CVE-2020-13160: AnyDesk Format String RCE Vulnerability

CVE-2020-13160 Overview

CVE-2020-13160 is a format string vulnerability in AnyDesk remote desktop software versions prior to 5.5.3 running on Linux and FreeBSD systems. This vulnerability allows remote attackers to achieve code execution by exploiting improper handling of format specifiers in user-controlled input. The flaw represents a significant security risk for organizations using AnyDesk for remote access and support operations.

Critical Impact

Remote attackers can exploit this format string vulnerability to execute arbitrary code on vulnerable AnyDesk installations without authentication, potentially gaining complete control over affected systems.

Affected Products

• AnyDesk versions prior to 5.5.3 on Linux
• AnyDesk versions prior to 5.5.3 on FreeBSD
• Systems running Linux kernel with vulnerable AnyDesk installations

Discovery Timeline

• 2020-06-09 - CVE-2020-13160 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-13160

Vulnerability Analysis

This vulnerability is classified as CWE-134 (Use of Externally-Controlled Format String). Format string vulnerabilities occur when user-supplied input is passed directly to formatting functions like printf() without proper sanitization. In AnyDesk's case, the GUI component improperly processes format specifiers embedded in input data, allowing attackers to read from or write to arbitrary memory locations.

The attack can be executed remotely over the network without requiring any authentication or user interaction. When successfully exploited, attackers can achieve full system compromise, including reading sensitive memory contents, corrupting application state, and executing arbitrary code with the privileges of the AnyDesk process.

Root Cause

The root cause of CVE-2020-13160 lies in the AnyDesk GUI component's failure to properly validate and sanitize user-controlled input before passing it to format string functions. When external data containing format specifiers (such as %s, %x, %n) is processed by functions expecting static format strings, attackers can manipulate the program's execution flow. The %n specifier is particularly dangerous as it allows writing values to memory addresses, enabling attackers to overwrite function pointers or return addresses to redirect execution.

Attack Vector

The vulnerability is exploitable via network-based attack vectors. An attacker can craft malicious input containing specially formatted strings that, when processed by the vulnerable AnyDesk components, trigger the format string vulnerability. The attack does not require prior authentication or user interaction, making it highly exploitable.

The exploitation process typically involves:

1. Sending a crafted payload containing format specifiers to the AnyDesk service
2. Using %x or %p specifiers to leak memory addresses and defeat ASLR
3. Calculating target addresses for code execution
4. Using %n specifiers to write shellcode addresses to critical memory locations
5. Redirecting execution flow to attacker-controlled code

Detailed technical analysis and exploitation techniques are documented in the Development Blog Post Analysis and the Packet Storm Advisory - Remote Code Execution.

Detection Methods for CVE-2020-13160

Indicators of Compromise

• Unusual network traffic patterns targeting AnyDesk service ports
• Unexpected crashes or restarts of the AnyDesk process
• Memory corruption artifacts in AnyDesk log files
• Suspicious strings containing multiple format specifiers (%x, %n, %s) in network traffic

Detection Strategies

• Monitor network traffic for payloads containing sequences of format specifiers destined for AnyDesk services
• Implement intrusion detection rules to identify format string attack patterns
• Deploy endpoint detection solutions that can identify format string exploitation attempts
• Analyze AnyDesk process behavior for signs of memory corruption or unexpected code execution

Monitoring Recommendations

• Enable verbose logging for AnyDesk services to capture potential exploitation attempts
• Configure SIEM alerting for anomalous AnyDesk process behavior including crashes and respawns
• Monitor for child processes spawned by AnyDesk that deviate from normal operational patterns
• Track outbound connections from AnyDesk processes to identify potential command and control activity

How to Mitigate CVE-2020-13160

Immediate Actions Required

• Update AnyDesk to version 5.5.3 or later immediately on all Linux and FreeBSD systems
• Restrict network access to AnyDesk services using firewall rules where possible
• Implement network segmentation to limit exposure of vulnerable systems
• Review systems for signs of compromise before applying patches

Patch Information

AnyDesk has addressed this vulnerability in version 5.5.3. Organizations should immediately upgrade all AnyDesk installations on Linux and FreeBSD systems to the patched version. The official changelog documenting the fix is available at the AnyDesk Changelog Document.

For environments where immediate patching is not possible, consider temporarily disabling or restricting access to AnyDesk services until updates can be applied.

Workarounds

• Implement strict firewall rules to limit AnyDesk network exposure to trusted IP addresses only
• Deploy network-level intrusion prevention systems capable of detecting format string attacks
• Consider using alternative remote desktop solutions until patching is complete
• Enable application whitelisting to prevent execution of unauthorized code

# Configuration example
# Restrict AnyDesk access using iptables (Linux)
# Allow AnyDesk only from trusted network ranges
iptables -A INPUT -p tcp --dport 7070 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 7070 -j DROP

# Verify AnyDesk version to ensure patch is applied
anydesk --version
# Expected output should be 5.5.3 or higher