5.3Medium
CVE-2026-86252
h3 versions before 1.15.9 fail to sanitize carriage return characters in EventStream data and comment fields, allowing attackers to inject arbitrary SSE events by including unsanitized carriage returns. Attackers can inject event type directives, split single push calls into multiple browser-parsed events, or escape comment fields to inject data, bypassing the prior CVE fix that only addressed newline injection.
What this means for your business
- An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
- FIRST's prediction model gives it a 0.2% chance of attack attempts being seen in the next 30 days, ranking above 12% of all known flaws.
What to do
- 1Ask your IT team or provider whether any of your systems use the affected product.
- 2If you do, follow the vendor's guidance. No patch reference has been published yet.
Not sure if your company is exposed?
Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.
Scoring
- CVSS
- 5.3 (v3.1)
- Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N- CVSS v4
- 6.9
- Weakness
- CWE-74
- Assigned by
- disclosure@vulncheck.com
Dates
- Published
- 2026-09-06
- Last modified
- 2026-09-08
- Sources
- NVD