LibreSSL ocspcheck: Memory exhaustion via unbounded OCSP HTTP response
Summary
The OCSP check utility ocspcheck in LibreSSL through 4.4.0 does not limit the size of HTTP headers and bodies it receives from an OCSP responder. A malicious responder, or an attacker who can intercept plain-HTTP AIA OCSP traffic, can stream endless data and exhaust memory, which denies certificate status checks. The attack requires network access to the OCSP exchange, with no authentication needed.
Affected
- LibreSSL 0 to 4.4.0 (OpenBSD LibreSSL, per CNA record)
- ocspcheck utility is affected; the default configuration is unaffected
- Not a server daemon; typically relevant on systems that run ocspcheck
Fixed in
- Fixed version: Unknown.
- Upstream OpenBSD commits referenced: bf5da63f53179ef423d18c56d6da89eaefb56228 and de4274c56c9ace88232591d4c919d79281a8de59 (release containing them not confirmed)
Workaround
- No vendor-documented workaround found.
- Limit use of ocspcheck to trusted network paths.
- Avoid plain-HTTP AIA OCSP fetches where possible and restrict outbound paths for these tools.
Exploitation
- Not known to be exploited in the wild.
- Not listed in CISA KEV.
- No public proof of concept found.
- EPSS: 0.21% (percentile 11).
Our recommendation
- Check whether any system runs LibreSSL's ocspcheck (for example OpenBSD hosts).
- If present, track OpenBSD errata and apply a confirmed fixed build once published.
- Limit ocspcheck use to trusted networks and restrict plain-HTTP OCSP traffic.
- For most SMEs that use OpenSSL or OS-supplied TLS stacks, the risk is low.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Official advisory (VulnCheck) www.vulncheck.com
- LibreSSL 4.4.0 release notes ftp.openbsd.org
- OpenBSD commit bf5da63f github.com
- OpenBSD commit de4274c5 github.com
- OpenBSD ocspcheck http.c github.com
- LibreSSL portable github.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 14:47.
Original description (NVD)
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.
NVD references (6)
- https://ftp.openbsd.org/pub/OpenBSD/LibreSSL/libressl-4.4.0-relnotes.txt
- https://github.com/libressl/portable
- https://github.com/openbsd/src/blob/f6f428241d469cf434e60b73fc834d0448f068d5/usr.sbin/ocspcheck/http.c#L419
- https://github.com/openbsd/src/commit/bf5da63f53179ef423d18c56d6da89eaefb56228
- https://github.com/openbsd/src/commit/de4274c56c9ace88232591d4c919d79281a8de59
- https://www.vulncheck.com/advisories/libressl-ocspcheck-through-4.4.0-memory-exhaustion-via-unbounded-ocsp-ht…
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
Affected? We can help.
We check whether your systems are affected, install updates and close the gap – continuously with the System Health Checker if you wish.