CVE-2023-2255: LibreOffice Auth Bypass Vulnerability

CVE-2023-2255 Overview

CVE-2023-2255 is an improper access control vulnerability in the editor components of The Document Foundation LibreOffice that allows attackers to craft malicious documents causing external links to load without user consent. In affected versions, documents utilizing "floating frames" linked to external files automatically load frame contents without prompting users for permission, contrary to how LibreOffice handles other linked content.

This vulnerability creates an inconsistent security model where floating frames bypass the expected user authorization workflow, potentially enabling attackers to exfiltrate sensitive information or load malicious content from external sources without the victim's knowledge.

Critical Impact

Attackers can craft malicious LibreOffice documents that automatically load external content without user consent, potentially leading to information disclosure or loading of malicious resources.

Affected Products

• LibreOffice 7.4 versions prior to 7.4.7
• LibreOffice 7.5 versions prior to 7.5.3
• Debian Linux 11.0

Discovery Timeline

• 2023-05-25 - CVE-2023-2255 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-2255

Vulnerability Analysis

The vulnerability resides in LibreOffice's editor components that handle floating frames (also known as IFrames in document context). Floating frames are document elements that can embed content from external sources within a LibreOffice document. The core issue is a failure to enforce consistent access control policies across different types of linked content.

When a user opens a document containing floating frames with external file links, LibreOffice automatically loads these external resources without displaying a security prompt. This behavior contradicts the security model applied to other types of linked content in LibreOffice, where users are typically prompted before external resources are fetched.

The improper access control (CWE-264) allows attackers to abuse this functionality for various malicious purposes, including tracking document opens via external callbacks, loading potentially malicious content from attacker-controlled servers, or exfiltrating data about the victim's network environment through external requests.

Root Cause

The root cause is an inconsistent implementation of access control checks in the floating frame component compared to other linked content handlers within LibreOffice. While other external content types trigger user authorization prompts, the floating frame implementation failed to include equivalent security checks, creating an exploitable gap in the security model.

This design flaw allowed documents to silently establish network connections to external resources, bypassing user awareness and consent mechanisms that should protect against such behavior.

Attack Vector

The attack vector for CVE-2023-2255 is network-based and requires no authentication or user interaction beyond opening the malicious document. An attacker can exploit this vulnerability through the following attack chain:

1. Document Crafting: The attacker creates a LibreOffice document containing floating frames with links pointing to attacker-controlled external resources
2. Distribution: The malicious document is distributed to victims via email attachments, file sharing, or other document delivery mechanisms
3. Execution: When the victim opens the document in a vulnerable LibreOffice version, the floating frames automatically load external content without any user prompt
4. Exploitation: The attacker's server receives the request, potentially revealing victim information (IP address, network details) or serving malicious content

The vulnerability can be exploited to create tracking pixels within documents, enumerate internal network resources through URL-based probing, or serve dynamic malicious content that changes based on the victim's environment.

Detection Methods for CVE-2023-2255

Indicators of Compromise

• Unusual outbound network connections originating from LibreOffice processes (soffice.bin, soffice.exe)
• Documents containing floating frame elements with external URLs pointing to suspicious or unknown domains
• Network traffic logs showing requests to external resources immediately after document opens
• LibreOffice documents with embedded frame elements referencing non-local file paths

Detection Strategies

• Monitor outbound network connections from LibreOffice application processes for connections to unknown or suspicious external hosts
• Implement document scanning solutions that analyze LibreOffice files for floating frame elements with external links
• Deploy network-level detection rules to identify unusual HTTP/HTTPS requests originating from office application processes
• Use endpoint detection to flag documents containing frame elements with URLs pointing to external resources

Monitoring Recommendations

• Enable logging of all network connections initiated by LibreOffice processes at the endpoint level
• Configure proxy logs to capture and analyze requests from office application user agents
• Implement alerting for LibreOffice processes making connections to newly observed or low-reputation domains
• Monitor for document files being opened followed by immediate network activity to external hosts

How to Mitigate CVE-2023-2255

Immediate Actions Required

• Update LibreOffice 7.4.x installations to version 7.4.7 or later immediately
• Update LibreOffice 7.5.x installations to version 7.5.3 or later immediately
• Apply Debian security updates via DSA-5415 for Debian-based systems
• Review and audit any documents received from untrusted sources before opening

Patch Information

The Document Foundation has released patched versions that address this vulnerability. Users should upgrade to:

• LibreOffice 7.4.7 or later for the 7.4 branch
• LibreOffice 7.5.3 or later for the 7.5 branch

Linux distribution users should apply vendor-specific security updates:

• Debian users should apply Debian Security Advisory DSA-5415
• Debian LTS users should refer to Debian LTS Advisory 2023-08
• Gentoo users should consult GLSA 202311-15

For detailed information, see the LibreOffice Security Advisory CVE-2023-2255.

Workarounds

• Disable or restrict network access for LibreOffice applications at the firewall level until patches can be applied
• Configure endpoint security solutions to prompt or block LibreOffice network connections to external hosts
• Implement document quarantine procedures that scan incoming LibreOffice files for suspicious frame elements
• Consider using document preview tools that don't execute embedded content for reviewing untrusted documents

# Example: Block LibreOffice network access using iptables (Linux)
# Identify LibreOffice process user and restrict outbound connections
iptables -A OUTPUT -m owner --uid-owner $USER -p tcp --dport 80 -m string --string "soffice" --algo bm -j DROP
iptables -A OUTPUT -m owner --uid-owner $USER -p tcp --dport 443 -m string --string "soffice" --algo bm -j DROP