As an introduction, MS15-034 is a critical security vulnerability discovered in Microsoft's HTTP.sys which, if exploited, can lead to a remote code execution (RCE) or a denial of service (DoS) attack. With the pervasiveness of Windows servers worldwide, the gravity of this security flaw cannot be overstated. But what is MS15-034? How does it impact systems and what are the steps required to mitigate it? This blog provides a deep dive into this topic.

The main body of this discussion starts with an overview of HTTP.sys. This is a kernel-mode device driver that listens for HTTP requests at the kernel level. By doing so, it offers improved performance over user-mode HTTP listeners. However, this advantage also implies a higher risk - kernel-level vulnerabilities like MS15-034 can be extremely harmful as they essentially provide the attacker with kernel-space access.

MS15-034 primarily exploits the Range HTTP header used for partial content requests. A specially crafted value in the Range header can cause HTTP.sys to parse it incorrectly. The vulnerability is mainly triggered by two aspects: a large 'Range' value and the absence of the 'If-Range' header. In essence, the attacker tricks the server into returning more data than it should, which causes a buffer overflow. This overflow can lead to a DoS condition or, potentially, unchecked code execution.

The impact of this vulnerability can be quite devastating. In a DoS scenario, the system crash can disrupt mission-critical services, posing a threat to organizational functionality. In a worse-case scenario, RCE can provide attacker a control over the system. The attacker can install programs; view, change, or delete data; or create new accounts with full user rights.

The vulnerability affects many Windows versions, including Windows 7, Windows Server 2008 R2, Windows 8, Windows Server 2012, Windows 8.1, and Windows Server 2012 R2. It is particularly dangerous for public-facing web servers where it can be exploited remotely.

It must be clarified that merely having HTTP.sys on a server does not make it vulnerable. HTTP.sys must be bound to an IP address and port, and must be listening for requests, for the vulnerability to be present. Microsoft has noted that this does not happen often, offering some respite.

Fortunately, there are several steps that systems administrators can take to mitigate this threat. Firstly, it's important to keep systems patched. Microsoft's regular security updates often include fixes for vulnerabilities like these, so it's crucial to apply these patches timely.

In other cases, restrictive firewall rules might help to block unsolicited traffic. Depending on the firewall’s capabilities, it might be possible to block requests with a large range header, which are usually indicative of an attack exploiting this vulnerability.

Another approach is to use a reverse proxy or load balancer. These can be configured to scrub incoming traffic, removing malicious headers before they reach HTTP.sys. However, this solution also has its drawbacks. For instance, legitimate users requesting large files might be blocked.

In conclusion, MS15-034 is a dangerous vulnerability, but it is possible to recover from its impact. Keeping systems updated, setting restrictive firewall rules, and, where applicable, using a reverse proxy or load balancer, are effective ways to mitigate this flaw. The key, as always, is persistent vigilance and proactive action. One cannot underscore enough the need to keep abreast with the information about the latest vulnerabilities and apply the necessary patches and updates regularly. The digital world evolves constantly, and so do the threats it conceals.