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Mail is an internet library for Ruby designed to handle email generation, parsing, and sending. Prior to version 2.9.1, the `Mail::Utilities.q_value_decode` and `Mail::Utilities.b_value_decode` methods contained a decoding flaw that could be exploited for email address spoofing.
Both decoders used a single `Stringmatch` against a pattern such as /=?(.+)??
?(.)?=/m</code>. This approach introduced two critical problems: <h2 style="color: blue;">1. Single Match, Dropped Remainder</h2> Only the first `=?charset?Q?...?=` (or <code>?B?</code>) encoded-word was decoded. Any additional encoded-words or surrounding text were not handled consistently, so the decoded result could silently omit or alter parts of the input. For example, if a display name contained multiple encoded-words, only the first one would be decoded, and the rest would be discarded. <h2 style="color: blue;">2. Greedy Charset Capture</h2> The pattern used an overly greedy `(.+)` capture, which matches across `?` delimiters. A malformed encoded-word could therefore span more of the string than a strict RFC 2047 parse would allow, changing the boundary between "encoded" and "literal" text. A crafted malformed encoded-word in an address display name or local part could cross `?` delimiters and cause the decoded <code>From</code>, <code>To</code>, or `Reply-To` header values to differ from the raw values inspected by a human reviewer or downstream parser. This mismatch enables attackers to spoof apparent sender or recipient addresses, facilitate phishing campaigns, or bypass authorization checks that rely on parsed address fields. The vulnerability is classified under CWE-436 (Interpretation Conflict) and carries a CVSS base score of 5.3 (Medium severity). No Remote Code Execution (RCE) is possible. The issue is fixed in version 2.9.1. <h2 style="color: blue;">DailyCVE Form:</h2> Platform: Ruby Mail gem Version: < 2.9.1 Vulnerability: Address spoofing Severity: Medium (CVSS 5.3) Date: 2026-09-01 <h2 style="color: blue;">Prediction: 2026-07-01 (Patched in v2.9.1)</h2> <h2 style="color: blue;">What Undercode Say:</h2> The vulnerability stems from improper decoding of RFC 2047 encoded-words within email headers. The fix implemented in version 2.9.1 switches both `q_value_decode` and `b_value_decode` to use `Stringgsub` instead of a single <code>Stringmatch</code>, ensuring every encoded-word in the string is decoded in place. The charset capture is tightened to `[^?]+` so it stops at the first `?` delimiter. Encoding-conversion failures are now rescued per encoded-word (falling back to UTF-8) instead of aborting the whole decode. The payload pattern `(?:[^?]|\?(?!\=))` permits a literal `?` inside the encoded text unless it is the `?=` terminator, so malformed/ambiguous words decode predictably rather than truncating the string. <h2 style="color: blue;">Bash command to check installed Mail gem version:</h2> [bash] gem list mail
Ruby code to check version programmatically:
require 'mail' puts Mail::VERSION
Vulnerable code pattern (lib/mail/utilities.rb before 2.9.1):
def self.q_value_decode(str) encoded = str.match(/=\?(.+)?\?[bash]\?(.)?\?=/m) Only first match decoded, greedy charset capture end
Patched code pattern (lib/mail/utilities.rb in 2.9.1):
def self.q_value_decode(str) str.gsub(/=\?([^?]+)\?[bash]\?(?:[^?]|\?(?!\=))\?=/) do |encoded| Decode each encoded-word in place end end
Exploit: (Educational Purposes!)
A crafted malformed encoded-word can be embedded in an address display name or local part. For example, a `From` header containing:
From: =?utf-8?Q?Legitimate?= <a href="mailto:attacker@example.com">attacker@example.com</a>
If a second encoded-word or malformed pattern follows, only the first word is decoded, and the remainder (including the actual address) may be dropped or misinterpreted. By crafting the encoded-word to cross `?` delimiters, an attacker can make the decoded `From` address appear as a trusted sender while the raw header contains a different address.
Example malformed payload:
From: =?utf-8?Q?Trusted?= =?utf-8?Q?Sender?= <a href="mailto:attacker@example.com">attacker@example.com</a>
The vulnerable decoder would decode only the first word ("Trusted") and drop the rest, potentially presenting "Trusted" as the display name while the actual address remains [email protected]. Applications that trust the decoded header value for display or authorization decisions could be deceived.
Protection:
- Upgrade the Mail gem to version 2.9.1 or later:
gem update mail
Or in your `Gemfile`:
gem 'mail', '>= 2.9.1'
- If upgrading immediately is not possible, configure email parsing to reject any token that contains a malformed RFC 2047 encoded-word, or enforce a stricter decoding routine that validates the encoded-word against RFC 2047 syntax.
- Enable detailed logging of any mismatch between decoded and raw address fields so that suspicious spoofed headers can be detected and reviewed.
- For applications that perform authorization checks based on email addresses, consider validating against the raw header values rather than solely relying on the decoded output.
Impact:
Applications using the Mail gem to parse and display or authorize based on decoded header values (From, To, Reply-To, etc.) may present or act on an address different from the one a validator inspecting the raw header would see. The primary risks are:
- Spoofing / Phishing: An attacker can make an email appear to come from a trusted sender when it actually originates from a malicious source.
- Authorization-Check Bypass: If an application relies on parsed address fields for access control or verification, the mismatch between decoded and raw values could allow unauthorized actions.
- No RCE: The vulnerability does not enable remote code execution.
The vulnerability affects all releases of the mikel:mail library before version 2.9.1. The CVSS score of 5.3 indicates moderate severity. The attack can be launched remotely with no authentication required.
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Sources:
Reported By: github.com
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