CVE-2026-39805: Inconsistent Interpretation of HTTP Requests vulnerability in mtrudel bandit allows HTTP request smuggling via duplicate Content-Length headers.

Inconsistent Interpretation of HTTP Requests vulnerability in mtrudel bandit allows HTTP request smuggling via duplicate Content-Length headers.

Inconsistent Interpretation of HTTP Requests vulnerability in mtrudel bandit allows HTTP request smuggling via duplicate Content-Length headers. 'Elixir.Bandit.Headers':get_content_length/1 in lib/bandit/headers.ex uses List.keyfind/3, which returns only the first matching header. When a request contains two Content-Length headers with different values, Bandit silently accepts it, uses the first value to read the body, and dispatches the remaining bytes as a second pipelined request on the same keep-alive connection. RFC 9112 §6.3 requires recipients to treat this as an unrecoverable framing error. When Bandit sits behind a proxy that picks the last Content-Length value and forwards the request rather than rejecting it, an unauthenticated attacker can smuggle requests past edge WAF rules, path-based ACLs, rate limiting, and audit logging. This issue affects bandit: before 1.11.0.

Description from the National Vulnerability Database, published by 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

What it means for a small business

If your business uses bandit, install the vendor's update. If you're not sure whether you use it, ask whoever manages your computers.

How likely is an attack? The Exploit Prediction Scoring System, run by FIRST (the global association of security response teams), puts the odds of this one being exploited in the next 30 days at 0.63%, higher than 49% of CVEs. That’s low, but low isn’t zero, and the odds jump the day someone publishes an exploit. Scored October 29, 2026, updated daily.

Check your own setup

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Server check: what your web server says about itself, and whether that version has known flaws.

Sources

Part of the Huttenlocher Technologies vulnerability database, refreshed twice a day from CISA and the NVD. The title, category and plain-language guidance are ours; check the linked record before acting on a detail. This is general guidance, not a review of your systems.

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