h2 allows HTTP Request Smuggling due to illegal characters in headers

Description

Summary

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.

Basic information

Type
reviewed
Severity
medium
Advisory on GitHub
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Repository advisory
Open repository advisory ↗
Source code
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Published (advisory)
2025-08-25 20:44:43 UTC
Updated
2025-11-03 21:34:26 UTC
GitHub reviewed
2025-08-25 20:44:43 UTC
NVD published
2025-08-25 21:15:37 UTC

EPSS Score

Score Percentile
0.07% 20.45%

CVSS Scores

Base score Version Severity Vector
6.9 4.0
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network.
Attack complexity (AC:L)
Exploitation conditions are straightforward and stable.
Attack requirements (AT:N)
No additional preconditions are required beyond normal reachability.
Privileges required (PR:N)
No privileges are required.
User interaction (UI:N)
No user interaction is required.
Vulnerable system confidentiality impact (VC:N)
No confidentiality impact on the vulnerable system.
Vulnerable system integrity impact (VI:N)
No integrity impact on the vulnerable system.
Vulnerable system availability impact (VA:N)
No availability impact on the vulnerable system.
Subsequent system confidentiality impact (SC:N)
No confidentiality impact on subsequent systems.
Subsequent system integrity impact (SI:L)
Limited integrity impact on subsequent systems.
Subsequent system availability impact (SA:N)
No availability impact on subsequent systems.

Identifiers

CWEs

CWE id Name
CWE-93 Improper Neutralization of CRLF Sequences ('CRLF Injection')

Credits

  • sebastianosrt (reporter)
  • mhils (remediation_verifier)

Affected packages (1)

Vulnerable version ranges and first patched releases as published by GitHub.

Ecosystem Package Vulnerable range First patched Vulnerable functions
pip h2 < 4.3.0 4.3.0

References

cvelogic Threat Intelligence