CVE-2026-28975: NIOExtras NIOHTTPRequestDecompressor Ratio Limit Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6ph5-fww6-vfwv",
  "modified" : "2026-06-12T15:08:04Z",
  "published" : "2026-06-12T15:08:04Z",
  "aliases" : [ "CVE-2026-28975" ],
  "summary" : "NIOExtras: NIOHTTPRequestDecompressor ratio limit bypass via inflated Content-Length",
  "details" : "### Impact\n\nWhen `NIOHTTPRequestDecompressor` is configured with `.ratio(N)`, the decompression limit is enforced using the `Content-Length` header value from the incoming request rather than the actual number of compressed bytes received. Since `Content-Length` is attacker-controlled, a malicious client can supply an inflated value that causes the ratio check to always pass, effectively disabling the configured decompression limit.\n\nThis allows an attacker to send a small, highly-compressed payload (a \"gzip bomb\") with a falsified `Content-Length` header to bypass the ratio-based protection entirely. The server will decompress the payload without limit, consuming unbounded memory and potentially causing denial of service.\n\nFor example, a gzip payload containing highly repetitive data can achieve amplification ratios of several hundred to one. Under `.ratio(10)` such a payload should be rejected, but if the attacker sets `Content-Length` to match the decompressed size, the check evaluates `decompressed > decompressed * 10` which is always false, and the payload is accepted without error.\n\nAcross repeated requests, this allows sustained memory amplification far exceeding the configured limits with no error raised.\n\n### Relationship to CVE-2020-9840\n\nGHSA-xhhr-p2r9-jmm7 (CVE-2020-9840) found that the `.size` limit checked compressed rather than decompressed bytes and recommended `.ratio` as a workaround. This advisory identifies a distinct flaw in the `.ratio` limit itself: it uses the attacker-supplied `Content-Length` header as the denominator rather than actual consumed compressed bytes. The two vulnerabilities are in the same decompression limit enforcement code but involve non-overlapping logic errors.\n\nUsers who followed the CVE-2020-9840 workaround by switching to `.ratio(N)` are affected by this vulnerability.\n\n### Patches\n\nFixed in swift-nio-extras 1.34.1. The fix unifies the request and response decompressor implementations so that both accumulate actual compressed bytes received (`compressedLength += part.readableBytes`) rather than relying on any header-supplied value.\n\n### Workarounds\n\nUse `.size(N)` instead of `.ratio(N)` if a fixed upper bound on decompressed output is acceptable for the application. The `.size` limit is not affected by this vulnerability as it does not reference `Content-Length`.\n\n### Credits\n\nNIOExtras is grateful to @nathanielmiller23 for their reporting and assistance with the process.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "SwiftURL",
      "name" : "github.com/apple/swift-nio-extras"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.34.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/apple/swift-nio-extras/security/advisories/GHSA-6ph5-fww6-vfwv"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/apple/swift-nio-extras"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-409", "CWE-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-12T15:08:04Z",
    "nvd_published_at" : null
  }
}