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<title>Featured Finding: Certificate path-length bypass | Zion Boggan</title>
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<meta name="description" content="A coordinated-disclosure vulnerability finding by Zion Boggan: an RFC 5280 certificate path-length constraint that a widely-deployed crypto library enforces only when an unrelated extension is present. Interactive in-browser demonstration of the bug class.">
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<meta property="og:title" content="Featured Finding: Certificate path-length bypass (coordinated disclosure)">
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<meta property="og:description" content="A logic-gating flaw in certificate-chain validation, found via variant analysis of a just-released security patch. Interactive demo of the bug class; full details after the fix ships.">
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<header class="hero"><div class="wrap">
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  <div class="crumb mono"><a href="/">home</a> / featured finding</div>
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  <div class="status mono"><span class="pulse"></span>Coordinated disclosure in progress</div>
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  <h1>A certificate path-length limit that disappears<br class="dtop">when you remove an unrelated field</h1>
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  <p class="lede">A widely-deployed open-source cryptography library enforces an <b>RFC&nbsp;5280
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    path-length constraint</b> (the rule that stops a subordinate CA from minting further
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    CAs) <b>only when the certificate also carries a separate, unrelated extension</b>. Drop
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    that extension and the limit is silently skipped: a CA that was explicitly forbidden from
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    delegating can issue rogue sub-CAs that the library accepts as valid. I found it by
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    <b>variant-hunting a security patch the maintainers had just shipped</b>. The fix closed
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    one instance of the gating mistake and left this one behind.</p>
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  <div class="facts mono">
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    <span>Class · <b>CWE-295</b> improper certificate validation</span>
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    <span>Standard · <b>RFC 5280</b> §6.1.4 / §4.2.1.9</span>
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    <span>Impact · <b>CA constraint bypass</b></span>
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    <span>Severity · <b>Medium</b> (~CVSS 6.0)</span>
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    <span>Confirmed on the <b>current shipped release</b></span>
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  </div>
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</div></header>
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<section><div class="wrap">
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  <div class="shead"><span class="idx mono">01</span><h2>See the bug in your browser</h2><span class="rule"></span></div>
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  <p class="body">This is a faithful, self-contained reproduction of the flaw's logic (no
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    library named yet; that waits for the fix). You're looking at a certificate chain where a
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    CA limited to <span class="mono">pathLenConstraint = 0</span>, meaning <i>"you may issue
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    end-entity certificates only, never another CA"</i>, has nonetheless issued a sub-CA.
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    A correct validator must reject this. <b>Toggle the <span class="mono">keyUsage</span>
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    extension</b> on that intermediate and watch the current library change its verdict on a chain that
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    should always be rejected, while the patched build stays put.</p>
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  <div class="lab">
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    <div class="labbar">
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      <span class="d r"></span><span class="d y"></span><span class="d g"></span>
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      <span class="dlabel mono">minipki · verifyCertificateChain()</span>
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    </div>
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    <div class="labgrid">
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      <div class="labcol left">
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        <h4>Certificate chain (attacker-supplied)</h4>
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        <div class="chain" id="chain"></div>
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        <div class="toggles">
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          <div class="tg" id="tgKU" onclick="toggle()">
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            <div class="sw"></div>
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            <div class="tt"><b>keyUsage</b> extension on the constrained intermediate
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              <small>An unrelated field. Real CAs include it; minimal tooling often omits it.</small></div>
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          </div>
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        </div>
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        <p class="labnote mono">Both validators re-evaluate the instant you flip the switch.</p>
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      </div>
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      <div class="labcol">
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        <h4>Same chain, two validators</h4>
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        <div class="vrow">
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          <div class="vbox" id="box_vuln">
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            <div class="vlabel">Current library</div>
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            <div class="vstate" id="st_vuln"></div>
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            <div class="vwhy" id="why_vuln"></div>
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          </div>
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          <div class="vbox" id="box_patch">
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            <div class="vlabel">With my one-line fix</div>
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            <div class="vstate" id="st_patch"></div>
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            <div class="vwhy" id="why_patch"></div>
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          </div>
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        </div>
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        <div class="vhint" id="hint"></div>
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      </div>
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    </div>
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  </div>
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  <p class="callout">One toggle, two verdicts. The current library lets the presence of an
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    unrelated extension decide whether it enforces the path-length limit, so flipping
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    <span class="mono">keyUsage</span> flips its answer on a chain that should always be rejected.
