CVE-2026-27729 in astro
Summary
by MITRE • 02/24/2026
Astro is a web framework. In versions 9.0.0 through 9.5.3, Astro server actions have no default request body size limit, which can lead to memory exhaustion DoS. A single large POST to a valid action endpoint can crash the server process on memory-constrained deployments. On-demand rendered sites built with Astro can define server actions, which automatically parse incoming request bodies (JSON or FormData). The body is buffered entirely into memory with no size limit — a single oversized request is sufficient to exhaust the process heap and crash the server. Astro's Node adapter (`mode: 'standalone'`) creates an HTTP server with no body size protection. In containerized environments, the crashed process is automatically restarted, and repeated requests cause a persistent crash-restart loop. Action names are discoverable from HTML form attributes on any public page, so no authentication is required. The vulnerability allows unauthenticated denial of service against SSR standalone deployments using server actions. A single oversized request crashes the server process, and repeated requests cause a persistent crash-restart loop in containerized environments. Version 9.5.4 contains a fix.
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Analysis
by VulDB Data Team • 02/24/2026
The vulnerability described in CVE-2026-27729 affects the Astro web framework version 9.0.0 through 9.5.3, specifically targeting server actions within standalone deployments. This issue represents a critical security flaw that enables unauthenticated denial of service attacks through memory exhaustion. The vulnerability stems from the absence of default request body size limits in Astro's server action implementation, creating a pathway for malicious actors to exploit memory constraints and crash server processes. Server actions in Astro automatically parse incoming request bodies including JSON and FormData formats, buffering these entire payloads into memory without any size restrictions, which directly violates security best practices for input validation and resource management.
The technical implementation of this vulnerability occurs within Astro's Node adapter when operating in standalone mode, where the HTTP server lacks any built-in protection against oversized request bodies. This design flaw creates a memory exhaustion condition where a single large POST request to a valid action endpoint can consume all available heap memory, causing the server process to crash. The vulnerability is particularly dangerous because action endpoint names are discoverable through HTML form attributes present on public pages, eliminating any requirement for authentication to exploit the flaw. This exposure means that any attacker with knowledge of the application's public pages can identify and target server actions without needing credentials or privileged access.
The operational impact of this vulnerability extends beyond simple server crashes to create persistent service disruption in containerized environments. When the server process crashes due to memory exhaustion, container orchestration systems automatically restart the crashed process, but repeated attacks can cause a continuous crash-restart loop that effectively renders the service unavailable. This behavior constitutes a denial of service attack that can persist as long as the attacker continues to send oversized requests, making it particularly problematic for applications deployed in cloud environments where automatic restart mechanisms are common. The vulnerability specifically affects on-demand rendered sites built with Astro, which rely on server actions for processing user input and form submissions, making these deployments especially vulnerable to exploitation.
This vulnerability aligns with CWE-400, which covers "Uncontrolled Resource Consumption" and specifically addresses the lack of input validation for request body sizes in web applications. The attack pattern follows the MITRE ATT&CK framework's T1499.004 technique for "Resource Hijacking" where adversaries consume system resources to prevent legitimate use of services. The lack of request size limiting in the framework's server action implementation creates a fundamental weakness in the application's resource management that directly enables the memory exhaustion attack vector. The fix implemented in version 9.5.4 addresses this by introducing default request body size limits, which prevents any single request from consuming excessive memory resources and eliminates the crash condition that previously occurred. This remediation aligns with industry standards for secure web application development that require proper input validation and resource management to prevent denial of service conditions. The vulnerability demonstrates the importance of implementing automatic resource limits and size validation for all incoming request data, particularly in frameworks that handle user-provided content without built-in safeguards.