CVE-2025-43857
ADVISORY - githubSummary
Summary
There is a possibility for denial of service by memory exhaustion when net-imap reads server responses. At any time while the client is connected, a malicious server can send can send a "literal" byte count, which is automatically read by the client's receiver thread. The response reader immediately allocates memory for the number of bytes indicated by the server response.
This should not be an issue when securely connecting to trusted IMAP servers that are well-behaved. It can affect insecure connections and buggy, untrusted, or compromised servers (for example, connecting to a user supplied hostname).
Details
The IMAP protocol allows "literal" strings to be sent in responses, prefixed with their size in curly braces (e.g. {1234567890}\r\n). When Net::IMAP receives a response containing a literal string, it calls IO#read with that size. When called with a size, IO#read immediately allocates memory to buffer the entire string before processing continues. The server does not need to send any more data. There is no limit on the size of literals that will be accepted.
Fix
Upgrade
Users should upgrade to net-imap 0.5.7 or later. A configurable max_response_size limit has been added to Net::IMAP's response reader. The max_response_size limit has also been backported to net-imap 0.2.5, 0.3.9, and 0.4.20.
To set a global value for max_response_size, users must upgrade to net-imap ~> 0.4.20, or > 0.5.7.
Configuration
To avoid backward compatibility issues for secure connections to trusted well-behaved servers, the default max_response_size for net-imap 0.5.7 is very high (512MiB), and the default max_response_size for net-imap ~> 0.4.20, ~> 0.3.9, and 0.2.5 is nil (unlimited).
When connecting to untrusted servers or using insecure connections, a much lower max_response_size should be used.
# Set the global max_response_size (only ~> v0.4.20, > 0.5.7)
Net::IMAP.config.max_response_size = 256 << 10 # 256 KiB
# Set when creating the connection
imap = Net::IMAP.new(hostname, ssl: true,
max_response_size: 16 << 10) # 16 KiB
# Set after creating the connection
imap.max_response_size = 256 << 20 # 256 KiB
# flush currently waiting read, to ensure the new setting is loaded
imap.noop
Please Note: max_response_size only limits the size per response. It does not prevent a flood of individual responses and it does not limit how many unhandled responses may be stored on the responses hash. Users are responsible for adding response handlers to prune excessive unhandled responses.
Compatibility with lower max_response_size
A lower max_response_size may cause a few commands which legitimately return very large responses to raise an exception and close the connection. The max_response_size could be temporarily set to a higher value, but paginated or limited versions of commands should be used whenever possible. For example, to fetch message bodies:
imap.max_response_size = 256 << 20 # 256 KiB
imap.noop # flush currently waiting read
# fetch a message in 252KiB chunks
size = imap.uid_fetch(uid, "RFC822.SIZE").first.rfc822_size
limit = 252 << 10
message = ((0..size) % limit).each_with_object("") {|offset, str|
str << imap.uid_fetch(uid, "BODY.PEEK[]<#{offset}.#{limit}>").first.message(offset:)
}
imap.max_response_size = 16 << 20 # 16 KiB
imap.noop # flush currently waiting read
References
- PR to introduce max_response_size: https://github.com/ruby/net-imap/pull/444
- Specific commit: 0ae8576c1 - lib/net/imap/response_reader.rb
- Backport to 0.4: https://github.com/ruby/net-imap/pull/445
- Backport to 0.3: https://github.com/ruby/net-imap/pull/446
- Backport to 0.2: https://github.com/ruby/net-imap/pull/447
Common Weakness Enumeration (CWE)
OWASP Top Ten 2017 Category A9 - Using Components with Known Vulnerabilities
Uncontrolled Resource Consumption
Asymmetric Resource Consumption (Amplification)
Allocation of Resources Without Limits or Throttling
Memory Allocation with Excessive Size Value
OWASP Top Ten 2013 Category A9 - Using Components with Known Vulnerabilities
GitHub
CVSS SCORE
6medium| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| net-imap | gem | - | - | <=0.2.4 | 0.2.5 |
| net-imap | gem | - | - | >=0.3.0,<=0.3.8 | 0.3.9 |
| net-imap | gem | - | - | >=0.4.0,<=0.4.19 | 0.4.20 |
| net-imap | gem | - | - | >=0.5.0,<=0.5.6 | 0.5.7 |
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.
There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.
NIST
CVSS SCORE
6mediumDebian
-
Ubuntu
3.9
CVSS SCORE
7.5lowAmazon
-
CVSS SCORE
N/AmediumRed Hat
2.8
CVSS SCORE
6.5mediumChainguard
CGA-22r6-gqwr-r3mx
-
Chainguard
CGA-2rcr-6f45-rc9q
-
Chainguard
CGA-pggm-rjff-9358
-
minimos
MINI-mfwq-779c-vj46
-
minimos
MINI-mj9j-p5vp-f5jh
-
minimos
MINI-w3r4-vv49-vf4r
-
minimos
MINI-xgxc-h9pc-qvf6
-