CVE-2025-59822Public exploitNetwork attack vector

Backend request injection via premature trailer termination

Published Sep 23, 2025 · Updated Sep 23, 2025

HTTP request smuggling in http4s before 0.23.31 and 1.0.0-M1 through 1.0.0-M44 allows remote attackers to inject backend requests. Ember's chunked-message trailer parser treats a single CRLF after a colonless trailer line as the end of trailers, leaving buffered bytes to be parsed as a second request. Exploitation requires an application behind a reverse proxy that forwards trailer headers; successful requests bypass front-end controls or poison web caches.

CVSS severity6.3
Medium
EPSS probability0.37%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in http4s before 0.23.31 and 1.0.0-M1 through 1.0.0-M44 allows remote attackers to inject backend requests. Ember's chunked-message trailer parser treats a single CRLF after a colonless trailer line as the end of trailers, leaving buffered bytes to be parsed as a second request. Exploitation requires an application behind a reverse proxy that forwards trailer headers; successful requests bypass front-end controls or poison web caches.

The record

CVE
CVE-2025-59822
Published
Sep 23, 2025
Updated
Sep 23, 2025
Vendor
http4s
Product
http4s
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 23, 2025CVE publishedPublication date reported by the CVE source.
  2. Sep 23, 2025Record 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.

  1. Affected versionversion=< 0.23.31
  2. Affected versionversion=>= 1.0.0-M1, < 1.0.0-M45

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

http4s · http4sversion=< 0.23.31; version=>= 1.0.0-M1, < 1.0.0-M45

Published CVSS scores

6.3GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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