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    The patched validator ignores that field and rejects the chain every time.</p>
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</div></div></section>
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<section><div class="wrap">
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  <div class="shead"><span class="idx mono">02</span><h2>Why it happens</h2><span class="rule"></span></div>
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  <p class="body">Certificate-chain validators decide whether each intermediate is allowed to
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    act as a CA. RFC&nbsp;5280 makes two of those decisions <b>independent</b>: §4.2.1.9 says a
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    <span class="mono">basicConstraints</span> <span class="mono">pathLenConstraint</span> caps
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    how many CAs may follow in the path, and §6.1.4 applies that cap to <b>every</b> intermediate
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    unconditionally. The presence of a <span class="mono">keyUsage</span> extension is governed
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    separately by §4.2.1.3 and has nothing to do with path-length processing.</p>
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  <p class="body">The vulnerable code couples them anyway: the path-length check sits behind a
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    guard that only runs <span class="mono">if (keyUsage extension is present)</span>. The author's
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    reasoning was that having parsed <span class="mono">keyUsage</span> they "know" the basic
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    constraints are present too, but that conflates two orthogonal extensions. Remove
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    <span class="mono">keyUsage</span> and a real, signed, constraint-bearing CA escapes its own
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    limit. It's the same <b>"gate a mandatory check on an optional field"</b> anti-pattern that a
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    recently-published advisory fixed for a sibling check. This is the instance that fix didn't reach.</p>
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  <div class="embargo">
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    <b>Held until the fix ships:</b> the exact library, file/line, the published advisory, and
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    the real end-to-end proof-of-concept against the live release. Those will
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    appear here once the maintainers release a patched version. Responsible disclosure first,
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    write-up second. <span class="pending">(Private report submitted to the maintainer.)</span>
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  </div>
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</div></div></section>
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<section><div class="wrap">
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  <div class="shead"><span class="idx mono">03</span><h2>What I ruled out</h2><span class="rule"></span></div>
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  <p class="body">Finding the bug was half the work; <b>not crying wolf on the others was the
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    rest</b>. I ran four more independent deep analyses against the same library's most
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    security-critical paths. Each produced a rigorous negative, and a negative I can defend is
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    worth more than a finding I can't. This is the discipline that keeps a report from getting
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    auto-rejected as noise.</p>
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  <div class="ruled">
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    <div class="rcard">
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      <div class="h"><span class="neg mono">RULED OUT</span><h3>ASN.1 validator desync</h3></div>
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      <p>A recently-rewritten schema validator does ignore trailing elements, but every
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        trust-bearing sink is guarded by an outer length check or sits under a real signature.
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        Not weaponizable.</p>
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    </div>
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    <div class="rcard">
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      <div class="h"><span class="neg mono">RULED OUT</span><h3>RSA PKCS#1 v1.5 forgery</h3></div>
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      <p>Probed the Bleichenbacher / low-exponent class with real e=3 keys. Tight padding +
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        full-byte parsing + exact-digest compare pin the hash to the low-order bytes. No
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        no-private-key forgery.</p>
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    </div>
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    <div class="rcard">
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      <div class="h"><span class="neg mono">RULED OUT</span><h3>Ed25519 malleability</h3></div>
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      <p>Differential-tested against OpenSSL across non-canonical, low-order and cofactor edge
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        cases. The S&lt;L fix plus byte-exact R compare make the accepted-signature set a
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        singleton. No divergence.</p>
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    </div>
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    <div class="rcard">
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      <div class="h"><span class="neg mono">RULED OUT</span><h3>X.509 chain confusion</h3></div>
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      <p>DN/issuer spoofing, trust-anchor confusion, critical-extension smuggling, algorithm
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        downgrade. Trust anchoring is byte-exact; no false-accept beyond the path-length issue
325
        above.</p>
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    </div>
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  </div>
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</div></div></section>
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<section><div class="wrap">
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  <div class="shead"><span class="idx mono">04</span><h2>Disclosure timeline</h2><span class="rule"></span></div>
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  <ul class="timeline">
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    <li><span class="when">2026-05-31</span><span class="what"><b>Discovered &amp; confirmed.</b> Variant analysis of a just-released security patch; working PoC on the current shipped release.</span></li>
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    <li><span class="when">2026-05-31</span><span class="what"><b>Reported privately</b> to the maintainer through their security-advisory channel.</span></li>
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    <li><span class="when">pending</span><span class="what pending">Maintainer triage &amp; fix.</span></li>
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    <li><span class="when">pending</span><span class="what pending">Maintainer publishes an advisory.</span></li>
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    <li><span class="when">pending</span><span class="what pending"><b>This page unlocks:</b> library named, file/line, advisory link, live PoC against the patched-vs-vulnerable release.</span></li>
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  </ul>
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</div></div></section>
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<footer><div class="wrap">
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  <div class="row">
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    <div class="links">
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      <a href="/">← Back to home</a>
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      <a href="/security-research-notebook/">Research notebook</a>
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      <a href="https://github.com/zionboggan">GitHub</a>
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    </div>
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  </div>
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  <p class="note">Research conducted against open-source code under coordinated disclosure. No
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    systems were attacked; all proofs run against local copies and lab reproductions. Full
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    technical detail is withheld until a fixed release is available.</p>
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</div></footer>
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<script>
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// disclosure-safe MiniPKI: two faithful validators over one attacker-supplied chain.
