CVE-2024-9393: Mozilla Firefox XSS Vulnerability

CVE-2024-9393 Overview

CVE-2024-9393 is a high-severity origin validation vulnerability [CWE-346] affecting Mozilla Firefox, Firefox ESR, and Thunderbird. An attacker can send a specially crafted multipart response that causes arbitrary JavaScript to execute under the resource://pdf.js origin. This origin context enables access to cross-origin PDF content rendered by the browser's built-in PDF viewer.

On desktop clients, the Site Isolation feature restricts the impact to same-site documents. On Android, however, attackers achieve full cross-origin access. The flaw affects Firefox versions prior to 131, Firefox ESR before 128.3 and 115.16, and Thunderbird before 128.3 and 131.

Critical Impact
Attackers can execute arbitrary JavaScript in the privileged resource://pdf.js origin and read cross-origin PDF content, with full cross-origin reach on Android.

Affected Products

Mozilla Firefox versions prior to 131
Mozilla Firefox ESR versions prior to 128.3 and prior to 115.16
Mozilla Thunderbird versions prior to 128.3 and prior to 131

Discovery Timeline

2024-10-01 - CVE-2024-9393 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-9393

Vulnerability Analysis

The vulnerability resides in how Firefox's built-in PDF viewer (pdf.js) handles multipart HTTP responses. When the browser fetches a PDF resource, a crafted multipart response can manipulate the viewer into executing attacker-controlled JavaScript within the resource://pdf.js origin context.

This origin is internal and privileged for the PDF viewer. Code running there can interact with PDF content loaded by the viewer, bypassing the Same-Origin Policy that normally prevents cross-origin document access. On Firefox for Android, where Site Isolation is not enforced, the bypass extends to any origin the user views.

Root Cause

The root cause is an origin validation failure [CWE-346] in the PDF viewer's handling of multipart streams. The viewer fails to enforce strict origin boundaries when processing multipart response parts, allowing script content to be parsed and executed in the privileged viewer origin. Refer to Mozilla Bug Report #1918301 for technical specifics.

Attack Vector

Exploitation requires a victim to load a PDF from an attacker-controlled URL or an upstream resource that returns a malicious multipart response. The attacker constructs response parts that smuggle script content interpreted by pdf.js. Once executed, the script reads PDF contents from other origins (same-site on desktop, any origin on Android). No authentication or user interaction beyond visiting the PDF is required.

No verified public proof-of-concept code is available. The vulnerability mechanism is documented in Mozilla Security Advisory MFSA-2024-46 and related advisories.

Detection Methods for CVE-2024-9393

Indicators of Compromise

HTTP responses to PDF requests with Content-Type: multipart/* headers containing unexpected script-bearing parts.
Web traffic where PDF URLs return multipart boundaries embedding HTML or JavaScript payloads.
Outbound connections from Firefox or Thunderbird processes immediately after rendering a PDF from an untrusted host.

Detection Strategies

Inspect proxy and web gateway logs for PDF responses delivered with multipart content types, which is uncommon for legitimate PDFs.
Monitor endpoint telemetry for Firefox and Thunderbird process versions and flag hosts running affected builds.
Correlate browser process activity with anomalous DOM script execution patterns sourced from resource://pdf.js.

Monitoring Recommendations

Track Firefox, Firefox ESR, and Thunderbird version inventory across managed endpoints and alert on builds below the patched versions.
Log and review user navigation to externally hosted PDF resources, particularly from email clients.
Enable browser telemetry forwarding to a central SIEM for analysis of unusual PDF viewer behavior.

How to Mitigate CVE-2024-9393

Immediate Actions Required

Update Firefox to version 131 or later on all desktop and Android devices.
Update Firefox ESR to 128.3 or 115.16 and Thunderbird to 128.3 or 131 as appropriate for your deployment channel.
Prioritize patching Android Firefox installations, where Site Isolation does not limit the impact.
Apply Debian package updates referenced in Debian LTS Announcement #4 and Debian LTS Announcement #6.

Patch Information

Mozilla released patches in Firefox 131, Firefox ESR 128.3, Firefox ESR 115.16, Thunderbird 128.3, and Thunderbird 131. Full advisory details are documented in Mozilla Security Advisory MFSA-2024-46, MFSA-2024-47, MFSA-2024-48, MFSA-2024-49, and MFSA-2024-50.

Workarounds

Configure Firefox to open PDFs in an external trusted viewer by setting pdfjs.disabled to true in about:config until patches are deployed.
Block or proxy-filter PDF responses that arrive with multipart/* content types at the network gateway.
Restrict use of Firefox for Android in high-risk environments until devices receive the 131 update.

bash

# Disable the built-in PDF viewer as a temporary workaround
# Navigate to about:config and set:
pdfjs.disabled = true

# Or deploy via enterprise policy (policies.json)
{
  "policies": {
    "Preferences": {
      "pdfjs.disabled": {
        "Value": true,
        "Status": "locked"
      }
    }
  }
}