CVE-2025-43857: Net imap Denial of Service Vulnerability

Q: What is CVE-2025-43857?

CVE-2025-43857 is a security vulnerability in Net imap involving net-imap rubygem vulnerable to possible DoS by memory exhaustion under CWE-405: Asymmetric Resource Consumption (Amplification).

Q: Which Net imap versions are affected?

Affected versions include releases prior to 0.5.7, 0.4.20, 0.3.9, 0.2.5 across affected packages: net-imap.

Q: What version fixes CVE-2025-43857?

Upgrading to 0.5.7, 0.4.20, 0.3.9, 0.2.5 (or the latest patched release for your respective branch) resolves the vulnerability.

Q: Is net-imap affected?

Yes, net-imap is affected in releases prior to 0.5.7, 0.4.20, 0.3.9, 0.2.5. Upgrade your dependency to 0.5.7, 0.4.20, 0.3.9, 0.2.5 or later.

Q: What CWE is associated with CVE-2025-43857?

CVE-2025-43857 is classified under CWE-405: Asymmetric Resource Consumption (Amplification) (The product does not properly control situations in which an adversary can cause the product to consume or produce excessive resources without requiring the adversary to invest equivalent work or otherwise prove authorization, i.e., the adversary's influence is "asymmetric.").

Q: How can I check RubyGems dependencies for CVE-2025-43857?

Run the dependency tree command: 'inspect dependencies for net-imap'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.

MODERATE

Advisory Identifier: GHSA-j3g3-5qv5-52mj

CWE-405 CWE-789 CWE-400 CWE-770

Published 2025-04-28

Advisory Summary

CVE-2025-43857 is a MODERATE denial of service vulnerability (CWE-405: Asymmetric Resource Consumption (Amplification)) affecting Net imap libraries in the RubyGems ecosystem. The issue involves net-imap rubygem vulnerable to possible dos by memory exhaustion. Remediation is available by upgrading affected packages to version 0.5.7, 0.4.20, 0.3.9, 0.2.5.

CVE-2025-43857 at a Glance

CVE Identifier CVE-2025-43857

GHSA Identifier GHSA-j3g3-5qv5-52mj

Severity MODERATE

CVSS v3.1 Base Score N/A

CWE Classification CWE-405: Asymmetric Resource Consumption (Amplification)

Affected Product Net imap

Package Ecosystem RubyGems

Published Date 2025-04-28

Last Updated 2025-11-21

How to Fix CVE-2025-43857

To remediate this vulnerability, upgrade all affected Net imap dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

net-imap fixed: 0.5.7

Upgrade net-imap to 0.5.7

net-imap fixed: 0.4.20

Upgrade net-imap to 0.4.20

net-imap fixed: 0.3.9

Upgrade net-imap to 0.3.9

net-imap fixed: 0.2.5

Upgrade net-imap to 0.2.5

Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release:

inspect dependencies for net-imap

How to Check if You Are Affected

To determine whether your application is affected by CVE-2025-43857, inspect your project dependencies for Net imap:

inspect dependencies for net-imap

Analyze the terminal output to locate any version of Net imap in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.5.7), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & RFC 5280 Specification

Under RFC 5280 specifications for X.509 certificates, the Name Constraints extension defines a namespace within which all subsequent subject names in a certification path must be located. Constraints apply to various name forms including directory names, IP addresses, rfc822Name (email addresses), and uniformResourceIdentifier (URI) entries. Specifically, during certificate path validation, affected versions of Net imap failed to properly normalize or enforce boundary checks when domain names contained trailing dot (.) characters. Because domain names with a trailing dot signify fully qualified domain names (FQDNs), this omission allowed non-conforming certificates to be improperly validated as compliant with the defined name constraints.

Source Advisory Statement

### 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. ```ruby # 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: ```ruby 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](https://github.com/ruby/net-imap/pull/444/commits/0ae8576c1a90bcd9573f81bdad4b4b824642d105#diff-53721cb4d9c3fb86b95cc8476ca2df90968ad8c481645220c607034399151462) * 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

CVE-2025-43857 Affected Net imap Versions

4 component(s)

Fixed Versions:

0.5.70.4.200.3.90.2.5

Product / Package	Ecosystem	Affected Versions	Fixed Version
net-imap gem/net-imap	RubyGems	≥ 0.5.0 < 0.5.7	0.5.7
net-imap gem/net-imap	RubyGems	≥ 0.4.0 < 0.4.20	0.4.20
net-imap gem/net-imap	RubyGems	≥ 0.3.0 < 0.3.9	0.3.9
net-imap gem/net-imap	RubyGems	< 0.2.5	0.2.5

References

GitHub Advisory / Wiki ADVISORY

https://github.com/ruby/net-imap/security/advisories/GHSA-j3g3-5qv5-52mj

NVD — CVE-2025-43857 NVD

https://nvd.nist.gov/vuln/detail/CVE-2025-43857

Net imap Source Repository REPOSITORY

https://github.com/ruby/net-imap/pull/442

Net imap Source Repository REPOSITORY

https://github.com/ruby/net-imap/pull/444

Net imap Source Repository REPOSITORY

https://github.com/ruby/net-imap/pull/444/commits/0ae8576c1a90bcd9573f81bdad4b4b824642d105#diff-53721cb4d9c3fb86b95cc8476ca2df90968ad8c481645220c607034399151462

Net imap Source Repository REPOSITORY

https://github.com/ruby/net-imap/pull/445

Net imap Source Repository REPOSITORY

https://github.com/ruby/net-imap/pull/446

Net imap Source Repository REPOSITORY

https://github.com/ruby/net-imap/pull/447

Net imap Source Repository REPOSITORY

https://github.com/ruby/net-imap

Net imap Source Repository REPOSITORY

https://github.com/rubysec/ruby-advisory-db/blob/master/gems/net-imap/CVE-2025-43857.yml

Frequently Asked Questions

What is CVE-2025-43857?

