CVE-2026-42246
ADVISORY - githubSummary
Summary
A man-in-the-middle attacker can cause Net::IMAP#starttls to return "successfully", without starting TLS.
Details
When using Net::IMAP#starttls to upgrade a plaintext connection to use TLS, a man-in-the-middle attacker can inject a tagged OK response with an easily predictable tag. By sending the response before the client finishes sending the command, the command completes "successfully" before the response handler is registered. This allows #starttls to return without error, but the response handler is never invoked, the TLS connection is never established, and the socket remains unencrypted.
This allows man-in-the-middle attackers to perform a STARTTLS stripping attack, unless the client code explicitly checks Net::IMAP#tls_verified?.
Impact
TLS bypass, leading to cleartext transmission of sensitive information.
Mitigation
- Upgrade to a patched version of net-imap that raises an exception whenever
#starttlsdoes not establish TLS. - Connect to an implicit TLS port, rather than use
STARTTLSwith a cleartext port. This is strongly recommended anyway:- RFC 8314: Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access
- NO STARTTLS: Why TLS is better without STARTTLS, A Security Analysis of STARTTLS in the Email Context
- Explicitly verify
Net::IMAP#tls_verified?istrue, before using the connection after#starttls.
GitHub
CVSS SCORE
7.6high| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| net-imap | gem | - | - | >=0.5.0,<=0.5.13 | 0.5.14 |
| net-imap | gem | - | - | <=0.3.9 | 0.3.10 |
| net-imap | gem | - | - | >=0.4.0,<=0.4.23 | 0.4.24 |
| net-imap | gem | - | - | >=0.6.0,<=0.6.3 | 0.6.4 |
CVSS:4 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component.
The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: A race condition must be won to successfully exploit the vulnerability. The successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker. The attack may need to be launched multiple times against a single target before being successful. Network injection. The attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
Successful exploitation of this vulnerability requires limited interaction by the targeted user with the vulnerable system and the attacker's payload. These interactions would be considered involuntary and do not require that the user actively subvert protections built into the vulnerable system. Examples include: utilizing a website that has been modified to display malicious content when the page is rendered (most stored XSS or CSRF) running an application that calls a malicious binary that has been planted on the system using an application which generates traffic over an untrusted or compromised network (vulnerabilities requiring an on-path attacker).
There is a total loss of confidentiality, resulting in all information within the Vulnerable System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the Vulnerable System. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the Vulnerable System.
There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
There is no impact to availability within the Vulnerable System.
There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.
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