CVE-2026-118374 in LibreSSL
Summary
by MITRE • 10/28/2026
LibreSSL ocspcheck through 4.4.0 contains a memory exhaustion vulnerability that allows malicious OCSP responders to exhaust memory by sending unbounded HTTP responses. Attackers controlling the responder or intercepting plain HTTP AIA OCSP traffic can stream endless headers or body data to http_head_read() and http_body_read(), denying certificate status checks.
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Analysis
by VulDB Data Team • 10/28/2026
The identified vulnerability in LibreSSL versions through 4.4.0 represents a critical resource exhaustion flaw within the Online Certificate Status Protocol (OCSP) checking mechanism, specifically affecting the functions responsible for parsing HTTP responses from OCSP responders. This issue stems from an insufficient validation of response sizes and lengths during the processing of headers and body data by the http_head_read and http_body_read routines. When LibreSSL initiates a certificate status check via the Authority Information Access (AIA) extension, it establishes an HTTP connection to retrieve the revocation status of a digital certificate. The flaw allows for unbounded memory allocation because the implementation fails to enforce strict limits on the amount of data read from the network stream before processing or storing it in memory buffers.
From a technical perspective, this vulnerability is classified under CWE-789 as a Memory Exhaustion issue, which occurs when software allocates an excessive amount of memory relative to available system resources due to flawed logic regarding input size constraints. An attacker who controls a malicious OCSP responder can exploit this by sending HTTP responses with extremely large headers or body payloads that exceed the expected limits for typical certificate status queries. Alternatively, in scenarios where plain HTTP is used without encryption and no integrity checks are enforced against man-in-the-middle attacks, an adversary intercepting the traffic can inject these oversized responses into the communication stream. The http_head_read function may attempt to allocate memory proportional to the header size, while http_body_read continues to consume resources for the body content, leading to a rapid depletion of available system memory on the host performing the OCSP check.
The operational impact of this vulnerability is primarily denial of service against systems relying on LibreSSL for certificate validation. By exhausting the memory resources of the application or operating system, an attacker can cause the process handling the OCSP request to crash or become unresponsive. This disruption prevents legitimate users from verifying the revocation status of certificates, which undermines the trust model of Public Key Infrastructure (PKI). In environments where automated systems continuously perform certificate checks, such as web servers validating client certificates or email clients checking sender authenticity, this attack can lead to widespread service outages and degraded security posture. The inability to complete OCSP checks may force applications to fall back to less secure validation methods or fail entirely, depending on their configuration policies regarding revocation status verification failures.
This vulnerability aligns with MITRE ATT&CK technique T1498 Network Denial of Service, specifically the sub-technique related to resource exhaustion through overwhelming input data. It also relates to CWE-20 Improper Input Validation, as the software fails to properly validate the size and structure of incoming HTTP responses before processing them. The attack vector is classified as remote with low complexity if plain HTTP is used for OCSP requests, or network adjacent if an attacker can intercept encrypted traffic through sophisticated man-in-the-middle techniques that bypass certificate validation checks prior to this specific flaw being triggered.
Mitigation strategies should focus on implementing strict size limits for both header and body data during the parsing phase of OCSP responses. Developers must ensure that memory allocation routines check available resources before committing large blocks of memory and enforce maximum byte thresholds consistent with RFC standards for HTTP and OCSP protocols. Upgrading to a patched version of LibreSSL beyond 4.4.0 is the primary remediation step, as these versions include fixes that validate response lengths against predefined limits. Additionally, organizations should configure their systems to use HTTPS for all OCSP requests whenever possible, reducing the risk of interception by unauthorized parties who might attempt to inject malicious payloads into unencrypted traffic streams.