CVE-2026-48802

python-engineio has unbound thread allocation that can cause denial of service

An attacker can cause the creation of unnecessary background threads in the python-engineio server by exploiting the heartbeat mechanism, which launches a thread when a new connection is received, and when the client sends a PONG packet.

Note: this issue primarily affects synchronous servers. Asynchronous servers allocate background tasks instead of physical threads, which are lightweight and less likely to cause denial of service. However, the fix that was implemented was also applied to the asynchronous case.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-48802 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.13.2); upgrading removes the vulnerable code path.

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Version 4.13.2 addresses this issue as follows:

• The initial background thread (or async task( for heartbeat management is only launched if a client passes authentication in the connect handler.
• The server now ensures that there is only one background heatbeat thread (or async task) per client at a given point in time. Out of sequence PONG packets are now discarded when an active heartbeat thread is already running.

1. What is CVE-2026-48802? CVE-2026-48802 is a high-severity allocation of resources without limits or throttling vulnerability in python-engineio (pip), affecting versions <= 4.13.1. It is fixed in 4.13.2. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-48802? CVE-2026-48802 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of python-engineio are affected by CVE-2026-48802? python-engineio (pip) versions <= 4.13.1 is affected.
4. Is there a fix for CVE-2026-48802? Yes. CVE-2026-48802 is fixed in 4.13.2. Upgrade to this version or later.
5. Is CVE-2026-48802 exploitable, and should I be worried? Whether CVE-2026-48802 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-48802 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-48802? Upgrade python-engineio to 4.13.2 or later.

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