NIOHTTPRequestDecompressor ratio limit bypass via inflated Content-Length

### Impact When `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. This 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. For 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. Across repeated requests, this allows sustained memory amplification far exceeding the configured limits with no error raised. ### Relationship to CVE-2020-9840 GHSA-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. Users who followed the CVE-2020-9840 workaround by switching to `.ratio(N)` are affected by this vulnerability. ### Patches Fixed 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. ### Workarounds Use `.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`. ### Credits We are very grateful to @nathanielmiller23 for their reporting and assistance with our process.

Severity from

GitHub (reviewed advisory)

• github.com/apple/swift-nio-extras/security/advisories/GHSA-6ph5-fww6-vfwv
• github.com/apple/swift-nio-extras