CVE-2026-77255: MCP Atlassian — HIGH

Q: Is CVE-2026-77255 actively exploited?

No confirmed active exploitation of CVE-2026-77255 has been reported, but organizations should still patch proactively.

Q: How to fix CVE-2026-77255?

Update to patched version: MCP Atlassian 0.22.0.

Q: What is the CVSS score for CVE-2026-77255?

CVE-2026-77255 has a CVSS v3.1 base score of 8.6 (HIGH).

### 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...

Full CISO analysis pending enrichment.

What systems are affected?

Package	Ecosystem	Vulnerable Range	Patched
MCP Atlassian	pip	< 0.22.0	`0.22.0`
1 dependents 85% patched ~3d to patch Full package profile →

Do you use MCP Atlassian? You're affected.

How severe is it?

CVSS 3.1

8.6 / 10

EPSS

N/A

Exploitation Status

No known exploitation

Sophistication

N/A

What is the attack surface?

AV Network

AC Low

PR None

UI None

S Changed

C High

I None

A None

What should I do?

Patch available

Update MCP Atlassian to version 0.22.0

Which compliance frameworks are affected?

Compliance analysis pending. Sign in for full compliance mapping when available.

Frequently Asked Questions

What is CVE-2026-77255?

### 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.

Is CVE-2026-77255 actively exploited?

No confirmed active exploitation of CVE-2026-77255 has been reported, but organizations should still patch proactively.

How to fix CVE-2026-77255?

Update to patched version: MCP Atlassian 0.22.0.

What is the CVSS score for CVE-2026-77255?

CVE-2026-77255 has a CVSS v3.1 base score of 8.6 (HIGH).

What are the technical details?

Original Advisory

### 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.

Weaknesses (CWE)

CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') Primary CWE-441 Unintended Proxy or Intermediary ('Confused Deputy') Primary CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') CWE-441 Unintended Proxy or Intermediary ('Confused Deputy')

CWE-22 — Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'): The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.

[Implementation] Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylis
[Architecture and Design] For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

Source: MITRE CWE corpus.