Description

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.

Weaknesses 1

References 4

• cna.erlef.org https://cna.erlef.org/cves/CVE-2026-39807.html
• github.com https://github.com/mtrudel/bandit/commit/45feea20dea8af7ffd7245271107b695c040e667
• github.com https://github.com/mtrudel/bandit/security/advisories/GHSA-375f-4r2h-f99j
• osv.dev https://osv.dev/vulnerability/EEF-CVE-2026-39807

Remediation