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GHSA-87pf-fpwv-p7m7 CVE-2026-42256

net-imap vulnerable to denial of service via high iteration count for `SCRAM-*` authentication

Medium RubyGems
Severity
0.0 / 10
Medium · CVSS 4.0
Exploit chance (EPSS)
0.30%
Chance of exploitation in the next 30 days. Higher than 22.60% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-1322 Use of Blocking Code in Single-threaded, Non-blocking Context
CWE-770 Allocation of Resources Without Limits or Throttling
Fix
0.6.4
Upgrade net-imap to this version or later

Description

Summary

When authenticating a connection with SCRAM-SHA1 or SCRAM-SHA256, a hostile server can perform a computational denial-of-service attack on the client process by sending a big iteration count value.

Details

A hostile IMAP server can send an arbitrarily large PBKDF2 iteration count in the SCRAM server-first-message, causing the client to perform an expensive OpenSSL::KDF.pbkdf2_hmac call. Because the PBKDF2 function is a blocking C extension and holds onto Ruby’s Global VM Lock, it can freeze the entire Ruby VM for the duration of the computation.

OpenSSL enforces an effective maximum by using a 32-bit signed integer for the iteration count, Depending on hardware capabilities and OpenSSL version, this iteration count may be sufficient for to block all Ruby threads in the process for over seven minutes.

This is listed as one of the "Security Considerations", in RFC 7804:

A hostile server can perform a computational denial-of-service attack on clients by sending a big iteration count value. In order to defend against that, a client implementation can pick a maximum iteration count that it is willing to use and reject any values that exceed that threshold (in such cases, the client, of course, has to fail the authentication).

Impact

During SCRAM authentication to a hostile server, the entire Ruby VM will be locked for the duration of the computation. Depending on hardware capabilities and OpenSSL version, this may take many minutes.

OpenSSL::KDF.pbkdf2_hmac is a blocking C function, so Timeout cannot be used to guard against this. And it retains the Global VM lock, so other ruby threads will also be unable to run.

Mitigation

  • Upgrade to a patched version of net-imap that adds the max_iterations option to the SASL-* authenticators, and call Net::IMAP#authenticate with a max_iterations keyword argument.

    NOTE: The default max_iterations is 2³¹ - 1, the maximum signed 32 bit integer, the maximum allowed by OpenSSL. To prevent a denial of service attack, this must be set to a safe value, depending on hardware and version of OpenSSL. It is the user's responsibility to enforce minimum and maximum iteration counts that are appropriate for their security context.

  • Alternatively, avoid SCRAM-* mechanisms when authenticating to untrusted servers.

Affected versions

RubyGems

PackageVulnerableFixed in
net-imap
>= 0.6.0, < 0.6.4
0.6.4
net-imap
>= 0.5.0, < 0.5.14
0.5.14
net-imap
>= 0.4.0, < 0.4.24
0.4.24

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

net-imap RubyGems → 0.6.4

bundler
bundle update net-imap # Gemfile: gem "net-imap", ">= 0.6.4"

Severity breakdown

CVSS 4.0 0.0

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

Attack vector
Network
Attack complexity
Low
Attack requirements
Present
Privileges required
None
User interaction
Passive
Confidentiality (vulnerable system)
None
Integrity (vulnerable system)
None
Availability (vulnerable system)
High
Confidentiality (subsequent systems)
None
Integrity (subsequent systems)
None
Availability (subsequent systems)
None

CVSS 3.1 6.5

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Weakness

CWE-1322: Use of Blocking Code in Single-threaded, Non-blocking Context

The product uses a non-blocking model that relies on a single threaded process for features such as scalability, but it contains code that can block when it is invoked.

How to prevent it
  • Implementation: Generally speaking, blocking calls should be replaced with non-blocking alternatives that can be used asynchronously. Expensive computations should be passed off to worker threads, although the correct approach depends on the framework being used.
  • Implementation: For expensive computations, consider breaking them up into multiple smaller computations. Refer to the documentation of the framework being used for guidance.

CWE-770: Allocation of Resources Without Limits or Throttling

The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

How to prevent it
  • Requirements: Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
  • Architecture and Design: Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
  • Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place, and it will help the administrator to identify who is committing the abuse. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
  • Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
  • Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
  • Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, typically by using increasing time delays uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution can be difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply requires more resources on the part of the attacker.
  • Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
  • Architecture and Design, Implementation: If the program must fail, ensure that it fails gracefully (fails closed). There may be a temptation to simply let the program fail poorly in cases such as low memory conditions, but an attacker may be able to assert control before the software has fully exited. Alternately, an uncontrolled failure could cause cascading problems with other downstream components; for example, the program could send a signal to a downstream process so the process immediately knows that a problem has occurred and has a better chance of recovery. Ensure that all failures in resource allocation place the system into a safe posture.
  • Operation, Architecture and Design: Use quotas or other resource-limiting settings provided by the operating system or environment. For example, when managing system resources in POSIX, setrlimit() can be used to set limits for certain types of resources, and getrlimit() can determine how many resources are available. However, these functions are not available on all operating systems. When the current levels get close to the maximum that is defined for the application (see CWE-770), then limit the allocation of further resources to privileged users; alternately, begin releasing resources for less-privileged users. While this mitigation may protect the system from attack, it will not necessarily stop attackers from adversely impacting other users. Ensure that the application performs the appropriate error checks and error handling in case resources become unavailable (CWE-703).

References

Advisories

Packages

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy it's related
GHSA-46q3-7gv7-qmgg
Net::IMAP: Command Injection via ID command argument
Medium Also affects net-imap
GHSA-c4fp-cxrr-mj66
Net::IMAP: Denial of Service via incomplete raw argument validation
Low Also affects net-imap
GHSA-8p34-64r3-mwg8
Net::IMAP: Command Injection via non-synchronizing literal in "raw" argument
Medium Also affects net-imap
GHSA-hm49-wcqc-g2xg
net-imap vulnerable to command Injection via "raw" arguments to multiple commands
Critical Also affects net-imap
GHSA-75xq-5h9v-w6px
net-imap vulnerable to command Injection via unvalidated Symbol inputs
Medium Also affects net-imap
GHSA-q2mw-fvj9-vvcw
net-imap has quadratic complexity when reading response literals
High Also affects net-imap
GHSA-vcgp-9326-pqcp
net-imap vulnerable to STARTTLS stripping via invalid response timing
High Also affects net-imap
GHSA-j3g3-5qv5-52mj
net-imap rubygem vulnerable to possible DoS by memory exhaustion
Medium Also affects net-imap
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