CVE-2024-7788: LibreOffice Signature Forgery Vulnerability

CVE-2024-7788 Overview

CVE-2024-7788 is an Improper Digital Signature Invalidation vulnerability affecting The Document Foundation's LibreOffice office suite. The flaw exists in the Zip Repair Mode functionality and enables signature forgery attacks, potentially allowing malicious actors to craft documents that appear to have valid digital signatures when they do not.

Digital signatures in office documents serve as critical trust indicators, validating document authenticity and integrity. When this verification process is compromised, attackers can distribute malicious or tampered documents that falsely appear to be legitimately signed by trusted entities, bypassing security controls that rely on signature validation.

Critical Impact

Attackers can forge digital signatures on LibreOffice documents, enabling distribution of malicious content that appears authentically signed, potentially leading to document tampering, malware distribution, or social engineering attacks.

Affected Products

• LibreOffice versions 24.2 through 24.2.4
• LibreOffice installations utilizing Zip Repair Mode functionality
• Document workflows relying on digital signature verification

Discovery Timeline

• 2024-09-17 - CVE-2024-7788 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-7788

Vulnerability Analysis

The vulnerability is classified under CWE-347 (Improper Verification of Cryptographic Signature). The core issue resides in how LibreOffice handles digital signature validation when processing documents through its Zip Repair Mode.

LibreOffice documents (ODT, ODS, ODP, etc.) are stored as ZIP archives containing XML content and associated resources. When a document's ZIP structure is damaged or malformed, LibreOffice's Zip Repair Mode attempts to recover the content. However, during this repair process, the signature validation mechanism fails to properly invalidate signatures that should no longer be trusted after document modification.

This improper invalidation allows an attacker to manipulate signed documents in a way that preserves the appearance of valid signatures despite content modifications. The vulnerability requires local access and user interaction—specifically, the victim must open a crafted malicious document.

Root Cause

The root cause stems from improper handling of cryptographic signature state during the ZIP archive repair process. When LibreOffice repairs a malformed document archive, the signature validation logic does not correctly detect that the repaired content may differ from the originally signed content. This creates a disconnect between the signature status displayed to the user and the actual integrity of the document contents.

The signature verification implementation fails to account for modifications made during the repair process, allowing the original signature to persist even when document contents have been altered. This represents a fundamental flaw in the trust chain between content integrity verification and signature display.

Attack Vector

The attack vector is local with user interaction required. An attacker must craft a malicious LibreOffice document with a specifically malformed ZIP structure that triggers the Zip Repair Mode while preserving signature validation bypass conditions.

A typical attack scenario involves:

1. Creating a legitimately signed LibreOffice document
2. Manipulating the document's ZIP structure to introduce specific malformations that trigger repair mode
3. Modifying the document content (inserting malicious macros, changing text, adding malicious links)
4. Distributing the crafted document to victims via email, file sharing, or other channels
5. When opened, LibreOffice's repair mode processes the document while incorrectly maintaining the signature as valid

The attacker can leverage this to distribute documents containing malicious content under the guise of trusted, signed documents—potentially bypassing organizational security policies that trust digitally signed documents.

Detection Methods for CVE-2024-7788

Indicators of Compromise

• LibreOffice documents that trigger Zip Repair Mode warnings upon opening
• Documents with digital signatures from unexpected or impersonated sources
• Unusual document metadata inconsistencies between signed content and actual content
• Documents exhibiting repair mode behavior combined with signature validation displays

Detection Strategies

• Monitor for LibreOffice Zip Repair Mode activation events in endpoint telemetry
• Implement document scanning solutions that validate signature integrity independently of LibreOffice
• Deploy behavioral detection for documents that exhibit characteristics of signature forgery attempts
• Audit received documents with digital signatures for structural anomalies before trusting

Monitoring Recommendations

• Enable logging of LibreOffice document processing events on managed endpoints
• Implement email gateway scanning for malformed ODF document structures
• Monitor for suspicious document sources claiming trusted signatures
• Establish baseline patterns for legitimate signed document workflows to identify anomalies

How to Mitigate CVE-2024-7788

Immediate Actions Required

• Upgrade LibreOffice to version 24.2.5 or later immediately
• Review any documents with digital signatures received during the vulnerable period
• Implement additional validation for signed documents in high-security workflows
• Alert users about the potential for forged signatures in LibreOffice documents

Patch Information

The Document Foundation has addressed this vulnerability in LibreOffice version 24.2.5 and later releases. Organizations should upgrade all LibreOffice installations to the patched version as soon as possible. Detailed information about the security fix is available in the LibreOffice Security Advisory.

For Debian-based systems, security updates have been released as detailed in the Debian LTS Announcement. Administrators should ensure package repositories are updated and patches are applied across all managed systems.

Workarounds

• Disable automatic Zip Repair Mode if configuration options allow
• Implement secondary signature verification using external tools for critical documents
• Treat all digitally signed documents with suspicion until LibreOffice is patched
• Consider blocking or quarantining LibreOffice documents with signatures at the email gateway until patches are deployed

# Verify LibreOffice version and upgrade on Debian/Ubuntu systems
libreoffice --version
sudo apt update && sudo apt upgrade libreoffice