Description

league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes ## 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

// 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;
}
...
Show more

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

An application enables the (commonly-used) AttributesExtension and sets allow_unsafe_links => false — the project's own documented hardening step for untrusted input.

An attacker submits Markdown: [Click me](javascript:alert(0)){href="java<TAB>script:alert(document.cookie)"} (TAB is one literal 0x09 byte).

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.

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.

Mitigation

Aliases

References