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// No third-party code; models the logic with public RFC 5280 concepts.
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// Chain (top to bottom): Demo Root CA (trust anchor) -> Constrained Intermediate
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// (pathLenConstraint=0) -> Rogue Sub-CA -> leaf. A pathLen=0 CA must NOT issue another CA,
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// so a correct validator always rejects this chain.
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var keyUsage = false; // start in the divergent state so the difference is visible on load
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// returns true if the chain is ACCEPTED. enforceAlways=true models the patched build;
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// the current build only enforces pathLen when a keyUsage extension is present (the bug).
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function accepts(enforceAlways){
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  var enforce = enforceAlways || keyUsage;
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  return enforce ? false : true; // one sub-CA follows pathLen=0 -> rejected when enforced
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}
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function extLine(role){
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  if(role==='leaf')  return 'basicConstraints: cA=false';
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  if(role==='inter') return 'basicConstraints: cA=true, pathLenConstraint=0' + (keyUsage ? '  +keyUsage' : '  (no keyUsage)');
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  if(role==='sub')   return 'basicConstraints: cA=true  +keyUsage';
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  return 'trust anchor · basicConstraints: cA=true';
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}
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function renderChain(flagged){
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  var rows = [
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    {cn:'Demo Root CA',              role:'root',  cls:'okca'},
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    {cn:'Constrained Intermediate',  role:'inter', cls:'okca'},
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    {cn:'Rogue Sub-CA',              role:'sub',   cls: flagged ? 'flagged' : 'rogue'},
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    {cn:'victim.example.com',        role:'leaf',  cls: flagged ? 'flagged' : ''}
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  ];
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  var html='';
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  rows.forEach(function(r,i){
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    if(i) html += '<div class="arrow">│ issues ▼</div>';
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    html += '<div class="node '+r.cls+'"><div class="cn">'+r.cn+'</div><div class="ext">'+extLine(r.role)+'</div></div>';
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  });
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  document.getElementById('chain').innerHTML = html;
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}
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function setBox(id,accepted,whyAccept,whyReject){
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  var box=document.getElementById('box_'+id);
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  box.className='vbox '+(accepted?'accept':'reject');
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  document.getElementById('st_'+id).textContent = accepted ? '✗ Chain accepted' : '✓ Chain rejected';
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  document.getElementById('why_'+id).textContent = accepted ? whyAccept : whyReject;
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}
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function evaluate(){
399
  var vuln  = accepts(false); // current library
400
  setBox('vuln', vuln,
401
    'A pathLenConstraint=0 CA issued a sub-CA and it was trusted. keyUsage is absent, so the path-length check never ran.',
402
    'keyUsage happens to be present, so this build runs the path-length check and rejects the sub-CA.');
403
  setBox('patch', accepts(true),
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    '',
405
    'pathLenConstraint is enforced whenever cA and pathLen are set, regardless of keyUsage.');
406
  renderChain(vuln); // when the current library wrongly accepts, flag the certs it should not have trusted
407
  document.getElementById('hint').innerHTML = keyUsage
408
    ? 'Now switch <b>keyUsage</b> off: the current library will accept a chain it just rejected. The fix will not move.'
409
    : 'Flip <b>keyUsage</b> back on: the current library suddenly rejects the very same chain. Its decision hinges on a field that has nothing to do with path limits. The fix never let it.';
410
}
411
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function toggle(){
413
  keyUsage = !keyUsage;
414
  document.getElementById('tgKU').classList.toggle('on', keyUsage);
415
  evaluate();
416
}
417
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evaluate();
419
</script>
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</body>
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</html>