Weakness Type (CWE)

CWE-807 — CWE-807 Reliance on Untrusted Inputs in a Security Decision

CAPEC

CVSS 4.0 Score

6.3

MEDIUM

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

Vulnerability 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.

Affected

pkg:hex/bandit

Module Source File Routine
Bandit.Pipeline lib/bandit/pipeline.ex Bandit.Pipeline.determine_scheme/2
Status Type Version Changes / Fixed in
affected semver ⓘ 1.0.0 < 1.11.0

pkg:github/mtrudel/bandit

Module Source File Routine
Bandit.Pipeline lib/bandit/pipeline.ex Bandit.Pipeline.determine_scheme/2
Status Type Version Changes / Fixed in
affected git ⓘ ff2f829326 < 1.11.0

Configurations

The vulnerable system must be accepting plaintext (non-TLS) HTTP connections, either directly or via h2c. Deployments that exclusively use TLS are not affected.

References

Credits

  • Finder: Peter Ullrich
  • Remediation developer: Mat Trudel
  • Analyst: Jonatan Männchen

CVE record as JSON:  GET /cves/CVE-2026-39807.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-39807.json