Emissary has a Path Traversal via Blacklist Bypass in Configuration API

Summary

The configuration API endpoint (/api/configuration/{name}) validated configuration names using a blacklist approach that checked for \, /, .., and trailing .. This could potentially be bypassed using URL-encoded variants, double-encoding, or Unicode normalization to achieve path traversal and read configuration files outside the intended directory.

Details

Vulnerable code — Configs.java (line 126)

protected static String validate(String config) {
    if (StringUtils.isBlank(config) || config.contains("\\") || config.contains("/")
        || config.contains("..") || config.endsWith(".")) {
        throw new IllegalArgumentException("Invalid config name: " + config);
    }
    return Strings.CS.appendIfMissing(config.trim(), CONFIG_FILE_ENDING);
}

Weakness

The blacklist blocked literal \, /, .., and trailing . but could potentially miss:

• URL-encoded variants (%2e%2e%2f) if decoded after validation
• Double-encoded sequences (%252e%252e%252f)
• Unicode normalization bypasses
• The approach relies on string matching rather than canonical path resolution

Impact

• Potential read access to configuration files outside the intended config directory
• Information disclosure of sensitive configuration values

Remediation

Fixed in PR #1292, merged into release 8.39.0.

The blacklist was replaced with an allowlist regex that only permits characters matching ^[a-zA-Z0-9._-]+$:

protected static final Pattern VALID_CONFIG_NAME = Pattern.compile("^[a-zA-Z0-9._-]+$");

protected static String validate(String config) {
    if (!VALID_CONFIG_NAME.matcher(config).matches() || config.contains("..") || config.endsWith(".")) {
        throw new IllegalArgumentException("Invalid config name: " + config);
    }
    return Strings.CS.appendIfMissing(config.trim(), CONFIG_FILE_ENDING);
}

This ensures that any character outside the allowed set — including encoded slashes, percent signs, and Unicode sequences — is rejected before the config name reaches the filesystem.

Tests were added to verify that URL-encoded (%2e%2e%2f), double-encoded (%252e%252e%252f), and Unicode () traversal attempts are blocked.