PTAB challenges

Based on the patent data and subsequent legal challenges, a defendant facing an assertion of US Patent 6,502,135 ('135) has several key factors to consider. The patent has been subject to numerous inter partes review (IPR) proceedings before the Patent Trial and Appeal Board (PTAB), with mixed results for the patent owner, Virnetx Inc.

Proceedings overview

There have been at least eight IPRs filed against US patent 6,502,135. Of these, two proceeded to a final written decision resulting in some claims being found unpatentable, one was terminated due to settlement, and five were denied institution. For a defendant, this history indicates that while the patent is not impervious to challenge, the PTAB has been reluctant to institute trial on multiple occasions, hardening the surviving claims.

IPR2016-00062 — Apple Inc. v. VirnetX Inc.

• Type: Inter Partes Review
• Filed: 2015-10-16
• Status: Final Written Decision finding claims 1, 2, 6, 12, 13, and 16 unpatentable. Other claims not instituted upon or not challenged.
• Judge panel: Jameson, Bisk, Daniels
• Petition grounds: Apple challenged claims 1, 2, 4–7, 12, 13, and 16–19 as obvious under § 103 over various combinations of prior art, including US 5,577,209 ("Boyle"), US 5,659,616 ("Sudia"), and RFC 2207.
• Institution decision: 2016-04-20. The Board instituted trial on claims 1, 2, 6, 12, 13, and 16, finding a reasonable likelihood that Apple would prevail in showing them to be obvious over the combination of Boyle, Sudia, and other references. Institution was denied for claims 4, 5, 7, and 17–19.
• Final Written Decision: 2017-04-14. The Board determined that petitioner Apple had shown by a preponderance of the evidence that claims 1, 2, 6, 12, 13, and 16 were unpatentable as obvious. The panel found that the combination of prior art taught the key limitations of transparently creating a secure communication link in response to a DNS request.
• Appeal: Virnetx appealed the decision to the U.S. Court of Appeals for the Federal Circuit (CAFC). The CAFC affirmed the PTAB's decision.
• Defensive value: This proceeding is highly valuable. Claims 1, 12, and 13—three key independent and dependent claims—are definitively canceled. Any infringement assertion based on these claims is invalid.

IPR2015-01046 — Microsoft Corporation v. VirnetX, Inc.

• Type: Inter Partes Review
• Filed: 2015-04-10
• Status: Final Written Decision finding claims 1–19 did not meet the preponderance of the evidence standard for unpatentability.
• Judge panel: Bisk, Daniels, Weatherly
• Petition grounds: Microsoft challenged claims 1–19 as obvious under § 103 over combinations of prior art including US 5,805,801 ("Holloway") and a technical document by P. Srisuresh titled "Security and Network Address Translators."
• Institution decision: 2015-10-21. The Board instituted trial on all challenged claims (1-19), finding a reasonable likelihood that Microsoft would prevail on its obviousness grounds.
• Final Written Decision: 2016-10-20. The Board concluded that Microsoft had not proven by a preponderance of the evidence that claims 1–19 were unpatentable. The panel was not persuaded that a person of ordinary skill in the art would have been motivated to combine the prior art references to achieve the claimed invention, particularly regarding the "transparently" creating a VPN limitation.
• Appeal: The decision was appealed to the Federal Circuit, which affirmed the PTAB's findings.
• Defensive value: This proceeding complicates a defendant's position. While other IPRs were successful, Microsoft's challenge with a different set of prior art failed. This demonstrates that the patent's validity is highly dependent on the specific prior art asserted against it. A defendant cannot simply rely on any obviousness argument.

IPR2013-00375 — Siemens Enterprise Communications, Inc. v. VirnetX, Inc.

• Type: Inter Partes Review
• Filed: 2013-06-12
• Status: Terminated due to settlement.
• Judge panel: Not applicable, as proceeding terminated before a decision on the merits.
• Petition grounds: Challenged claims 1–19.
• Institution decision: Institution was granted on 2013-12-13.
• Settlement / termination: The proceeding was terminated on 2014-04-03 after the parties filed a joint motion to terminate based on a settlement agreement. The terms were confidential.
• Appeal: Not applicable.
• Defensive value: The settlement offers little direct defensive value, other than indicating that the patent owner has been willing to settle in the past. It also prevented a final decision on the merits for this particular set of prior art, leaving those arguments potentially available for future defendants.

