CVE-2024-21411: Skype Remote Code Execution Vulnerability

CVE-2024-21411 Overview

CVE-2024-21411 is a remote code execution vulnerability in Skype for Consumer. Microsoft published the advisory on March 12, 2024. An attacker can exploit the issue over the network when a user interacts with crafted content, leading to code execution in the context of the Skype client.

The vulnerability is tracked under [CWE-453] (Insecure Default Variable Initialization) and carries an EPSS probability of 5.042% at the 89.923 percentile, indicating elevated likelihood of exploitation attempts compared to most CVEs.

Critical Impact

Successful exploitation grants attackers code execution on the victim system with full impact to confidentiality, integrity, and availability after user interaction.

Affected Products

• Skype for Consumer (desktop client)
• Component identifier: skype:skype
• Refer to the Microsoft Security Update Guide for affected build numbers

Discovery Timeline

• 2024-03-12 - CVE-2024-21411 published to NVD by Microsoft
• 2024-12-27 - Last updated in NVD database

Technical Details for CVE-2024-21411

Vulnerability Analysis

CVE-2024-21411 is a remote code execution flaw in the Skype for Consumer client. Microsoft classifies the underlying weakness as [CWE-453], insecure default variable initialization. The issue allows an attacker to influence application state in a way that results in arbitrary code execution within the Skype process.

Exploitation requires user interaction, such as opening or previewing attacker-supplied content delivered through Skype. No prior authentication on the target system is required. Successful exploitation yields execution under the privileges of the Skype user, providing a foothold for follow-on activity including credential theft, lateral movement, and persistence.

Root Cause

The root cause is improper initialization of variables during processing of untrusted input received by the Skype client. When the client handles a malformed object or message, default state is used in a path that should require validated values, allowing the attacker to influence control flow and trigger code execution.

Attack Vector

The attack vector is network-based. An attacker sends crafted content through Skype messaging or media channels. The victim must interact with the content for the exploit to succeed. Because Skype is widely deployed on consumer endpoints, phishing-style delivery is the most plausible exploitation path.

No public proof-of-concept code or exploit module is currently published, and the CVE is not listed in the CISA Known Exploited Vulnerabilities catalog. See the Microsoft Security Update Guide for vendor-supplied technical context.

Detection Methods for CVE-2024-21411

Indicators of Compromise

• Unexpected child processes spawned by Skype.exe, particularly command interpreters such as cmd.exe, powershell.exe, or wscript.exe
• Outbound network connections from the Skype process to non-Microsoft infrastructure following inbound messages with attachments or rich content
• New persistence entries (Run keys, scheduled tasks) created shortly after Skype activity
• Anomalous file writes by the Skype process to user-writable directories such as %APPDATA% or %TEMP%

Detection Strategies

• Hunt for process-tree anomalies where Skype is the parent of scripting or living-off-the-land binaries
• Correlate Skype client versions across the fleet against the patched build listed in the Microsoft advisory
• Inspect endpoint telemetry for memory protection violations or exception events inside the Skype process

Monitoring Recommendations

• Centralize Skype client logs and EDR telemetry for retrospective hunting once new indicators emerge
• Track Skype version inventory continuously to surface unpatched endpoints
• Alert on suspicious DNS or HTTP egress originating from the Skype process

How to Mitigate CVE-2024-21411

Immediate Actions Required

• Apply the Skype for Consumer update referenced in the Microsoft Security Update Guide on all endpoints
• Identify Skype installations across managed and BYOD systems and prioritize patching for high-risk users
• Block or quarantine Skype messages from untrusted contacts where business policy allows

Patch Information

Microsoft addressed CVE-2024-21411 in the March 2024 servicing cycle for Skype for Consumer. The fix is delivered through the standard Skype auto-update mechanism. Confirm the installed client version matches or exceeds the build listed in the vendor advisory before considering an endpoint remediated.

Workarounds

• Restrict acceptance of files and rich media in Skype to known contacts only
• Educate users to avoid interacting with unsolicited Skype messages, links, or attachments
• Where Skype for Consumer is not required, remove the application and standardize on a managed communications platform