Loading story…
Researchers disclosed a critical zero-click vulnerability chain in Anthropic’s Claude Chrome extension that allowed a malicious website to inject prompts into the AI assistant without any user interaction. The attack relied on the extension trusting any *.claude.ai origin and a DOM-based XSS flaw in an older Arkose Labs CAPTCHA component hosted on a-cdn.claude.ai; by loading the vulnerable component in a hidden iframe and sending a crafted postMessage, attacker-controlled JavaScript could appear to come from a trusted Claude origin and submit prompts as the victim.
Successful exploitation could have exposed more than 3 million users to account and data compromise, including theft of Gmail OAuth tokens, access to Gmail, Google Drive, and Claude chat history, and invisible actions such as sending emails on the user’s behalf. KOI Security researcher Oren Yomtov reported the issue to Anthropic in late December 2025, Anthropic patched the extension by tightening origin validation to https://claude.ai in January 2026, and Arkose Labs separately fixed the XSS issue in February 2026 after the vulnerable resource was disabled.
Mallory correlates global threat intelligence with your attack surface — know if you’re exposed before adversaries strike.
5 events from the most recent confirmed update back to the earliest known activity.
Researchers publicly revealed that the chained flaws could have allowed silent prompt injection against Claude Chrome extension users, potentially exposing conversation history, Gmail and Drive data, OAuth tokens, and enabling actions such as sending emails on a victim's behalf. Reports said the issue affected a user base of more than 3 million before the fixes were applied.
Arkose Labs patched the vulnerable CAPTCHA component, and the previously exploitable URL later returned HTTP 403. This remediation closed the XSS portion of the exploit chain used against the Claude extension.
The DOM-based XSS vulnerability in the Arkose Labs CAPTCHA component hosted on a-cdn.claude.ai was separately disclosed to Arkose Labs. This second flaw was necessary for the full exploit chain because it enabled attacker-controlled JavaScript execution from a trusted Claude subdomain.
Anthropic fixed the extension-side issue by tightening trusted origin checks from any claude.ai subdomain to exactly https://claude.ai. This patch blocked malicious websites from abusing the extension's overly broad allowlist to send trusted messages without user interaction.
KOI Security researcher Oren Yomtov responsibly disclosed a zero-click prompt-injection vulnerability chain affecting Anthropic's Claude Chrome extension. The issue combined an overly broad *.claude.ai trusted-origin allowlist with a DOM-based XSS flaw in an Arkose Labs CAPTCHA component hosted on a-cdn.claude.ai.
Vulnerabilities, threat actors, malware, products, organizations, and breaches Mallory has linked to this story.
3 references tracked. Mallory keeps watching after this page renders.
socradar.io
Open sourcecybersecuritynews.com
Open sourcethehackernews.com
Open sourceMap indicators from this story to your assets and identify affected systems in minutes.
Every observed campaign, victim, and pivot linked to actors named in this story.
Malware, exploits, and IOCs connected to the activity described here.
YARA, Sigma, and Snort rules deployed to your SIEM as soon as they’re published.
Get matching new stories delivered to your team as they break — not the next morning.
Ask questions about this story and take action on the answers.
Other stories Mallory is tracking that overlap on entities, sources, or topic labels.
Researchers at LayerX disclosed a critical flaw in Anthropic’s Claude Chrome extension that allowed **any other Chrome extension**, including ones with no declared permissions, to send hidden instructions to Claude and abuse its browser automation features. The issue reportedly stemmed from the extension trusting messages associated with the `claude.ai` origin without properly authenticating the true sender, creating a cross-extension privilege escalation path. In proof-of-concept attacks, LayerX said it used the weakness to extract files from Google Drive, read and send Gmail messages, steal code from connected private GitHub repositories, and even summarize or delete recent emails to conceal activity. Anthropic said the bug had already been identified internally and released version `1.0.70` with mitigations, including approval flows for privileged actions, but LayerX reported that the fix was incomplete and that takeover remained possible through privileged mode and side-panel initialization paths. The researchers said the flaw exposed a broader trust-boundary problem in AI browser assistants, where interface-based approvals and safety guardrails can be bypassed if extensions fail to verify message origin and execution context. Separate White House discussions about tighter oversight of advanced AI models, including Anthropic systems with offensive cyber capabilities, underscored growing concern over how powerful AI tools are secured before deployment.
Jun 29, 2026
Researchers at Manifold disclosed two unpatched vulnerabilities in Anthropic’s **Claude for Chrome** extension that can let a malicious browser extension trigger Claude actions and access sensitive Google Workspace content, including **Gmail messages, Google Docs data and comments, and Google Calendar entries**. The researchers said the flaws were reported in May and remained reproducible in version `1.0.80`, with one issue allowing another extension that has script access on `claude.ai` to simulate a trusted user click and launch hardcoded Claude prompts on the victim’s behalf. The weaknesses stem in part from Anthropic’s earlier mitigation for **ClaudeBleed**, which limited prompts to pre-approved tasks but did not verify that task-triggering clicks came from a real user. Manifold also identified a separate design flaw in which Claude’s side panel can enter a privileged **no-confirmation** mode through a URL parameter, creating a potential privilege-escalation path if a future bug allows outside control of that parameter. The risk is highest when users enable Claude’s **“Act without asking”** mode, because the extension may perform actions without visible consent prompts; Anthropic acknowledged the reports but had not issued fixes, saying one issue was already tracked internally and the other was not externally reachable.
Jul 17, 2026
Researchers at **Oasis Security** disclosed an attack chain dubbed **"Claudy Day"** that combines three flaws in Anthropic’s **Claude** platform to enable covert data theft. The chain starts with attacker-controlled Google search results or fake Claude ads that abuse an **open redirect** on `claude.com`, leading victims to what appears to be a legitimate Claude page. From there, a pre-filled chat URL can carry **hidden prompt-injection instructions** embedded in HTML so the user sees benign text while Claude processes malicious commands in the background. According to the reports, the chained flaws allow attackers to silently instruct Claude to search prior conversations for sensitive information and exfiltrate it through the **Anthropic Files API**, creating an end-to-end path from victim luring to unauthorized data exposure. The issue is notable because it does not require extra tooling, integrations, or MCP servers; built-in Claude features are sufficient for exploitation. A separate report about Claude autonomously exploiting vulnerable websites is **not** the same incident, and a report on the **SideWinder** espionage campaign is unrelated.
Jun 29, 2026