CVE-2021-38189
📋 TL;DR
This vulnerability in the lettre Rust crate allows attackers to inject arbitrary SMTP commands into email message bodies. By placing a period character after two CRLF sequences, attackers can bypass input validation and execute unauthorized commands on SMTP servers. This affects any Rust application using lettre versions before 0.9.6 for sending email.
💻 Affected Systems
- lettre Rust crate
📦 What is this software?
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
Lettre by Lettre
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution on SMTP servers, email spoofing, data exfiltration, or complete system compromise through command injection.
Likely Case
Email spoofing, unauthorized email sending, SMTP command manipulation leading to service disruption or information disclosure.
If Mitigated
Limited impact with proper input validation and SMTP server hardening, potentially only causing email formatting issues.
🎯 Exploit Status
Exploit requires ability to control email message body content. Public advisory includes technical details.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: lettre 0.9.6 and later
Vendor Advisory: https://rustsec.org/advisories/RUSTSEC-2021-0069.html
Restart Required: Yes
Instructions:
1. Update Cargo.toml to specify 'lettre = "^0.9.6"' or higher. 2. Run 'cargo update lettre'. 3. Rebuild and redeploy your application. 4. Restart any running services using the updated application.
🔧 Temporary Workarounds
Input validation filter
allAdd custom validation to filter or escape CRLF sequences followed by periods in email message bodies
// Rust code example: let sanitized_body = body.replace("\r\n\r\n.", "\r\n\r\n ");
Use alternative email library
allTemporarily switch to a different Rust email library while waiting for patching
// In Cargo.toml: replace 'lettre' dependency with alternative like 'rust-imap' or 'async-smtp'
🧯 If You Can't Patch
- Implement network segmentation to isolate SMTP servers from untrusted networks
- Deploy WAF or application firewall rules to block email messages containing suspicious CRLF patterns
🔍 How to Verify
Check if Vulnerable:
Check Cargo.lock or run 'cargo tree | grep lettre' to see installed version. If version is below 0.9.6, you are vulnerable.Check Version:
cargo tree | grep lettre | head -1Verify Fix Applied:
After update, verify with 'cargo tree | grep lettre' shows version 0.9.6 or higher. Test email sending functionality works correctly.📡 Detection & Monitoring
Log Indicators:
- Unusual SMTP commands in email server logs
- Multiple failed email sending attempts with malformed bodies
- Unexpected command execution in application logs
Network Indicators:
- SMTP traffic containing CRLF.CRLF patterns
- Unusual outbound email volume from applications
- SMTP command injection patterns in network captures
SIEM Query:
source="*smtp*" AND ("\r\n\r\n." OR "command injection" OR "unexpected command")