CVE-2026-63379Network attack vector

Authorization bypass via merged HTTP trailers

Published Aug 20, 2026 · Updated Aug 25, 2026

HTTP request smuggling in Libevent before 2.1.13 and 2.2.0-alpha through 2.2.1-alpha allows remote attackers to bypass controls. The evhttp_read_trailer path parses chunked trailers into req->input_headers, treating attacker-supplied trailer fields as ordinary request headers. Exploitation requires a Libevent HTTP deployment behind an upstream proxy that interprets those fields differently, enabling authorization bypass, proxy-header spoofing, or cache poisoning.

CVSS severity6.3
Medium
EPSS probability0.52%
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 Libevent before 2.1.13 and 2.2.0-alpha through 2.2.1-alpha allows remote attackers to bypass controls. The evhttp_read_trailer path parses chunked trailers into req->input_headers, treating attacker-supplied trailer fields as ordinary request headers. Exploitation requires a Libevent HTTP deployment behind an upstream proxy that interprets those fields differently, enabling authorization bypass, proxy-header spoofing, or cache poisoning.

The record

CVE
CVE-2026-63379
Published
Aug 20, 2026
Updated
Aug 25, 2026
Vendor
Libevent
Product
Libevent
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 20, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 25, 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=< 2.1.13
  2. Affected versionversion=>= 2.2.0-alpha, < 2.2.2-alpha

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Libevent · Libeventversion=< 2.1.13; version=>= 2.2.0-alpha, < 2.2.2-alpha

Published CVSS scores

6.3GitHub CNACVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
6.5Amazon LinuxCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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 exploitUnconfirmed

Weakness, pattern, technique

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

Public exploit references

No public exploit references are available in this record.

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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