HTTP/2 cookie crumb join expands a small HPACK header block into an unbounded allocation in Bandit

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  "url": "https://www.cve.org/CVERecord?id=CVE-2026-109584",
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    "kind": "advisory",
    "title": "HTTP/2 cookie crumb join expands a small HPACK header block into an unbounded allocation in Bandit",
    "summary": "Improper Handling of Highly Compressed Data (Data Amplification) vulnerability in the HTTP/2 implementation of mtrudel bandit allows an unauthenticated remote attacker to exhaust server memory via a small HPACK-compressed HEADERS block. The block stores one large cookie value in the HPACK dynamic table and then references it tens of thousands of times with 1-byte indexed fields. In the reporter's run, a 49 KB block made a server with default settings allocate a 183 MB cookie binary before any application code ran, and every concurrent stream can repeat the cost. Bandit.HTTP2.Connection caps only the compressed block (max_header_block_size), and its check of the decoded list against max_header_list_size never fires because that setting defaults to :infinity. combine_cookie_crumbs/1 in lib/bandit/http2/stream.ex then joins every crumb into one binary. This issue affects bandit: from 0.3.3 before 1.14.0.",
    "evidence": "cve:CVE-2026-109584",
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            "version": "4.0",
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            "baseScore": 8.7,
            "Automatable": "NOT_DEFINED",
            "attackVector": "NETWORK",
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            "valueDensity": "NOT_DEFINED",
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            "providerUrgency": "NOT_DEFINED",
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      "affected": [
        {
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            "cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*"
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          "collectionURL": "https://repo.hex.pm",
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      "modified": "2026-10-26T15:35:51.081Z",
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      "references": [
        "https://github.com/mtrudel/bandit/security/advisories/GHSA-7gfj-7gvq-3xqj",
        "https://cna.erlef.org/cves/CVE-2026-109584.html",
        "https://osv.dev/vulnerability/EEF-CVE-2026-109584",
        "https://github.com/mtrudel/bandit/commit/44bf969cd59851622fb6ee4d16227fbc39a77fb4",
        "https://github.com/mtrudel/bandit/commit/220b9dfd27f9a21305be2afc75cbe585ddc24e7a"
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        "value": "CVE-2026-109584",
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  "published_at": "2026-10-26T13:32:22.359+00:00"
}