CVE-2026-42245
ADVISORY - githubSummary
Summary
Net::IMAP::ResponseReader has quadratic time complexity when reading large responses containing many string literals. A hostile server can send responses which are crafted to exhaust the client's CPU for a denial of service attack.
Details
For each literal in a response, ResponseReader rescans the entire growing response buffer. The regular expression that is used to scan the response buffer runs in linear time. With many literals, this becomes O(n²) total work. The regular expression should run in constant time: it is anchored to the end and only the last 23 bytes of the buffer are relevant.
Because the algorithmic complexity is super-linear, this bypasses protection from max_response_size: a response can stay well below the default size limit while still causing very large CPU cost.
Net::IMAP::ResponseReader runs continuously in the receiver thread until the connection closes.
Impact
This consumes disproportionate CPU time in the client's receiver thread. A hostile server could use this to exhaust the client's CPU for a denial of service attack.
For a response near the default max_response_size, each individual regexp scan could take between 100 to 200ms on common modern hardware, and this may be repeated 200k times per megabyte of response. While the regexp is scanning, it retains the Global VM lock, preventing other threads from running.
Although other threads should not be completely blocked, their run time will be significantly impacted.
Mitigation
- Upgrade to a patched version of net-imap that reads responses more efficiently.
- Do not connect to untrusted IMAP servers.
- When connecting to untrusted servers, a much smaller
max_response_size(for example: 8KiB) will limit the impact. Although this is too small for fetching unpaginated message bodies, it should be enough for most other operations.
Common Weakness Enumeration (CWE)
Inefficient Algorithmic Complexity
GitHub
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CVSS SCORE
2.3low| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| net-imap | gem | - | - | <=0.4.23 | 0.4.24 |
| net-imap | gem | - | - | >=0.5.0,<=0.5.13 | 0.5.14 |
| 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 no loss of confidentiality within the Vulnerable System.
There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
There is no loss of integrity within the Vulnerable System.
There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the Vulnerable System are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to 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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