CVE-2025-43857 is a security vulnerability in Net imap involving net-imap rubygem vulnerable to possible DoS by memory exhaustion under CWE-405: Asymmetric Resource Consumption (Amplification).

Which Net imap versions are affected?

Affected versions include releases prior to 0.5.7, 0.4.20, 0.3.9, 0.2.5 across affected packages: net-imap.

What version fixes CVE-2025-43857?

Upgrading to 0.5.7, 0.4.20, 0.3.9, 0.2.5 (or the latest patched release for your respective branch) resolves the vulnerability.

Is net-imap affected?

Yes, net-imap is affected in releases prior to 0.5.7, 0.4.20, 0.3.9, 0.2.5. Upgrade your dependency to 0.5.7, 0.4.20, 0.3.9, 0.2.5 or later.

What CWE is associated with CVE-2025-43857?

CVE-2025-43857 is classified under CWE-405: Asymmetric Resource Consumption (Amplification) (The product does not properly control situations in which an adversary can cause the product to consume or produce excessive resources without requiring the adversary to invest equivalent work or otherwise prove authorization, i.e., the adversary's influence is "asymmetric.").

How can I check RubyGems dependencies for CVE-2025-43857?

Run the dependency tree command: 'inspect dependencies for net-imap'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.

Machine-readable OSV JSON OSV v1.5 Schema

Normalized schema output

View Raw JSON

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  "summary" : "net-imap rubygem vulnerable to possible DoS by memory exhaustion",
  "details" : "### Summary\n\nThere 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.\n\nThis 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).\n\n### Details\n\nThe 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.\n\n### Fix\n#### Upgrade\nUsers 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.\n\nTo set a global value for `max_response_size`, users must upgrade to `net-imap` ~> 0.4.20, or > 0.5.7.\n\n#### Configuration\n\nTo 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).\n\nWhen connecting to untrusted servers or using insecure connections, a much lower `max_response_size` should be used.\n```ruby\n# Set the global max_response_size (only ~> v0.4.20, > 0.5.7)\nNet::IMAP.config.max_response_size = 256 << 10 # 256 KiB\n\n# Set when creating the connection\nimap = Net::IMAP.new(hostname, ssl: true,\n                     max_response_size: 16 << 10) # 16 KiB\n\n# Set after creating the connection\nimap.max_response_size = 256 << 20 # 256 KiB\n# flush currently waiting read, to ensure the new setting is loaded\nimap.noop\n```\n\n_**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.\n\n#### Compatibility with lower `max_response_size`\n\nA 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:\n\n```ruby\nimap.max_response_size = 256 << 20 # 256 KiB\nimap.noop # flush currently waiting read\n\n# fetch a message in 252KiB chunks\nsize = imap.uid_fetch(uid, \"RFC822.SIZE\").first.rfc822_size\nlimit = 252 << 10\nmessage = ((0..size) % limit).each_with_object(\"\") {|offset, str|\n  str << imap.uid_fetch(uid, \"BODY.PEEK[]<#{offset}.#{limit}>\").first.message(offset:)\n}\n\nimap.max_response_size = 16 << 20 # 16 KiB\nimap.noop # flush currently waiting read\n```\n\n### References\n\n* PR to introduce max_response_size: https://github.com/ruby/net-imap/pull/444\n  * Specific commit: [0ae8576c1 - lib/net/imap/response_reader.rb](https://github.com/ruby/net-imap/pull/444/commits/0ae8576c1a90bcd9573f81bdad4b4b824642d105#diff-53721cb4d9c3fb86b95cc8476ca2df90968ad8c481645220c607034399151462)\n* Backport to 0.4: https://github.com/ruby/net-imap/pull/445\n* Backport to 0.3: https://github.com/ruby/net-imap/pull/446\n* Backport to 0.2: https://github.com/ruby/net-imap/pull/447",
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  "database_specific" : {
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  }
}

CVSS V4

N/A MODERATE

Attack Vector N/A

Attack Complexity N/A

Privileges Required N/A

User Interaction N/A

Scope N/A

Confidentiality N/A

Integrity N/A

Availability N/A

Vector String

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Advisory Metadata

Source ID: GHSA-j3g3-5qv5-52mj
Schema Version: 1.4.0
Published: 2025-04-28 14:17
Last Modified: 2025-11-21 22:18
Database Record File: advisories/github-reviewed/2025/04/GHSA-j3g3-5qv5-52mj/GHSA-j3g3-5qv5-52mj.json