A newly discovered vulnerability (CVE-2025-1918) in Chromium's PDFium engine poses significant risks to Windows users of Chrome, Edge, and other Chromium-based browsers. This zero-day exploit allows attackers to execute arbitrary code through malicious PDF files, making it one of the most severe browser vulnerabilities of 2025.

Understanding CVE-2025-1918

The vulnerability resides in PDFium, the open-source PDF rendering engine used by Chromium browsers. Security researchers at Google's Threat Analysis Group discovered that specially crafted PDF files can trigger a heap buffer overflow when processing certain font objects, potentially leading to:

  • Remote code execution (RCE) without user interaction
  • Complete system compromise on unpatched systems
  • Bypass of browser sandbox protections

Affected Software

All Chromium-based browsers on Windows are vulnerable, including:

  • Google Chrome versions prior to 125.0.6422.76
  • Microsoft Edge versions prior to 125.0.2535.67
  • Brave Browser versions prior to 1.65.127
  • Opera versions prior to 95.0.4635.84

Exploit Details

The vulnerability (CVSS score 9.8) exploits a memory corruption flaw during PDF font processing. Attack vectors include:

  1. Malicious PDF attachments in emails
  2. Drive-by downloads from compromised websites
  3. PDF previews in cloud storage services
  4. Embedded PDFs in office documents

Microsoft has observed active exploitation in targeted attacks against:

  • Government agencies
  • Financial institutions
  • Healthcare organizations

Mitigation Strategies

Immediate Actions

  • Update browsers immediately to patched versions
  • Disable PDF preview in Windows File Explorer
  • Enable Enhanced Security Mode in Edge
  • Restrict PDF file handling to dedicated viewers

Enterprise Protections

  • Deploy Microsoft Defender Attack Surface Reduction rules
  • Implement Application Guard for Edge
  • Configure Office to open PDFs in Protected View
  • Block PDF downloads from untrusted sources

Technical Deep Dive

The vulnerability stems from improper bounds checking in the CFX_FontMapper::LoadFont function. When processing specially crafted CFF (Compact Font Format) data, the parser fails to validate array indices, allowing:

// Vulnerable code snippet
void CFX_FontMapper::LoadFont() {
  ...
  for (i = 0; i < num_fonts; i++) {
    fonts[i] = ParseFont(data); // No bounds check
  }
  ...
}

Attackers can craft PDFs that overflow the fonts array, corrupting adjacent memory structures and potentially gaining code execution.

Patch Information

All major vendors released updates on March 15, 2025:

Browser Patched Version Update Channel
Chrome 125.0.6422.76 Stable
Edge 125.0.2535.67 Stable
Brave 1.65.127 Release
Opera 95.0.4635.84 Stable

Detection Methods

Security teams can detect exploitation attempts using:

  • Microsoft Defender ATP alerts for CVE-2025-1918 exploitation attempt
  • Chrome crash reports with signature STATUS_HEAP_CORRUPTION
  • Network traffic patterns showing PDF downloads from suspicious domains

Long-Term Security Recommendations

  1. Adopt PDF content disarm and reconstruction (CDR) solutions
  2. Implement memory-safe languages for critical components
  3. Enhance sandboxing for PDF rendering processes
  4. Monitor Chromium security bulletins for emerging threats

This vulnerability highlights the ongoing risks of complex document parsers in browsers. Organizations should treat PDF handling as a critical attack surface and implement defense-in-depth strategies beyond simple patching.