CVE-2017-12425 in Cache
Summary
by MITRE
An issue was discovered in Varnish HTTP Cache 4.0.1 through 4.0.4, 4.1.0 through 4.1.7, 5.0.0, and 5.1.0 through 5.1.2. A wrong if statement in the varnishd source code means that particular invalid requests from the client can trigger an assert, related to an Integer Overflow. This causes the varnishd worker process to abort and restart, losing the cached contents in the process. An attacker can therefore crash the varnishd worker process on demand and effectively keep it from serving content - a Denial-of-Service attack. The specific source-code filename containing the incorrect statement varies across releases.
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Analysis
by VulDB Data Team • 08/11/2022
The vulnerability identified as CVE-2017-12425 represents a critical denial-of-service flaw affecting multiple versions of Varnish HTTP Cache, a widely deployed HTTP caching system used by organizations to improve web performance and reduce server load. This vulnerability specifically targets the varnishd worker process that handles client requests and manages cached content, making it particularly dangerous for production environments where continuous availability is essential. The flaw exists in the source code of Varnish versions ranging from 4.0.1 through 4.0.4, 4.1.0 through 4.1.7, 5.0.0, and 5.1.0 through 5.1.2, indicating a persistent issue that affected several major release lines of the software. The vulnerability manifests through an incorrect if statement within the varnishd source code that fails to properly validate client request parameters, creating a condition where malformed requests can trigger unexpected behavior in the caching system.
The technical root cause of this vulnerability stems from an integer overflow condition that occurs when processing specific client requests, as classified under CWE-191 Integer Underflow (Wrap or Wraparound). The problematic code structure contains a conditional statement that does not properly account for boundary conditions when handling integer values, leading to an assertion failure that terminates the worker process. This assertion failure is particularly significant because it occurs during normal request processing rather than in error handling code, meaning that legitimate requests can inadvertently trigger the condition. The integer overflow vulnerability is exacerbated by the fact that the flawed code logic does not include proper validation of request parameters before they are processed, allowing malicious or malformed inputs to exploit the condition. The specific filename containing the incorrect statement varies across different Varnish releases, which complicates patching efforts and requires careful analysis of each affected version.
The operational impact of this vulnerability is severe and directly translates to a denial-of-service attack capability that can be exploited by remote attackers without requiring authentication or special privileges. When an attacker sends a specially crafted request that triggers the integer overflow condition, the varnishd worker process terminates abruptly and restarts, resulting in complete loss of cached content for the duration of the restart process. This behavior effectively removes the caching benefits that organizations rely on for performance optimization and can lead to significant service degradation or complete unavailability of web applications. The restart process also causes temporary disruption to ongoing user sessions and can trigger cascading effects throughout the application infrastructure, particularly in high-traffic environments where caching serves as a critical performance optimization layer. The vulnerability's exploitability is enhanced by the fact that it can be triggered on demand with minimal effort, making it an attractive target for attackers seeking to disrupt services.
Organizations affected by this vulnerability should prioritize immediate patching of all Varnish installations to the latest available versions that contain the corrected source code. The recommended mitigation strategy involves upgrading to Varnish versions that have addressed the integer overflow condition in the if statement logic, typically through version 4.1.8, 5.1.3, or later releases depending on the specific version in use. System administrators should also implement network-level controls such as rate limiting and request filtering to reduce the impact of potential exploitation attempts, though these measures do not provide complete protection against the vulnerability. Additionally, monitoring should be enhanced to detect abnormal worker process restart patterns that may indicate exploitation attempts, and security teams should conduct thorough vulnerability assessments to identify any other potentially affected systems within their infrastructure. From an ATT&CK framework perspective, this vulnerability aligns with techniques related to denial-of-service attacks and can be categorized under the T1499.004 technique for network denial of service, while also potentially enabling subsequent attack phases through service disruption that can mask other malicious activities.