Catastrophic backtracking in URL authority parser when passed URL containing many @ characters
### Impact
When provided with a URL containing many `@` characters in the authority component the authority regular expression exhibits catastrophic backtracking causing a denial of service if a URL were passed as a parameter or redirected to via an HTTP redirect.
### Patches
The issue has been fixed in urllib3 v1.26.5.
### References
- CVE-2021-33503
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-33503
- JVNVU#92413403 (English)
https://jvn.jp/en/vu/JVNVU92413403/
- JVNVU#92413403 (Japanese)
https://jvn.jp/vu/JVNVU92413403/
- urllib3 v1.26.5
https://github.com/urllib3/urllib3/releases/tag/1.26.5
### For more information
If you have any questions or comments about this advisory:
* Ask in our community Discord
https://discord.gg/urllib3
* Email [sethmichaellarson@gmail.com](mailto:sethmichaellarson@gmail.com)