USN-8556-1: Ruby vulnerabilities

unknownCVE-2026-42258 CVE-2026-27820

It was discovered that the Net::IMAP client in Ruby did not properly sanitize Symbol arguments passed to IMAP commands. A remote attacker controlling a malicious IMAP server, or able to influence command arguments, could use this to inject arbitrary IMAP commands via CRLF sequences. (CVE-2026-42258) It was discovered that the Zlib::GzipReader in Ruby did not correctly ensure sufficient buffer capacity in the zstream_buffer_ungets function. An attacker could use this to craft a gzip stream that, when processed, could cause a buffer overflow, resulting in memory corruption and possibly arbitrary code execution. (CVE-2026-27820)

CSIRTS triage

vendor: Ruby product: Ruby Memory corruptionOther

What: Vulnerabilities in Ruby could allow remote code execution and memory corruption.
Who is affected: Users of Ruby applications that utilize the affected components.
Urgency: Remediation is urgent due to the potential for severe impacts from exploitation.
Action: Users should update Ruby to the latest version.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Ubuntu Security Notices (INTL · vendor-psirt · site)
Severity: unknown
Published: 2026-07-16
Exploitation: Not in CISA KEV at last sync

Original advisory: https://ubuntu.com/security/notices/USN-8556-1

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-422580.78% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 52% of all scored CVEs.
Low exploitation riskCVE-2026-278200.56% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 43% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-42258	coverage & exploitation status	NVD · CVE.org
CVE-2026-27820	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

unknownMultiples vulnérabilités dans les produits IBM (31 juillet 2026)cert-fr-avis
high[NEW] [high] Ruby: Multiple vulnerabilitiescert-bund
medium[UPDATE] [medium] Ruby: Vulnerability allows Denial of Servicecert-bund
medium[UPDATE] [medium] Red Hat Enterprise Linux (ruby: net-imap): Multiple vulnerabilitiescert-bund
high[UPDATE] [high] HCL BigFix Compliance (Ruby): Multiple vulnerabilitiescert-bund
mediumGHSA-75xq-5h9v-w6px: net-imap vulnerable to command Injection via unvalidated Symbol inputsghsa