CVE-2021-34801 in Valine
Summary
by MITRE • 06/16/2021
Valine 1.4.14 allows remote attackers to cause a denial of service (application outage) by supplying a ua (aka User-Agent) value that only specifies the product and version.
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Analysis
by VulDB Data Team • 06/19/2021
The vulnerability identified as CVE-2021-34801 affects Valine version 1.4.14, a popular comment system for static websites. This issue represents a denial of service vulnerability that can be exploited by remote attackers through carefully crafted User-Agent headers. The flaw specifically manifests when attackers provide a User-Agent value that contains only the product name and version information without any additional identifying details. This particular configuration triggers an application outage that disrupts normal service operations.
The technical root cause of this vulnerability stems from improper input validation within the Valine comment system's user agent parsing mechanism. When the application encounters a User-Agent string that consists solely of product and version information, the parsing logic fails to handle this edge case correctly. This parsing failure leads to an application crash or unhandled exception that results in service disruption. The vulnerability demonstrates a classic lack of robust input sanitization and error handling in web applications, where the system does not anticipate or properly manage malformed input data.
From an operational perspective, this vulnerability poses significant risk to websites utilizing Valine comment systems, particularly those with high traffic volumes or mission-critical content. The denial of service impact means that legitimate users cannot access comment functionality, potentially leading to complete service unavailability for comment-related features. Attackers can exploit this vulnerability with minimal technical expertise by simply crafting a specific User-Agent header that meets the described criteria. The attack requires no authentication and can be executed from any network location, making it particularly dangerous for public-facing applications.
The vulnerability aligns with CWE-20, "Improper Input Validation," which addresses weaknesses that occur when applications fail to validate input data properly. This classification indicates that the system does not adequately validate the structure or format of User-Agent headers before processing them. Additionally, the issue demonstrates characteristics consistent with ATT&CK technique T1499.004, "Endpoint Denial of Service," where attackers target application endpoints to cause service disruption. The vulnerability also reflects poor error handling practices that fall under ATT&CK technique T1595.001, "Network Device Discovery," as the attack vector targets application behavior rather than network infrastructure.
Mitigation strategies for this vulnerability should include immediate patching of the Valine application to version 1.4.15 or later, which contains the necessary fixes for the User-Agent parsing issue. Organizations should also implement input validation measures at the web application firewall level to filter out malformed User-Agent headers before they reach the application layer. Additionally, monitoring systems should be configured to detect unusual patterns in User-Agent header usage that might indicate exploitation attempts. Regular security assessments of third-party components and dependency updates should be enforced to prevent similar vulnerabilities from being introduced into the application stack.