UltraVNC Repeater HTTP Server Buffer Overflow

Executive Summary

This vulnerability exists in UltraVNC repeater versions up to 1.8.2.2 in its embedded HTTP administration server. Specifically, the functions wi_senderr() and wi_replyhdr() write the HTTP request URI into a fixed-size 1000-byte global buffer without proper bounds checking. Since the HTTP receive buffer can accept URIs up to approximately 150 KB, an attacker can send a specially crafted HTTP request with a URI longer than 1500 bytes to overflow this buffer by at least 500 bytes.

The overflow corrupts adjacent global variables in memory before any authentication check is performed, allowing a remote, unauthenticated attacker who can reach the repeater's HTTP port (default TCP 80) to execute arbitrary code on the host running the repeater.

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Detection Guidance

This vulnerability involves a buffer overflow in the UltraVNC repeater's embedded HTTP administration server, which listens on the default TCP port 80. Detection can focus on monitoring HTTP requests to this port for unusually long URI strings (1500 bytes or longer) that could trigger the overflow.

You can detect potential exploitation attempts by capturing and analyzing network traffic targeting the UltraVNC repeater HTTP port. For example, using tcpdump or Wireshark to filter HTTP requests with long URIs.

• Use tcpdump to capture HTTP requests on port 80: tcpdump -i <interface> 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) > 1500)'
• Use Wireshark to filter HTTP requests with long URIs by applying a display filter for HTTP requests and inspecting the URI length manually.

Additionally, scanning the system for the presence of UltraVNC repeater versions up to 1.8.2.2 can help identify vulnerable hosts.

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Impact Analysis

This vulnerability can have severe impacts including complete compromise of the affected system. Because it allows remote, unauthenticated attackers to execute arbitrary code, an attacker could gain full control over the host running the UltraVNC repeater.

• Unauthorized access to sensitive data or system resources.
• Potential disruption or denial of service by crashing the application or system.
• Use of the compromised system as a foothold for further attacks within the network.

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Compliance Impact

The vulnerability in UltraVNC repeater allows unauthenticated remote attackers to execute arbitrary code on the host system by exploiting a global buffer overflow in the embedded HTTP administration server. This can lead to unauthorized access and potential compromise of sensitive data.

Such a security flaw can negatively impact compliance with common standards and regulations like GDPR and HIPAA, which require protection of personal and sensitive data against unauthorized access and breaches.

If exploited, this vulnerability could result in data breaches or unauthorized control over systems handling protected information, thereby violating requirements for data confidentiality, integrity, and availability mandated by these regulations.

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Mitigation Strategies

Immediate mitigation steps include restricting access to the UltraVNC repeater HTTP administration port (default TCP 80) to trusted networks only, such as by using firewall rules or network segmentation.

Disabling or blocking the HTTP administration server if it is not required can prevent exploitation.

Updating UltraVNC repeater to a version later than 1.8.2.2 that addresses this buffer overflow vulnerability is recommended once a patch is available.

Monitoring logs and network traffic for suspicious HTTP requests with abnormally long URIs can help detect exploitation attempts.

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