USN-8563-4: nginx regression

unknownpublic exploitCVE-2026-42533 CVE-2026-56434 CVE-2026-60005

USN-8563-3 fixed a vulnerability in nginx. The fix introduced a regression in certain environments. This update reverts the fix for CVE-2026-42533 pending further investigation. We apologize for the inconvenience. Original advisory details: It was discovered that nginx incorrectly handled certain map directives using regex matching and capture variables. A remote attacker could use this issue to cause nginx to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2026-42533) It was discovered that nginx had a use-after-free vulnerability in the ngx_http_ssi_module module when configured with Server-Side Includes, proxy_pass, and proxy buffering disabled directives. An attacker able to intercept traffic and control responses from an upstream server could possibly use this issue to cause nginx to crash, resulting in a denial of service. (CVE-2026-56434) It was discovered that nginx incorrectly handled certain requests in the ngx_http_slice_module module. A remote attacker could possibly use this issue to obtain sensitive information or cause nginx to crash, resulting in a denial of service. (CVE-2026-60005)

CSIRTS triage

vendor: nginx product: nginx Denial of service Memory corruption Remote code executionaffected: affected by USN-8563-3 fix

What: Regression introduced by CVE-2026-42533 fix requiring revert pending further investigation; original vulnerabilities include incorrect regex map handling and use-after-free in SSI module.
Who is affected: nginx installations with map directives using regex capture variables, or SSI with proxy_pass and buffering disabled.
Urgency: Medium; regression fix reverts protection against DoS and RCE but vulnerability details require investigation.
Action: Apply USN-8563-4 to revert problematic fix and monitor for corrected patch addressing CVE-2026-42533 properly.

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: 2026-08-19
Exploitation: Not in CISA KEV at last sync

Original advisory: https://ubuntu.com/security/notices/USN-8563-4

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.

Moderate exploitation riskCVE-2026-425333.5% 30-day exploitation probability. Patch within normal cadence, watch for KEV listing. Riskier than 88% of all EPSS-scored CVEs.
Low exploitation riskCVE-2026-564340.45% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 38% of all EPSS-scored CVEs.
Low exploitation riskCVE-2026-600050.71% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 51% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-42533	coverage & exploitation status	NVD · CVE.org
CVE-2026-56434	coverage & exploitation status	NVD · CVE.org
CVE-2026-60005	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

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

unknownUSN-8563-3: nginx vulnerabilityubuntu
high[NEW] [high] NGINX NGINX Plus: Multiple vulnerabilitiescert-bund
unknownUSN-8563-2: nginx regressionubuntu
unknownUSN-8563-1: nginx vulnerabilitiesubuntu
unknownMultiple vulnerabilities in F5 products (July 16, 2026)cert-fr-avis
highCVE-2026-60005: NGINX Plus and NGINX Open Source have a vulnerability in the ngx_http_slice_module module. Whe…nvd
mediumCVE-2026-56434: NGINX Plus and NGINX Open Source have a vulnerability in the ngx_http_ssi_module module. This …nvd
highCVE-2026-42533: A vulnerability exists in NGINX Plus and NGINX Open Source when a map directive uses regex mat…nvd
highCVE-2026-42533: NGINX Map directive and Regex matching vulnerabilitymsrc
mediumCVE-2026-56434: NGINX ngx_http_ssi_module vulnerabilitymsrc
highCVE-2026-60005: NGINX ngx_http_slice_module vulnerabilitymsrc