Craft CMS from 4.0.0-RC1 contains an authenticated path traversal vulnerability in the assets/icon endpoint where the extension parameter is not validated before file existence checks. Attackers can bypass extension validation by passing traversal sequences that resolve to existing SVG files, allowing local file read access.
Craft CMS versions >= 5.0.0-RC1, <= 5.9.13 and >= 4.0.0-RC1, <= 4.17.7 contain an authorization bypass in the assets/preview-file endpoint. The action does not enforce per-asset view authorization before returning preview content, allowing an authenticated low-privileged user to supply a controlled assetId for an asset they are not permitted to view and still receive preview response data (previewHtml), including a private preview image route containing the target private assetId. Fixed in 5.9.14 and 4.17.8.
## Summary The `AttributesExtension`'s `href`/`src` unsafe-link filter (`AttributesHelper::filterAttributes()`) can be bypassed by embedding control bytes in a `javascript:` URL that browsers discard before parsing the scheme. Two variants: - **Tab/newline inside the scheme** — a literal ASCII TAB (0x09), CR (0x0D), or LF (0x0A), e.g. `java<TAB>script:alert(1)`. Per the WHATWG URL Standard's "basic URL parser" step 3, browsers "remove all ASCII tab or newline from input". - **Leading C0 controls** — e.g. `<0x01>javascript:alert(1)`. Per step 1 of the same algorithm, browsers remove any leading or trailing C0 control or space. (A leading *space* alone does not bypass, because `parseAttributes()` already `trim()`s the value; other C0 bytes are not trimmed.) The filter is a literal anchored-prefix regex (`RegexHelper::isLinkPotentiallyUnsafe()` / `REGEX_UNSAFE_PROTOCOL`) that matches neither obfuscated form, so in both cases the browser still executes `javascript:alert(1)`. **This is confirmed reproducible even with `allow_unsafe_links => false` set** — i.e. even applications that have followed the library's own documented hardening guidance for untrusted input remain exploitable. This is a *sibling gap* in the same defense that CVE-2025-46734 (GHSA-3527-qv2q-pfvx) fixed in v2.7.0 — that fix made `href`/`src` respect `allow_unsafe_links`, but did not normalize control bytes before checking, so these obfuscation techniques were never covered. ## Vulnerability **Files**: - `src/Util/RegexHelper.php:69` (`REGEX_UNSAFE_PROTOCOL`), `:239-242` (`isLinkPotentiallyUnsafe()`) - `src/Extension/Attributes/Util/AttributesHelper.php:149-179` (`filterAttributes()`) **CWE**: CWE-79 (Improper Neutralization of Input During Web Page Generation / XSS) — primary - CWE-692 (Incomplete Denylist to Cross-Site Scripting) — the anchored-prefix denylist in `REGEX_UNSAFE_PROTOCOL` is incomplete. This is a composite of CWE-184 and CWE-79, so it captures the full "incomplete denylist → XSS" chain on its own. - CWE-86 (Improper Neutralization of Invalid Characters in Identifiers in Web Pages) — the specific evasion technique: control bytes embedded within the URI scheme identifier, which the browser strips before resolving it. ### Root Cause ```php // src/Util/RegexHelper.php public const REGEX_UNSAFE_PROTOCOL = '/^(?:javascript|vbscript|file|data):/i'; public static function isLinkPotentiallyUnsafe(string $url): bool { return \preg_match(self::REGEX_UNSAFE_PROTOCOL, $url) !== 0 && \preg_match(self::REGEX_SAFE_DATA_PROTOCOL, $url) === 0; } // src/Extension/Attributes/Util/AttributesHelper.php foreach ($attributes as $name => $value) { $attrNameLower = \strtolower($name); if (! $allowUnsafeLinks && ($attrNameLower === 'href' || $attrNameLower === 'src') && \is_string($value) && RegexHelper::isLinkPotentiallyUnsafe($value)) { unset($attributes[$name]); continue; } ... ``` The Attributes extension's own quote-value grammar (`PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"'`) accepts any byte except `"` inside quotes, including raw tab/CR/LF and other C0 controls, and `parseAttributes()` only `trim()`s (leading/trailing, and only the default charlist `" \t\n\r\0\x0B"` — so a leading `\x01` survives). Critically, **the core Markdown link-destination path (`LinkParserHelper` → `UrlEncoder::unescapeAndEncode()`) percent-encodes every control byte before this same safety check ever runs — but the Attributes extension's `href`/`src` handling has no equivalent normalization step**, so the raw control byte reaches both the check and the final HTML output (`Xml::escape()` only escapes `& < > " '`, not tab/CR/LF, since they're legal bytes inside an HTML attribute). ### Attack Scenario 1. An application enables the (commonly-used) `AttributesExtension` and sets `allow_unsafe_links => false` — the project's own documented hardening step for untrusted input. 2. An attacker submits Markdown: `[Click me](javascript:alert(0)){href="java<TAB>script:alert(document.cookie)"}` (TAB is one literal 0x09 byte). 3. The library emits `<a href="java<TAB>script:alert(document.cookie)">Click me</a>` — `isLinkPotentiallyUnsafe()` doesn't match the tab-split scheme, so the filter takes no action. 4. A victim viewing/clicking the link has the browser strip the embedded TAB and execute `javascript:alert(document.cookie)` in the victim's session — stored XSS, cookie theft, account takeover potential. **Why the payload needs an unsafe core destination.** Step 2 above deliberately uses `[Click me](javascript:alert(0))` rather than a normal link. `LinkRenderer` overwrites `attrs['href']` with the node's own URL *unless* that URL is itself judged unsafe — so `[x](https://example.com){href="java<TAB>script:..."}` renders the harmless `href="https://example.com"`, and an empty destination `[x](){href="..."}` renders `href=""`. The attacker therefore supplies a core destination that the filter *does* catch, which suppresses the overwrite and lets the attribute-supplied `href` reach the final tag. This is no obstacle in practice — the attacker writes the entire Markdown document. Two related forms that are **not** exploitable, noted so the fix isn't over-scoped: - Attaching the attribute to a non-link block — `hi {href="java<TAB>script:alert(1)"}` — does bypass the filter and emits `<p href="java<TAB>script:alert(1)">`, but `href` on a `<p>` is inert: there is nothing to navigate. (An earlier draft of this report described this as a "simpler, unconditional variant" of the attack; it is a filter bypass, not an XSS.) - `<img src>` is unaffected, since `ImageRenderer` unconditionally overwrites `src` from the core URL regardless of the safety verdict. ### Recommended Fix Normalize inside `RegexHelper::isLinkPotentiallyUnsafe()` before testing, mirroring the WHATWG URL parser's own normalization. This covers both variants, fixes every call site at once (`LinkRenderer`, `ImageRenderer`, and any third-party callers), and needs no changes in the Attributes extension. ## Affected Versions **`>= 1.5.0, <= 2.8.3`** - every release that ships the `AttributesExtension`. Verified by installing each version and rendering the payloads with `allow_unsafe_links => false`. The attribute-value grammar (`PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"'`) has accepted raw control bytes since the extension was introduced, and none of the intervening parser rewrites narrowed it. ## Prior Related Advisories GHSA-3527-qv2q-pfvx / CVE-2025-46734 fixed a different Attributes-extension XSS (unallowlisted `on*` handlers, `href`/`src` not respecting `allow_unsafe_links` at all) in v2.7.0. This issue bypasses the specific `href`/`src` protection that fix introduced (the control-byte normalization gap was not part of that fix) - but the obfuscated inputs also work on older versions.