GHSA-gj48-438w-jh9v

### Summary

Bleach `clean()` / `Cleaner()` fails to sanitize dangerous URI schemes in allowed `formaction` attributes.

Bleach applies URI protocol sanitization only to attributes listed in `attr_val_is_uri`. While URI-bearing attributes such as `action`, `href`, `src`, and `poster` are included in that set, `formaction` is not. As a result, if a downstream application explicitly allows `formaction` on submit-capable controls in untrusted HTML, Bleach preserves dangerous values such as `javascript:alert(1)` instead of stripping them.

This can lead to **submit-triggered JavaScript execution** in applications that rely on Bleach to sanitize untrusted HTML and allow the relevant tag/attribute combination.

---

### Details

The issue appears to be a URI-sanitization coverage gap in Bleach’s sanitizer logic.

Relevant code paths:

* `bleach/sanitizer.py` — `BleachSanitizerFilter.allow_token` (around line 553) * `bleach/_vendor/html5lib/filters/sanitizer.py` — `attr_val_is_uri` (around line 525)

In `BleachSanitizerFilter.allow_token`, URI protocol sanitization is only applied when:

```python id="pft79m" if namespaced_name in self.attr_val_is_uri: ```

However, `(None, 'formaction')` is currently missing from `attr_val_is_uri`.

This creates an inconsistency where `action` is protocol-sanitized, but `formaction` is not.

As a result, if a downstream application allows:

* tags such as `<button>` or `<input>` * the `formaction` attribute

then Bleach preserves dangerous URI schemes such as `javascript:` in `formaction`.

Examples of affected submit-capable controls include:

* `<button>` (default submit behavior unless `type="button"` is set) * `<input type="submit">` * `<input type="image">`

This appears to be a real library-side sanitizer gap rather than only an application misuse issue, because Bleach already treats similar URI-bearing attributes (such as `action`) as protocol-sensitive and sanitizes them.

Suggested minimal fix:

Add:

```python id="4v4fkn" (None, 'formaction') ```

to `attr_val_is_uri` in:

* `bleach/_vendor/html5lib/filters/sanitizer.py`

I also prepared a minimal patch and focused regression tests if helpful.

---

### PoC

Below are minimal reproductions using `bleach.clean()`.

#### 1) `<button>`

```python id="d3g0v7" from bleach import clean

print(clean( '<form><button formaction="javascript:alert(1)">go</button></form>', tags={'form', 'button'}, attributes={'button': ['formaction']}, )) ```

**Actual output:**

```html id="i4nd7s" <form><button formaction="javascript:alert(1)">go</button></form> ```

**Expected output:**

```html id="g4d2r1" <form><button>go</button></form> ```

---

#### 2) `<input type="submit">`

```python id="l4dy0j" print(clean( '<form><input type="submit" formaction="javascript:alert(1)" value="go"></form>', tags={'form', 'input'}, attributes={'input': ['type', 'formaction', 'value']}, )) ```

**Actual output:**

```html id="h8lgbt" <form><input type="submit" formaction="javascript:alert(1)" value="go"></form> ```

**Expected output:**

```html id="6y8mws" <form><input type="submit" value="go"></form> ```

---

#### 3) `<input type="image">`

```python id="g8q0x8" print(clean( '<form><input type="image" formaction="javascript:alert(1)" src="/foo.png"></form>', tags={'form', 'input'}, attributes={'input': ['type', 'formaction', 'src']}, )) ```

**Actual output:**

```html id="fd22kg" <form><input type="image" formaction="javascript:alert(1)" src="/foo.png"></form> ```

**Expected output:**

```html id="z6t6je" <form><input type="image" src="/foo.png"></form> ```

---

### Impact

This is a **client-side HTML sanitization bypass / dangerous URI preservation issue**.

If an application relies on Bleach to sanitize untrusted HTML and explicitly allows:

* `formaction` * and submit-capable controls such as `<button>` or `<input>`

then Bleach can emit sanitized output that still contains a dangerous `javascript:` URI in `formaction`.

That can lead to **submit-triggered JavaScript execution** when the user activates the control.

Impact is limited to configurations that explicitly allow the relevant tag/attribute combination, but the issue is still security-relevant because:

* `formaction` is a real browser sink * Bleach already protocol-sanitizes similar URI-bearing attributes like `action` * the omission creates inconsistent sanitizer coverage for dangerous URI schemes

I would currently assess this as **Medium severity**.

If useful, I also have:

* a minimal patch * focused regression tests for:

* `<button formaction="javascript:...">` * `<input type="submit" formaction="javascript:...">` * `<input type="image" formaction="javascript:...">` * a safe control case where `formaction="/submit"` is preserved