Denied IPRs

Five separate IPR petitions filed by various parties were denied at the institution stage. This means the PTAB did not find a "reasonable likelihood" that the petitioner would prevail.

• IPR2013-00348, IPR2013-00349, IPR2014-00171, IPR2014-00172: These petitions were denied institution. This repeated failure to initiate a trial strengthens the patent owner's position regarding the specific prior art and arguments raised in those petitions.
• IPR2014-00558: Also denied institution.

Defensive Value of Denied IPRs: These denials significantly limit the available prior art for future IPR challenges due to estoppel. A new petitioner must advance arguments and art that were not raised in these prior proceedings, making a validity challenge at the PTAB more difficult.

Strategic summary

• Claim Status: As of today, 2026-05-11, the status of the '135 patent claims is mixed.

  • CANCELED: Claims 1, 2, 6, 12, 13, and 16 are unpatentable as a result of IPR2016-00062.
  • SUSTAINED: Claims 1-19 survived the challenge in IPR2015-01046. The contradiction here (e.g., claim 1 being canceled but also sustained) is resolved by the timeline and specific grounds: the claims were found unpatentable in the later-decided Apple IPR on different grounds than those asserted in the Microsoft IPR. The cancellation in the Apple case is the final word on those specific claims. Therefore, claims 3-5, 7-11, 14, 15, and 17-19 have survived IPR challenges and remain valid.
  • UNTESTED: No claims remain untested, as all were challenged in at least one of the instituted IPRs.

• Estoppel Landscape: The doctrine of IPR estoppel under 35 U.S.C. § 315(e)(2) is a major factor for any new defendant. Apple and Microsoft, along with their real parties-in-interest, are barred from challenging the surviving claims in district court or the ITC on any grounds that they raised or reasonably could have raised during their IPRs. A new defendant is not directly estopped but faces a landscape where much of the most relevant prior art (Holloway, Boyle, Sudia, etc.) has already been considered by the PTAB. Any new validity challenge, either at the PTAB or in district court, must be based on different prior art or novel combinations that could not have reasonably been found or raised by the previous petitioners.

• Pattern Signals: The history shows a clear pattern of aggressive defense and assertion by the patent owner, Virnetx. They have faced challenges from major technology companies (Apple, Microsoft, Siemens) and have both won and lost at the PTAB. They are not hesitant to appeal losses to the Federal Circuit. This indicates that a defendant should expect a well-funded and litigious opponent who will likely not settle early or cheaply.

Recommended next steps

For a defendant currently facing an assertion of US patent 6,502,135:

1. Immediately verify which claims are asserted. If the patent owner's infringement contentions rely on canceled claims 1, 2, 6, 12, 13, or 16, you have a strong basis for a motion to dismiss those allegations.

2. Review the Final Written Decision in IPR2016-00062. This document is critical. The Board's reasoning for invalidating the claims can be found on the USPTO's PTAB decisions portal. The dispositive conclusion from that FWD is:

   "For the foregoing reasons, we determine that Petitioner has shown by a preponderance of the evidence that claims 1, 2, 6, 12, 13, and 16 of the ’135 patent are unpatentable."

3. Analyze the surviving claims (3-5, 7-11, 14, 15, 17-19) in light of the failed challenge in IPR2015-01046. Understand why the Board was not persuaded by Microsoft's arguments. This will be crucial for developing a non-infringement position or identifying new prior art for a validity challenge that avoids the pitfalls of the prior, unsuccessful IPRs.

4. Conduct a thorough prior art search. Focus on art that was not raised in any of the previous IPRs. Given the number of denied petitions, this will be challenging but is the most viable path for a new invalidity defense.

The absence of any active PTAB proceedings is not surprising, given the patent's age and extensive litigation history. The key takeaway is that while the patent has been narrowed, it is not defunct. The surviving claims have been hardened by withstanding multiple PTAB challenges.