CVE-2026-77357 in mesop
Summary
by MITRE • 08/26/2026
Mesop is a Python-based UI framework that allows users to build web applications. Prior to 1.3.3, applications running in debug mode expose a GET /hot-reload endpoint whose unbounded loop depends on the user-supplied counter parameter, allowing an unauthenticated attacker to hold worker threads with high counter values until the worker pool is exhausted and the server becomes unavailable. A single unauthenticated attacker can crash the Mesop server with minimal effort. Because the attack leverages worker exhaustion, the server remains unresponsive until it is manually restarted. This issue is fixed in version 1.3.3.
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Analysis
by VulDB Data Team • 08/26/2026
The vulnerability identified in Mesop versions prior to 1.3.3 represents a critical resource exhaustion flaw rooted in improper input validation and lack of rate limiting within the framework's development infrastructure. As a Python-based UI framework designed for building web applications, Mesop includes specific endpoints intended solely for local development convenience, such as hot-reloading functionality. However, when an application is executed in debug mode, these internal mechanisms are exposed to external network traffic without adequate authentication or access controls. The core technical flaw lies within the GET /hot-reload endpoint, which processes a user-supplied counter parameter that dictates the number of iterations for an unbounded loop. Because there is no upper limit imposed on this value, an attacker can supply arbitrarily large integers to force the server-side worker threads into prolonged execution cycles. This design oversight transforms what should be a transient development tool into a significant attack vector against service availability.
From an operational perspective, this vulnerability facilitates a straightforward Denial of Service (DoS) scenario that requires minimal effort and no authentication credentials from the attacker. By repeatedly sending requests with high counter values to the /hot-reload endpoint, an unauthenticated actor can systematically occupy all available worker threads in the server's pool. Since Mesop relies on these workers to handle incoming HTTP requests for normal application functionality, exhausting them effectively paralyzes the service. The impact is severe because the server does not merely slow down; it becomes completely unresponsive to legitimate user traffic. Unlike some DoS attacks that can be mitigated by scaling resources or temporary throttling, this specific exhaustion of worker threads often requires manual intervention to resolve. Administrators must typically restart the Mesop process manually to reclaim the occupied threads and restore service availability, leading to significant downtime during incident response efforts.
This vulnerability aligns with CWE-787: Out-of-bounds Write in terms of resource consumption logic, though more accurately it maps to CWE-400: Uncontrolled Resource Consumption. The exploitation technique is consistent with ATT&CK tactic T1499: Endpoint Denial of Service, specifically under the sub-technique of resource exhaustion via application layer attacks. The lack of input validation on numeric parameters that drive server-side loops is a common pattern in web frameworks where development endpoints are inadvertently left exposed to production-like environments or public-facing networks. The severity is heightened by the fact that debug modes are sometimes enabled during deployment due to misconfiguration, making this an accessible risk for organizations not strictly isolating their development and production environments.
To mitigate this vulnerability, immediate action involves upgrading Mesop to version 1.3.3 or later, where the developers have implemented fixes to address the unbounded loop issue. In addition to patching, administrators should enforce strict environment separation by ensuring that debug mode is never enabled in any non-development environment. Network-level controls such as firewalls and web application firewalls can be configured to block access to development-specific endpoints like /hot-reload from external networks or unauthorized IP ranges. Furthermore implementing rate limiting on all API endpoints provides an additional layer of defense against automated exhaustion attacks, ensuring that even if a similar flaw exists in future versions, the impact is contained by restricting the frequency and volume of requests any single source can make within a given timeframe.