CVE-2024-49393: Mutt Email Header Information Disclosure

CVE-2024-49393 Overview

CVE-2024-49393 affects the Mutt and NeoMutt terminal-based email clients. The vulnerability stems from the failure to include the To and Cc email headers in the cryptographic signature scope. An attacker who intercepts a signed message can modify these recipient headers and add themselves as an additional recipient. The victim's mail client can then encrypt a reply to the attacker's key, compromising message confidentiality. The flaw is categorized as [CWE-347] Improper Verification of Cryptographic Signature.

Critical Impact

An in-path attacker can manipulate signed email recipient headers to be included in encrypted replies, exposing the confidentiality of downstream correspondence.

Affected Products

• Mutt (all versions prior to the upstream fix)
• NeoMutt (all versions prior to the upstream fix)
• Red Hat Enterprise Linux 8 and 9 (mutt package)

Discovery Timeline

• 2024-11-12 - CVE-2024-49393 published to NVD
• 2026-06-26 - Last updated in NVD database

Technical Details for CVE-2024-49393

Vulnerability Analysis

Mutt and NeoMutt support PGP and S/MIME cryptographic signatures on outgoing messages. The signing process protects the message body and a defined subset of headers. The To and Cc headers are not part of the signed scope. An attacker who intercepts a signed message in transit can rewrite these headers without invalidating the cryptographic signature. When the recipient's mail client processes the modified message and the user replies with encryption enabled, the reply is encrypted to keys corresponding to the attacker-controlled recipient list. The attacker is then able to decrypt sensitive reply content that the sender intended only for the legitimate recipients.

Root Cause

The root cause is an incomplete header protection policy in the signature generation and verification logic. Cryptographic signatures cover the message body but omit the addressing headers that downstream client logic uses to determine recipients for replies and encryption target selection. This mismatch between signed content and trust-influencing content violates the guarantee callers expect from a valid signature.

Attack Vector

Exploitation requires network-level interception of a signed email message, for example through a compromised mail relay, an ISP-level adversary, or a hostile mailing list infrastructure. The attacker modifies the To or Cc header to include an address they control, then forwards the message to the original recipient. The signature remains valid because the modified headers are not covered. Any subsequent encrypted reply from the recipient will include the attacker as a recipient. No user interaction beyond a normal reply workflow is required, though the attack complexity is high because it depends on message interception and a follow-up reply by the victim.

No public proof-of-concept has been published. See the NeoMutt GitHub Issue #4223 and the Red Hat advisory for upstream discussion.

Detection Methods for CVE-2024-49393

Indicators of Compromise

• Signed inbound messages where the To or Cc header lists unexpected external addresses not referenced in the message body or thread context.
• Outbound encrypted replies addressed to recipients that were not present in the original conversation history.
• Mail transport logs showing header rewrites between the sending MTA and the receiving MTA for signed messages.

Detection Strategies

• Correlate signed message headers against expected recipient lists derived from historical thread participants and organizational directories.
• Audit mail gateway logs for signed messages where the envelope recipients diverge from the header recipient set.
• Inspect PGP/MIME and S/MIME messages for the presence of protected header extensions (memoryhole / RFC-style header protection) and flag messages lacking them.

Monitoring Recommendations

• Enable verbose logging on mail gateways for cryptographically signed traffic and retain header state before and after relay.
• Alert on outbound encrypted messages where the recipient key set expanded compared to the parent thread.
• Track Mutt and NeoMutt package versions across Linux fleets to identify unpatched hosts.

How to Mitigate CVE-2024-49393

Immediate Actions Required

• Apply distribution updates for the mutt and neomutt packages on all Linux systems, including Red Hat Enterprise Linux 8 and 9.
• Enable protected headers (crypt_protected_headers_write) in Mutt and NeoMutt configuration so the To, Cc, and Subject headers are included within the signed and encrypted MIME part.
• Instruct users to visually verify recipient lists before sending encrypted replies to signed messages from external senders.

Patch Information

Upstream fixes are tracked in NeoMutt Pull Request #4221 and NeoMutt Pull Request #4227. Distribution-level patch status for Red Hat Enterprise Linux is documented in Red Hat Bug #2325317 and the Red Hat CVE page. Apply vendor package updates as they become available.

Workarounds

• Configure Mutt and NeoMutt to write and require protected headers on all signed and encrypted messages.
• Disable automatic recipient inheritance on reply so users must confirm the recipient list for encrypted responses.
• Route inbound signed mail through a gateway that validates recipient headers against the signed body context before delivery.

# Configuration example: enable protected headers in ~/.muttrc or ~/.neomuttrc
set crypt_protected_headers_write = yes
set crypt_protected_headers_read = yes
set crypt_protected_headers_save = yes
set crypt_protected_headers_subject = "..."