string fullDestDirPath = Path.GetFullPath(destDirectory + Path.DirectorySeparatorChar);
11+
if (!destFileName.StartsWith(fullDestDirPath)) {
12+
throw new System.InvalidOperationException("Entry is outside the target dir: " +
13+
destFileName);
14+
}
15+
entry.ExtractToFile(destFileName);
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}
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}

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