CVE-2026-66357Public exploitNetwork attack vectorHinoki check available

Request smuggling via promoted obs-fold header

Published Sep 1, 2026 · Updated Sep 1, 2026

HTTP request smuggling in Erlang/OTP inets httpd before 27.3.4.17, 28.5.0.6, or 29.0.6 allows remote attackers to alter requests. The parser strips leading whitespace from an obs-fold continuation and promotes its content to a standalone header, exposing a hidden Transfer-Encoding header. Exploitation requires a folding-aware reverse proxy in front of httpd and can desynchronize request boundaries so leftover bytes become the next request.

CVSS severity8.3
High
EPSS probability0.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
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Summary

What happened

HTTP request smuggling in Erlang/OTP inets httpd before 27.3.4.17, 28.5.0.6, or 29.0.6 allows remote attackers to alter requests. The parser strips leading whitespace from an obs-fold continuation and promotes its content to a standalone header, exposing a hidden Transfer-Encoding header. Exploitation requires a folding-aware reverse proxy in front of httpd and can desynchronize request boundaries so leftover bytes become the next request.

The record

CVE
CVE-2026-66357
Published
Sep 1, 2026
Updated
Sep 1, 2026
Vendor
Erlang
Product
Erlang/OTP
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 1, 2026CVE publishedPublication date reported by the CVE source.
  2. Sep 1, 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.

  1. Affected versionversion=17.0 <27.3.4.17
  2. Affected versionversion=28.0 <28.5.0.6
  3. Affected versionversion=29.0 <29.0.6
  4. Affected versionversion=5.10 <9.3.2.7
  5. Affected versionversion=84adefa331c4159d432d22840663c38f155cd4c1 <* changes=[{"at":"220d618d2479a7180b4a06d0c5aacfaa4af4a85b","status":"unaffected"},{"at":"273d8958de38ff2a7fe0c5dcc5b6bfe6f65717f3","status":"unaffected"},{"at":"e640d599287d2eba919e6f13b91f042d7dbe6ff4","status":"unaffected"}]
  6. Affected versionversion=9.4 <9.6.2.3
  7. Affected versionversion=9.7 <9.7.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Erlang · Erlang/OTPversion=17.0 <27.3.4.17; version=28.0 <28.5.0.6; version=29.0 <29.0.6; version=5.10 <9.3.2.7; version=84adefa331c4159d432d22840663c38f155cd4c1 <* changes=[{"at":"220d618d2479a7180b4a06d0c5aacfaa4af4a85b","status":"unaffected"},{"at":"273d8958de38ff2a7fe0c5dcc5b6bfe6f65717f3","status":"unaffected"},{"at":"e640d599287d2eba919e6f13b91f042d7dbe6ff4","status":"unaffected"}]; version=9.4 <9.6.2.3; version=9.7 <9.7.2

Published CVSS scores

8.3Erlang/OTP GitHub Security AdvisoryCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:H/VA:N/SC:N/SI:L/SA: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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