NIOExtras: 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
NIOExtras is grateful to @nathanielmiller23 for their reporting and assistance with the process.