USN-8563-5: nginx vulnerability
Ubuntu ships improved nginx fix for CVE-2026-42533 after earlier patch regression; flaw allows remote crash and possible code execution.
USN-8563-5 provides a better fix for CVE-2026-42533 after the original fix from USN-8563-1 was backed out in USN-8563-2 because it caused a regression. The flaw stems from nginx incorrectly handling certain map directives using regex matching and capture variables, allowing a remote attacker to crash nginx, causing denial of service, or possibly execute arbitrary code. The original advisory also described a use-after-free in the ngx_http_ssi_module when configured with Server-Side Includes, proxy_pass, and proxy buffering.
- Improved fix for CVE-2026-42533 replaces regression-prone earlier patch
- Remote attacker can crash nginx or possibly execute arbitrary code
- Trigger involves map directives with regex matching and capture variables
- Third revision of the original USN-8563-1 nginx advisory
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2026-42533 | A vulnerability exists in NGINX Plus and NGINX Open Source when a map directive uses regex matching and a string expression references the map's regex capture v A vulnerability exists in NGINX Plus and NGINX Open Source when a map directive uses regex matching and a string expression references the map's regex capture variables before referencing the map output variable. Alternatively, the same result could be achieved by using a non-cacheable variable in a string expression under certain conditions. An unauthenticated attacker along with conditions beyond their control can exploit this vulnerability by sending crafted HTTP requests. This may cause a heap buffer overflow in the NGINX worker process leading to a restart. Additionally, attackers can execute code on systems with Address Space Layout Randomization (ASLR) disabled or when the attacker can bypass ASLR. Impact: This vulnerability may allow remote attackers to cause a denial-of-service (DoS) on the NGINX system or to possibly trigger a code execution. There is no control plane exposure; this is a data plane issue only. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated. NVD description · AI analysis pending | 9.2 | 4% | PoC |
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USN-8563-1 fixed vulnerabilities in nginx. The fix for CVE-2026-42533 was backed out in USN-8563-2 because it could cause a regression. This update includes a better fix for CVE-2026-42533. 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…
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