CVE-2025-57804

h2 allows HTTP Request Smuggling due to illegal characters in headers

Description

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.

Category

6.9
CVSS
Severity: Medium
CVSS 4.0 •
EPSS 0.05%
Affected: python-hyper h2
Published at:
Updated at:

References

Frequently Asked Questions

What is the severity of CVE-2025-57804?
CVE-2025-57804 has been scored as a medium severity vulnerability.
How to fix CVE-2025-57804?
To fix CVE-2025-57804, make sure you are using an up-to-date version of the affected component(s) by checking the vendor release notes. As for now, there are no other specific guidelines available.
Is CVE-2025-57804 being actively exploited in the wild?
As for now, there are no information to confirm that CVE-2025-57804 is being actively exploited. According to its EPSS score, there is a ~0% probability that this vulnerability will be exploited by malicious actors in the next 30 days.
What software or system is affected by CVE-2025-57804?
CVE-2025-57804 affects python-hyper h2.
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