CVE-2026-105660

Unvalidated HTTP/1 request-target authority lets a client choose conn.host and conn.port in Bandit

Published Oct 15, 2026 CVSS 4.0 · 6.9 Medium EPSS < 1% KEV · Not listed
Description
Improper Validation of Syntactic Correctness of Input vulnerability in mtrudel bandit allows an unauthenticated remote attacker to set conn.host and conn.port to values outside the RFC 3986 host grammar via the authority of an HTTP/1 absolute-form or authority-form request target. Bandit validates the Host header and answers 400 when it is malformed, but when the request target carries its own authority that authority takes precedence, as RFC 9112 requires, and Bandit.Pipeline.determine_host_and_port!/4 in lib/bandit/pipeline.ex passes it to the application unchecked. A request line such as GET http://good.example@evil.example/x HTTP/1.1 with a valid Host header yields a conn.host of good.example@evil.example, and the same path admits NUL, DEL and non-ASCII bytes, invalid IP literals and ports outside 1..65535. The HTTP/1.0 clause needs no Host header at all, and CONNECT places the unparsed port string in conn.port.

Applications that build absolute URLs from conn.host, such as through Plug.Conn.request_url/1 or Phoenix URL helpers, emit links whose authority the attacker chose. Message framing is unaffected, and no CR or LF can reach conn.host.

This issue affects bandit: from 0.6.0 before 1.13.0.

Remediation

Vendor Workaround

Add a plug ahead of any code that reads conn.host or conn.port that compares conn.host against an allow-list of the hostnames the application serves, checks that conn.port is an integer, and halts the request with a 400 response otherwise. Do not derive absolute URLs from conn.host until that check has passed.

Tracking

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Advisories

No advisories available
History
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
Description Improper Validation of Syntactic Correctness of Input vulnerability in mtrudel bandit allows an unauthenticated remote attacker to set conn.host and conn.port to values outside the RFC 3986 host grammar via the authority of an HTTP/1 absolute-form or authority-form request target. Bandit validates the Host header and answers 400 when it is malformed, but when the request target carries its own authority that authority takes precedence, as RFC 9112 requires, and Bandit.Pipeline.determine_host_and_port!/4 in lib/bandit/pipeline.ex passes it to the application unchecked. A request line such as GET http://good.example@evil.example/x HTTP/1.1 with a valid Host header yields a conn.host of good.example@evil.example, and the same path admits NUL, DEL and non-ASCII bytes, invalid IP literals and ports outside 1..65535. The HTTP/1.0 clause needs no Host header at all, and CONNECT places the unparsed port string in conn.port. Applications that build absolute URLs from conn.host, such as through Plug.Conn.request_url/1 or Phoenix URL helpers, emit links whose authority the attacker chose. Message framing is unaffected, and no CR or LF can reach conn.host. This issue affects bandit: from 0.6.0 before 1.13.0.
Title Unvalidated HTTP/1 request-target authority lets a client choose conn.host and conn.port in Bandit
First Time appeared Mtrudel
Mtrudel bandit
Weaknesses CWE-1286
CPEs cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*
Vendors & Products Mtrudel
Mtrudel bandit
References
Metrics cvssV4_0

{'score': 6.9, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
EEF
Published
Updated
Oct 16, 2026, 8:02 PM
Reserved
Oct 10, 2026, 1:15 PM
Status
Deferred
Published
Oct 15, 2026, 7:17 PM
Modified
Oct 16, 2026, 9:17 PM