Proxy trust bypass via promoted HTTP trailers
Published Aug 12, 2026 · Updated Aug 12, 2026
Header injection in http4s Blaze before 0.23.18 and 1.0.0-M1 through 1.0.0-M41 allows remote attackers to bypass proxy trust controls. Http1ServerParser appends trailer fields to the same header buffer before Request.headers is snapshotted instead of keeping trailers separate. Exploitation requires HTTP/1.1 chunked requests to cross a proxy that removes request headers but forwards trailers, enabling spoofed trust headers or pooled-connection termination.
Summary
What happened
Header injection in http4s Blaze before 0.23.18 and 1.0.0-M1 through 1.0.0-M41 allows remote attackers to bypass proxy trust controls. Http1ServerParser appends trailer fields to the same header buffer before Request.headers is snapshotted instead of keeping trailers separate. Exploitation requires HTTP/1.1 chunked requests to cross a proxy that removes request headers but forwards trailers, enabling spoofed trust headers or pooled-connection termination.
The record
- CVE
- CVE-2026-73495
- Published
- Aug 12, 2026
- Updated
- Aug 12, 2026
- Vendor
- http4s
- Product
- Blaze
- Classifications
- CWE-444, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 12, 2026CVE publishedPublication date reported by the CVE source.
- Aug 12, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 0.23.18
- Affected versionversion=>= 1.0.0-M1, < 1.0.0-M42
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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