CVE-2026-93421: Mesop: Unauthenticated ANSI Escape Sequence Injection in CSP Reporting Endpoint
Summary
The /csp endpoint accepts unauthenticated JSON reports and logs user-controlled values directly to stdout using print() without escaping control characters.
A remote attacker can include ANSI/VT100 escape sequences in fields such as blocked-uri or document-uri. When the logs are viewed in an ANSI-capable terminal, these sequences can manipulate the displayed output (e.g., clear the screen, hide text, or inject misleading messages), affecting the integrity of operator-facing logs.
Details
The CSP reporting endpoint accepts arbitrary JSON and prints several request fields directly:
mesop/server/staticfileserving.py python @app.route(prefixbaseurl("/csp"), methods=["POST"]) def cspreport(): report = request.getjson(force=True)
documenturi = report["csp-report"]["document-uri"] blockeduri = report["csp-report"]["blocked-uri"] violateddirective = report["csp-report"]["violated-directive"]
print(f"... Blocked URL: {blockeduri} ...")
Since these values are written to stdout without sanitization, ANSI escape sequences supplied by a remote client are preserved and interpreted by ANSI-compatible terminals.
PoC
Send the following request:
http POST /csp Content-Type: application/json
{ "csp-report": { "document-uri": "https://victim.example", "blocked-uri": "\u001b[2J\u001b[H\u001b[32m SECURITY OK - No CSP violations found \u001b[0m\n\u001b[8mhttps://evil.example", "violated-directive": "script-src-elem" } }
The request is accepted (HTTP 204), and the injected escape sequences are written to stdout unchanged.
When the captured output is rendered in a VT100-compatible terminal (verified using pyte), the original CSP warning is visually replaced with attacker-controlled content.
Expected output
text Content Security Policy Error
Directive: script-src-elem Blocked URL: ... App path: /app
Rendered output
text SECURITY OK - No CSP violations found https://evil.example App path: /app
Impact
An unauthenticated remote attacker can submit a crafted request to the /csp endpoint containing ANSI/VT100 escape sequences. Because these values are written directly to stdout without sanitization, an attacker can:
1. Inject forged log messages that appear to originate from the application. 2. Manipulate the terminal display by clearing the screen, moving the cursor, or overwriting previously displayed log output. 3. Hide or disguise security-relevant log entries using terminal formatting sequences such as colors, hidden text, or cursor positioning. 4. Mislead administrators during monitoring or incident response by displaying attacker-controlled messages (e.g., fake "SECURITY OK" notifications). 5. Reduce the integrity and trustworthiness of operator-facing logs, making troubleshooting and security investigations less reliable.
Other sources
Mesop is a Python-based UI framework that allows users to build web applications. Prior to 1.3.4, the unauthenticated /csp endpoint passes attacker-controlled document-uri, blocked-uri, and violated-directive values to the cspreport handler in mesop/server/staticfileserving.py, which prints them to standard output without neutralizing terminal control sequences. When an operator views the resulting logs in an ANSI-capable terminal, injected ANSI or VT100 sequences can clear or reposition the display, hide text, or present forged messages, reducing the integrity of monitoring and incident-response output. This issue is fixed in version 1.3.4.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/mesopto a version that resolves this vulnerability.Fixed in 1.3.4 - Upgrade
Upgrade
Mesopto a version that resolves this vulnerability.Fixed in 1.3.4
Event History
Frequently Asked Questions
Who can exploit this issue?
Any unauthenticated party able to send requests to the /__csp__ endpoint can supply malicious document-uri, blocked-uri, or violated-directive values. Exploitation affects operators who view the resulting standard-output logs in an ANSI-capable terminal.
What is the impact of successful exploitation?
Injected ANSI or VT100 control sequences can clear or reposition terminal output, hide legitimate log text, or display forged messages. This compromises the integrity of monitoring and incident-response output rather than directly authenticating as a user or modifying application data.
Are default deployments affected?
The affected endpoint is described as unauthenticated, so deployments running a Mesop version before 1.3.4 may be exposed if attackers can reach /__csp__ and operators inspect its logs in an ANSI-capable terminal.
What should teams do if they cannot update immediately?
Avoid viewing untrusted CSP-report log output in ANSI-capable terminals, since the harmful sequences are rendered when the logs are displayed. Restricting access to the unauthenticated /__csp__ endpoint can also reduce who can submit malicious values.
How can responders identify possible exploitation?
Review CSP-report log entries for unexpected escape or terminal-control sequences in document-uri, blocked-uri, or violated-directive values. Visual anomalies such as cleared, repositioned, hidden, or apparently forged terminal messages may also indicate malicious log content.