CVE-2013-1331

What is Microsoft Office?

Microsoft Office is the world's most widely used productivity suite. Office documents (Word, Excel, PowerPoint) support embedded images, including PNG files. The PNG image parser within Microsoft Office is responsible for decoding PNG image data embedded in documents — and a flaw in this parser can be triggered by any Office document containing a maliciously crafted PNG image, regardless of which Office application opens the document.

Overview

CVE-2013-1331 is a buffer overflow vulnerability (CWE-120) in Microsoft Office's PNG image parser. When Office processes a crafted Office document containing a specially malformed PNG image, a classic buffer overflow condition is triggered, allowing arbitrary code execution in the context of the user who opened the document. Microsoft patched this in Security Bulletin MS13-051 on June 11, 2013.

Affected Versions

Note: Office 2007, 2010, and 2013 were not affected — the vulnerable PNG parsing code was specific to Office 2003 and Office for Mac 2011.

Technical Details

PNG (Portable Network Graphics) is a lossless image format with a complex internal structure — chunks with length fields, various filter types, and optional extensions. Buffer overflow vulnerabilities in PNG parsers typically occur when a length or size field in the PNG data is used to allocate or write to a buffer without proper bounds validation.

In CVE-2013-1331, a malformed PNG chunk in an embedded image triggers an out-of-bounds write (CWE-120) in Office 2003's PNG parsing routine. The overflow corrupts heap memory, which can be leveraged for code execution by a skilled attacker using heap spray techniques to shape the memory layout.

Attack delivery: A malicious Office document (.doc, .xls, .ppt) containing a specially crafted embedded PNG image. The victim opens the document, Office renders the PNG, and the overflow is triggered. RTF or DOC files with embedded PNG images were the typical delivery vehicle, sent via spear-phishing emails.

The Local attack vector reflects document-based delivery from the local filesystem (email attachment).

Discovery

The vulnerability was discovered through security research and coordinated with Microsoft, resulting in inclusion in the June 2013 Patch Tuesday cycle (MS13-051).

Exploitation Context

CISA confirmed in-the-wild exploitation. Office 2003 remained widely deployed in enterprise environments long past its end-of-life, making PNG parsing vulnerabilities in Office 2003 an attractive target for attackers. CVE-2013-1331 was used in spear-phishing campaigns targeting organizations still running Office 2003.

Remediation

1. Apply MS13-051 on all systems running Office 2003 SP3 or Office for Mac 2011
2. Microsoft Office 2003 reached end-of-life on April 8, 2014 — any remaining Office 2003 deployments should be replaced with supported Office versions immediately
3. Upgrade to Office 2016 or later, which receives Active Security Updates and includes modern memory-safety mitigations
4. Enable Office Protected View for documents received from email or downloaded from the internet — this opens documents in a sandboxed read-only mode before the user enables editing