CVE-2026-48142: NGINX Charset Module Buffer Over-Read Vulnerability

Technical summary

CVE-2026-48142 is a heap buffer over-read vulnerability in the ngx_http_charset_module of NGINX Plus and NGINX Open Source. The flaw manifests when a location block specifies both source_charset utf-8; and an alternative charset directive simultaneously. Under these conditions, an unauthenticated remote attacker can craft requests that cause the NGINX worker process to read past allocated heap memory boundaries. This results in either limited memory disclosure (information leak) or worker process termination. The vulnerability is classified as CWE-125 (Out-of-bounds Read).

Business impact

The primary business risk is service disruption through worker process crashes, which can degrade or temporarily disable affected NGINX instances. Secondary risk includes potential leakage of sensitive data resident in memory, such as session tokens, API keys, or user information, though the disclosed amount per incident is expected to be small. Organizations relying on NGINX for critical web services, API gateways, or reverse proxies should assess whether they match the vulnerable configuration profile and prioritize patching accordingly.

Affected systems

Affected F5 products include NGINX Plus, NGINX Open Source, NGINX Ingress Controller, NGINX Gateway Fabric, NGINX Instance Manager, and certain deployments of F5 WAF and F5 DoS modules that incorporate the vulnerable NGINX codebase. The vulnerability applies only to configurations explicitly using source_charset utf-8; combined with a non-UTF-8 target charset directive. Systems without this specific dual-charset setup are not vulnerable.

Exploitability

While remote and unauthenticated access is required, the vulnerability has a High attack complexity (AC:H in CVSS terms), meaning attackers need conditions partially beyond their control to reliably trigger the buffer over-read. No authentication is needed to attempt exploitation, but the precise request conditions and server state necessary to cause observable harm are not trivial to arrange. Public exploit code is not known to be available, reducing the likelihood of mass exploitation in the near term.

Remediation

Organizations must patch their NGINX installations to versions containing the security fix once available from F5. Mitigation prior to patching includes disabling the vulnerable dual-charset configuration by removing either the source_charset utf-8; directive or the alternative charset directive, though this may impact character encoding handling for affected traffic. Verify the specific patched version numbers in the F5 security advisory, as versions vary by product line and deployment model (open source vs. Plus vs. controller variants).

Patch guidance

Monitor F5's security advisories and NGINX release notes for patched versions addressing CVE-2026-48142. Patching typically follows product lines: NGINX Open Source updates are distributed through standard package managers, while NGINX Plus customers should consult their support portal. NGINX Ingress Controller, Gateway Fabric, and Instance Manager users must update their respective components. Test patches in a staging environment to confirm character encoding behavior remains correct before production rollout. Prioritize systems serving production traffic or handling sensitive user data.

Detection guidance

Monitor NGINX worker process restarts and error logs for sudden termination without explicit configuration reloads. Network-based detection is difficult without knowing the exact request patterns needed to trigger the flaw; however, instrumentation of NGINX with debugging symbols may reveal heap corruption signatures in crash dumps. Log aggregation systems should flag unusual worker recycling patterns as a potential indicator. Memory sanitizers (AddressSanitizer) on test deployments can help confirm the vulnerability before exploitation occurs in production.

Why prioritize this

Despite a CVSS score of 4.8 (Medium), this vulnerability warrants timely attention because it affects ubiquitous infrastructure software in a large installed base and combines both confidentiality and availability impact. The high attack complexity provides some breathing room, but the remote, unauthenticated attack vector and potential for service disruption make it a suitable candidate for early patching cycles, particularly for external-facing or high-availability deployments. Organizations already using the vulnerable configuration pattern should prioritize this above lower-impact medium-risk issues.

Risk score, explained

The CVSS 3.1 score of 4.8 reflects a medium-severity vulnerability with network-accessible attack surface (AV:N), high attack complexity (AC:H), no privilege or user interaction required (PR:N, UI:N), and limited impact to confidentiality and availability (C:L, A:L). The score does not increment for scope change, indicating impacts are contained to the vulnerable component. The moderate complexity requirement—specific configuration and request circumstances—prevents this from scoring higher despite the remote attack vector.

Frequently asked questions