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CVE-2026-77357 – Denial of Service via Unbounded Hot-Reload Loop – Mesop before 1.3.3

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Summary

CVE-2026-77357 is a Denial-of-Service vulnerability in Mesop, a Python-based UI framework, caused by an unbounded loop in the debug-mode hot-reload endpoint. An unauthenticated network attacker can exhaust the server’s worker threads and render the application unresponsive. The vulnerability is rated HIGH severity with a CVSS v4.0 base score of 8.7.

Technical details

  • Root cause: the GET /__hot-reload__ endpoint (referred to in some sources as /hot-reload) contains a while loop that depends on a user-supplied counter parameter without any bound, timeout, or request limit.
  • Trigger conditions: the vulnerable code path is only reachable when a Mesop application is run in debug/editor mode; production deployments are not affected.
  • Attack vector: an unauthenticated attacker sends one or more requests to the endpoint with an extremely large counter value (e.g. ?counter=999999999), causing the handling thread to loop indefinitely.
  • Impact: sending parallel requests exhausts the worker thread pool, making the server unresponsive; the application does not recover on its own and requires a manual restart.

Affected software

  • Mesop (mesop-dev/mesop), all versions prior to 1.3.3, when running in debug/editor mode

Severity

  • CVSS Base Score: 8.7 (HIGH)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: upgrade Mesop to version 1.3.3 or later, which adds bounds/timeout handling to the hot-reload loop.
  • If no patch can be applied immediately: ensure Mesop applications are not run in debug/editor mode on internet-facing or production systems, and restrict network access to the /__hot-reload__ endpoint (e.g., via reverse proxy or firewall rules) until upgraded.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Page title: Mesop
  • Raw response body: <mesop-app, window.__MESOP_EXPERIMENTS__ =, window.__MESOP_BASE_URL_PATH__ =

References

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Map your entire attack surface (continously)

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4

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Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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