{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.8.3"
      },
      "package": {
        "ecosystem": "Packagist",
        "name": "league/commonmark"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.5.0"
            },
            {
              "fixed": "2.9.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-71478"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79",
      "CWE-86",
      "CWE-692"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-06T20:30:39Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "\ufeff## Summary\n\nThe `AttributesExtension`\u0027s `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:\n\n- **Tab/newline inside the scheme** \u2014 a literal ASCII TAB (0x09), CR (0x0D), or LF (0x0A), e.g. `java\u003cTAB\u003escript:alert(1)`. Per the WHATWG URL Standard\u0027s \"basic URL parser\" step 3, browsers \"remove all ASCII tab or newline from input\".\n- **Leading C0 controls** \u2014 e.g. `\u003c0x01\u003ejavascript: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.)\n\nThe 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)`.\n\n**This is confirmed reproducible even with `allow_unsafe_links =\u003e false` set** \u2014 i.e. even applications that have followed the library\u0027s own documented hardening guidance for untrusted input remain exploitable.\n\nThis is a *sibling gap* in the same defense that CVE-2025-46734 (GHSA-3527-qv2q-pfvx) fixed in v2.7.0 \u2014 that fix made `href`/`src` respect `allow_unsafe_links`, but did not normalize control bytes before checking, so these obfuscation techniques were never covered.\n\n## Vulnerability\n\n**Files**:\n- `src/Util/RegexHelper.php:69` (`REGEX_UNSAFE_PROTOCOL`), `:239-242` (`isLinkPotentiallyUnsafe()`)\n- `src/Extension/Attributes/Util/AttributesHelper.php:149-179` (`filterAttributes()`)\n\n**CWE**: CWE-79 (Improper Neutralization of Input During Web Page Generation / XSS) \u2014 primary\n- CWE-692 (Incomplete Denylist to Cross-Site Scripting) \u2014 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 \u2192 XSS\" chain on its own.\n- CWE-86 (Improper Neutralization of Invalid Characters in Identifiers in Web Pages) \u2014 the specific evasion technique: control bytes embedded within the URI scheme identifier, which the browser strips before resolving it.\n\n### Root Cause\n```php\n// src/Util/RegexHelper.php\npublic const REGEX_UNSAFE_PROTOCOL = \u0027/^(?:javascript|vbscript|file|data):/i\u0027;\n\npublic static function isLinkPotentiallyUnsafe(string $url): bool\n{\n    return \\preg_match(self::REGEX_UNSAFE_PROTOCOL, $url) !== 0 \u0026\u0026 \\preg_match(self::REGEX_SAFE_DATA_PROTOCOL, $url) === 0;\n}\n\n// src/Extension/Attributes/Util/AttributesHelper.php\nforeach ($attributes as $name =\u003e $value) {\n    $attrNameLower = \\strtolower($name);\n    if (! $allowUnsafeLinks \u0026\u0026 ($attrNameLower === \u0027href\u0027 || $attrNameLower === \u0027src\u0027) \u0026\u0026 \\is_string($value) \u0026\u0026 RegexHelper::isLinkPotentiallyUnsafe($value)) {\n        unset($attributes[$name]);\n        continue;\n    }\n    ...\n```\nThe Attributes extension\u0027s own quote-value grammar (`PARTIAL_DOUBLEQUOTEDVALUE = \u0027\"[^\"]*\"\u0027`) 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\"` \u2014 so a leading `\\x01` survives). Critically, **the core Markdown link-destination path (`LinkParserHelper` \u2192 `UrlEncoder::unescapeAndEncode()`) percent-encodes every control byte before this same safety check ever runs \u2014 but the Attributes extension\u0027s `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 `\u0026 \u003c \u003e \" \u0027`, not tab/CR/LF, since they\u0027re legal bytes inside an HTML attribute).\n\n### Attack Scenario\n1. An application enables the (commonly-used) `AttributesExtension` and sets `allow_unsafe_links =\u003e false` \u2014 the project\u0027s own documented hardening step for untrusted input.\n2. An attacker submits Markdown: `[Click me](javascript:alert(0)){href=\"java\u003cTAB\u003escript:alert(document.cookie)\"}` (TAB is one literal 0x09 byte).\n3. The library emits `\u003ca href=\"java\u003cTAB\u003escript:alert(document.cookie)\"\u003eClick me\u003c/a\u003e` \u2014 `isLinkPotentiallyUnsafe()` doesn\u0027t match the tab-split scheme, so the filter takes no action.\n4. A victim viewing/clicking the link has the browser strip the embedded TAB and execute `javascript:alert(document.cookie)` in the victim\u0027s session \u2014 stored XSS, cookie theft, account takeover potential.\n\n**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[\u0027href\u0027]` with the node\u0027s own URL *unless* that URL is itself judged unsafe \u2014 so `[x](https://example.com){href=\"java\u003cTAB\u003escript:...\"}` 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 \u2014 the attacker writes the entire Markdown document.\n\nTwo related forms that are **not** exploitable, noted so the fix isn\u0027t over-scoped:\n\n- Attaching the attribute to a non-link block \u2014 `hi {href=\"java\u003cTAB\u003escript:alert(1)\"}` \u2014 does bypass the filter and emits `\u003cp href=\"java\u003cTAB\u003escript:alert(1)\"\u003e`, but `href` on a `\u003cp\u003e` 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.)\n- `\u003cimg src\u003e` is unaffected, since `ImageRenderer` unconditionally overwrites `src` from the core URL regardless of the safety verdict.\n\n### Recommended Fix\n\nNormalize inside `RegexHelper::isLinkPotentiallyUnsafe()` before testing, mirroring the WHATWG URL parser\u0027s 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.\n\n## Affected Versions\n\n**`\u003e= 1.5.0, \u003c= 2.8.3`** - every release that ships the `AttributesExtension`. Verified by installing each version and rendering the payloads with `allow_unsafe_links =\u003e false`. The attribute-value grammar (`PARTIAL_DOUBLEQUOTEDVALUE = \u0027\"[^\"]*\"\u0027`) has accepted raw control bytes since the extension was introduced, and none of the intervening parser rewrites narrowed it.\n\n## Prior Related Advisories\n\nGHSA-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.",
  "id": "GHSA-29pj-957v-52mc",
  "modified": "2026-08-06T20:30:40Z",
  "published": "2026-08-06T20:30:39Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/thephpleague/commonmark/security/advisories/GHSA-29pj-957v-52mc"
    },
    {
      "type": "WEB",
      "url": "https://github.com/thephpleague/commonmark/commit/493a5aa7d65754b73846006eaff9c2c4431a8e2c"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/thephpleague/commonmark"
    },
    {
      "type": "WEB",
      "url": "https://github.com/thephpleague/commonmark/releases/tag/2.9.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes"
}