Researchers Daan Keuper and Thijs Alkemade of Computest demonstrated a Zoom client exploit at Pwn2Own on April 7, 2021. SecurityWeek reported that they received $200,000 for it. The chain combined a heap-based buffer overflow with flaws involving GIPHY message paths and Zoom Marketplace URLs. It could run without the victim clicking a message, but it was not an attack on any stranger at random: the victim had to accept an external connection request or share a multi-user chat with the attacker.
What the exploit did
The principal vulnerability let a remote attacker execute code in the context of the current user on an affected Zoom client. In plain terms, the client mishandled data while processing an encrypted message: it failed to validate the supplied data length before copying it into a fixed-size heap buffer. ZDI described this as a heap-based buffer overflow.
The exploit chain also used two other weaknesses: a directory traversal issue in the handling of GIPHY messages and an information-disclosure issue involving URLs used to access Zoom Marketplace resources. ZDI described these latter flaws as usable in conjunction with other vulnerabilities. They were supporting parts of the chain, distinct from the buffer overflow that enabled code execution.
Could it really work without a click?
Yes, in the limited sense that the victim did not need to click a crafted message to trigger the exploit. But “zero-click” did not mean “no contact or access required.” SecurityWeek reported that the target still had to accept an external connection request or be in a multi-user chat shared with the attacker. That precondition matters: the story was about a serious client flaw reachable through particular messaging relationships, not an arbitrary stranger silently attacking every Zoom user.
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How the chain was discovered and disclosed
Keuper and Alkemade explored Zoom’s messaging functionality and advanced chat encryption, found a buffer-handling weakness, and developed the chain further. Their technical account also describes the GIPHY path traversal technique. Their write-up, “Zoom RCE from Pwn2Own 2021,” is dated August 23, 2021. It states: “On April 7 2021, Thijs Alkemade and Daan Keuper demonstrated a zero-click remote code execution exploit in the Zoom video client during Pwn2Own 2021.”
The disclosure and patch timeline was:
- April 7, 2021: The researchers demonstrated the exploit at Pwn2Own and reported it to the vendor, according to ZDI and Computest.
- April 19, 2021: SecurityWeek reported that Zoom Client for Meetings 5.6.3 fixed the main client flaw. It also reported a server-side fix a few days after the demonstration.
- August 17, 2021: ZDI publicly released three related advisories.
- August 23–24, 2021: Computest published its technical account on August 23; SecurityWeek published its report on August 24.
- November 6, 2023: NVD’s record update rejected the identifier CVE-2021-34407 as a duplicate and directed readers to CVE-2021-30480.
Which CVE identifies the issue?
For current identification, use CVE-2021-30480. Contemporary SecurityWeek coverage and ZDI material called the main issue CVE-2021-34407, but NVD now marks that identifier as rejected because it duplicates CVE-2021-30480. Zoom’s archive also lists CVE-2021-30480 in its August 2021 entry.
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What Zoom users should do now
This is a historical disclosure, and version 5.6.3 is a 2021 fix—not a current update recommendation. Keep Zoom updated to receive current fixes, using the guidance in Zoom’s security bulletin archive. The original report does not establish that this particular exploit remains usable on updated clients.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep this exploit separate from later Zoom research
Google Project Zero’s later article discusses separate vulnerabilities from its own Zoom investigation and says those issues were fixed on November 24, 2021. They are not part of the Pwn2Own chain described here; see Project Zero’s account for that separate work.
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