CVSS 4.0
6.9 mediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:NCVE-2025-57804
Medium (6.9). EPSS 1.9% chance of exploitation in the next 30 days. Fixed in h2 4.3.0.
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.
Severity
- EPSS
- 1.9% chance of exploitation in the next 30 days; higher than 79% of CVEs (scores of 2026-10-31)
- CWE
- CWE-93
- Published
- Modified
- Record from
- the NVD, synced
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-847f-9342-265h | PyPI | h2 | <4.3.0 | 4.3.0 |
| PYSEC-2026-1435 | PyPI | h2 | <4.3.0 | 4.3.0 |