CVE-2026-42256Network attack vector

Client freeze via unbounded SCRAM iteration count

Published May 9, 2026 · Updated May 9, 2026

Denial of service in net-imap 0.4.0 through 0.4.23, 0.5.0 through 0.5.13, and 0.6.0 through 0.6.3 allows remote attackers to freeze clients. During SCRAM-SHA1 or SCRAM-SHA256 authentication, the client accepts an arbitrarily large server-supplied iteration count and passes it to blocking OpenSSL::KDF.pbkdf2_hmac while the C extension holds Ruby's Global VM Lock. Exploitation requires the client to authenticate to an attacker-controlled server with SCRAM; the computation can stop every Ruby thread for minutes without disclosing or modifying data.

CVSS severity6.0
Medium
EPSS probability0.30%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in net-imap 0.4.0 through 0.4.23, 0.5.0 through 0.5.13, and 0.6.0 through 0.6.3 allows remote attackers to freeze clients. During SCRAM-SHA1 or SCRAM-SHA256 authentication, the client accepts an arbitrarily large server-supplied iteration count and passes it to blocking OpenSSL::KDF.pbkdf2_hmac while the C extension holds Ruby's Global VM Lock. Exploitation requires the client to authenticate to an attacker-controlled server with SCRAM; the computation can stop every Ruby thread for minutes without disclosing or modifying data.

The record

CVE
CVE-2026-42256
Published
May 9, 2026
Updated
May 9, 2026
Vendor
libwww-perl
Product
net-imap
Classifications
CWE-770, CWE-1322, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 9, 2026CVE publishedPublication date reported by the CVE source.
  2. May 9, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 0.4.0, < 0.4.24
  2. Affected versionversion=>= 0.5.0, < 0.5.14
  3. Affected versionversion=>= 0.6.0, < 0.6.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

libwww-perl · net-imapversion=>= 0.4.0, < 0.4.24; version=>= 0.5.0, < 0.5.14; version=>= 0.6.0, < 0.6.4

Published CVSS scores

6.0GitHub, Inc.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
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-770Allocation of Resources Without Limits or Throttling
CWE-1322Use of Blocking Code in Single-threaded, Non-blocking Context
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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