Machine-written summary
Summary written by Claude Haiku from the source advisory. Not reviewed by a person.
The h2 package, versions before 4.4.1, accepts HTTP/2 header blocks that contain more than one Host header and passes all of them to the application. An attacker who can send such requests to a server built on h2 could carry out request smuggling when the request is downgraded to HTTP/1.1. Upgrade h2 to version 4.4.1. The source's workaround is to check HTTP semantics in application code, which is only a partial safeguard.
Not analysed yet
We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.
Severity and exploitation
| Score | Rating | CVSS | Vector | Source |
|---|---|---|---|---|
| 5.3 | Medium | 3.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L | CNA (GitHub_M) |
- CISA KEV Not listed.
- EPSS 0.0042, the 36th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.
Timeline
- 31 Oct 2026, 14:10 UTC First seen by agentprime
- 31 Oct 2026, 14:10 UTC Confirmed by OSV