mtrudel bandit up to 1.10.x on Untrusted TCP Connection lib/bandit/pipeline.ex Elixir.bandit.Pipeline:determine_scheme reliance on untrusted inputs in a security decision

Summaryinfo

A vulnerability, which was classified as critical, has been found in mtrudel bandit up to 1.10.x on Untrusted. The affected element is the function Elixir.bandit.Pipeline:determine_scheme in the library lib/bandit/pipeline.ex of the component TCP Connection Handler. Performing a manipulation results in reliance on untrusted inputs in a security decision. This vulnerability is known as CVE-2026-39807. Remote exploitation of the attack is possible. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, was found in mtrudel bandit up to 1.10.x on Untrusted. This affects the function Elixir.bandit.Pipeline:determine_scheme in the library lib/bandit/pipeline.ex of the component TCP Connection Handler. The manipulation with an unknown input leads to a reliance on untrusted inputs in a security decision vulnerability. CWE is classifying the issue as CWE-807. The product uses a protection mechanism that relies on the existence or values of an input, but the input can be modified by an untrusted actor in a way that bypasses the protection mechanism. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

Reliance on Untrusted Inputs in a Security Decision vulnerability in mtrudel bandit allows unauthenticated transport-state spoofing on plaintext HTTP connections. 'Elixir.Bandit.Pipeline':determine_scheme/2 in lib/bandit/pipeline.ex returns the client-supplied URI scheme verbatim, ignoring the transport's secure? flag. HTTP/1.1 absolute-form request targets (e.g. GET https://victim/path HTTP/1.1) and the HTTP/2 :scheme pseudo-header are both attacker-controlled strings that flow through this function. Over a plaintext TCP connection, a client can declare https and Bandit will set conn.scheme = :https even though no TLS was negotiated. Downstream Plug consumers that branch on conn.scheme are silently misled: Plug.SSL's already-secure branch skips its HTTP→HTTPS redirect, cookies emitted with secure: true are sent over plaintext, audit logs record requests as having arrived over HTTPS, and CSRF/SameSite gating may make incorrect decisions. This issue affects bandit: from 1.0.0 before 1.11.0.

The weakness was published by Peter Ullrich and Jonatan Männchen as GHSA-375f-4r2h-f99j. It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-39807 since 04/07/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 1.11.0 eliminates this vulnerability. Applying the patch 45feea20dea8af7ffd7245271107b695c040e667 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-26714). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Vendor

• mtrudel

Name

• bandit

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6
• 1.7
• 1.8
• 1.9
• 1.10

License

• open-source

Website

• Product: https://github.com/mtrudel/bandit/

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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