Arbitrary file access during archive extraction (”Zip Slip”)¶
ID: js/zipslipKind: path-problemSecurity severity: 7.5Severity: errorPrecision: highTags: - security - external/cwe/cwe-022Query suites: - javascript-code-scanning.qls - javascript-security-extended.qls - javascript-security-and-quality.qls
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Extracting files from a malicious zip file, or similar type of archive, is at risk of directory traversal attacks if filenames from the archive are not properly validated. archive paths.
Zip archives contain archive entries representing each file in the archive. These entries include a file path for the entry, but these file paths are not restricted and may contain unexpected special elements such as the directory traversal element (..). If these file paths are used to create a filesystem path, then a file operation may happen in an unexpected location. This can result in sensitive information being revealed or deleted, or an attacker being able to influence behavior by modifying unexpected files.
For example, if a zip file contains a file entry..\sneaky-file, and the zip file is extracted to the directoryc:\output, then naively combining the paths would result in an output file path ofc:\output\..\sneaky-file, which would cause the file to be written toc:\sneaky-file.
Recommendation¶
Ensure that output paths constructed from zip archive entries are validated to prevent writing files to unexpected locations.
The recommended way of writing an output file from a zip archive entry is to check that".." does not occur in the path.
Example¶
In this example an archive is extracted without validating file paths. Ifarchive.zip contained relative paths (for instance, if it were created by something likeziparchive.zip../file.txt) then executing this code could write to locations outside the destination directory.
constfs=require('fs');constunzip=require('unzip');fs.createReadStream('archive.zip').pipe(unzip.Parse()).on('entry',entry=>{constfileName=entry.path;// BAD: This could write any file on the filesystem.entry.pipe(fs.createWriteStream(fileName));});
To fix this vulnerability, we need to check that the path does not contain any".." elements in it.
constfs=require('fs');constunzip=require('unzip');fs.createReadStream('archive.zip').pipe(unzip.Parse()).on('entry',entry=>{constfileName=entry.path;// GOOD: ensures the path is safe to write to.if(fileName.indexOf('..')==-1){entry.pipe(fs.createWriteStream(fileName));}else{console.log('skipping bad path',fileName);}});
References¶
Snyk:Zip Slip Vulnerability.
OWASP:Path Traversal.
Common Weakness Enumeration:CWE-22.