CVE-2026-48809

python-engineio has possible denial of service due to maximum payload size sometimes not being enforced

There are two specific configurations of the python-engineio server in which the size of incoming messages is not checked before the messages are loaded into memory. An attacker can take advantage of these to cause unnecessary memory allocations in the python-engineio server. The two cases are:

• POST requests, when using ASGI with the long polling transport
• WebSocket messages, when using Aiohttp with the WebSocket transport

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-2026-48809 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:

• ASGI severs now only load the body of incoming requests into memory after the client is confirmed to be known and authenticated, and the payload size is below the maximum allowed size. Requests that do not comply with these requirements are discarded.
• Aiohttp servers configure the maximum payload size in the underlying WebSocket layer from Aiohttp, so that large messages are discarded by Aiohttp before they are delivered to python-engineio.

1. What is CVE-2026-48809? CVE-2026-48809 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-2026-48809? CVE-2026-48809 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-2026-48809? python-engineio (pip) versions <= 4.13.1 is affected.
4. Is there a fix for CVE-2026-48809? Yes. CVE-2026-48809 is fixed in 4.13.2. Upgrade to this version or later.
5. Is CVE-2026-48809 exploitable, and should I be worried? Whether CVE-2026-48809 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-2026-48809 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-2026-48809? Upgrade python-engineio to 4.13.2 or later.

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