Arbitrary File Write via Archive Extraction (Zip Slip) Affecting github.com/u-root/u-root/pkg/cpio package, versions <0.9.0

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

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

• GitHub Commit
• GitHub PR
• GitHub PR