Discourse DoS through Onebox favicon URL

Vulnerability Description

Discourse is an open source platform for community discussion. In versions 3.1.0 through 3.1.2 of the `stable` branch and versions 3.1.0,beta6 through 3.2.0.beta2 of the `beta` and `tests-passed` branches, Redis memory can be depleted by crafting a site with an abnormally long favicon URL and drafting multiple posts which Onebox it. The issue is patched in version 3.1.3 of the `stable` branch and version 3.2.0.beta3 of the `beta` and `tests-passed` branches. There are no known workarounds.

References

• https://github.com/discourse/discourse/security/advisories/GHSA-77cw-xhj8-hfp3
• https://github.com/discourse/discourse/commit/95a82d608d6377faf68a0e2c5d9640b043557852
• https://github.com/discourse/discourse/commit/e910dd09140cb4abc3a563b95af4a137ca7fa0ce