CVE-2026-77255 is a high-severity (CVSS 8.6) Path Traversal vulnerability in mcp-atlassian. A fix is available for mcp-atlassian — see the affected versions and patch details below.
MCP Atlassian: Arbitrary File Read & Exfiltration (Confused Deputy) in JIRA update_issue
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-77255.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-77255 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
mcp-atlassianReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
A critical Confused Deputy (Arbitrary File Read & Exfiltration) vulnerability in the Atlassian MCP server (Python) allows an AI agent to exfiltrate sensitive host files and environment secrets. By providing absolute system paths to the attachments parameter of the update_issue tool, an agent can force the privileged MCP process to read and upload any file it has access to—including its own environment variables and host configuration—directly to a Jira ticket.
Details
The vulnerability is located in the update_issue tool handler. The attachments parameter accepts a list of file paths that are passed directly to the Jira API's upload method without any sanitization or validation.
Specifically, the implementation fails to: 1. Restrict paths to a safe workspace: It accepts absolute paths starting with /. 2. Prevent directory traversal: It does not filter for ../ sequences. 3. Validate ownership: It allows the process to read system-level files (like /proc/self/environ) that the calling agent is normally forbidden from accessing due to sandbox restrictions.
Because the MCP server typically runs with higher privileges than the AI agent (holding the JIRA_API_TOKEN and having wider filesystem access), it acts as a "Confused Deputy," performing exfiltration on behalf of a restricted agent.
PoC
To reproduce the vulnerability in an environment where the agent is sandboxed (e.g., as a restricted Unix user) but the MCP server has host-level access:
1. Exfiltrate MCP Secrets: The agent calls the update_issue tool with the following payload: { "issue_key": "SECURITY-1", "attachments": ["/proc/self/environ"] } Result: The MCP server reads its own process environment—containing the JIRA_API_TOKEN and other sensitive keys in plain text—and attaches it to the Jira issue.
2. Exfiltrate Host Configuration: The agent calls the tool to target global assistant settings: { "issue_key": "SECURITY-1", "attachments": ["/home/hermes/.hermes/config.yaml"] } Result: The host's global configuration file (containing tokens for GitLab, Slack, and other services) is successfully exfiltrated to the cloud.
Impact
* Critical Credential Theft: Immediate exposure of the JIRA_API_TOKEN and any other secrets injected into the MCP server's environment. * Arbitrary File Access: Total exfiltration of any data the host user running the MCP server can read (SSH keys, databases, local source code). * Privilege Escalation & Lateral Movement: Attackers can move from a restricted AI sandbox to full host-system control by stealing tokens or configuration files.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | mcp-atlassian | all versions | 0.22.0pip install --upgrade 'mcp-atlassian==0.22.0' |
Affected Products
mcp atlassianmcp-atlassianDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mcp-atlassian, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update mcp-atlassian to 0.22.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-77255 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.
Frequently Asked Questions
Is CVE-2026-77255 in your dependencies?
Find it across PyPI, including transitive dependencies.