OpenSSL HollowByte: A DoS Hiding in 11 Bytes
Every so often, a vulnerability reminds us how deeply our digital infrastructure relies on foundational libraries. Recently, the Okta Red Team discovered HollowByte, a Denial of Service (DoS) vulnerability in OpenSSL. By sending a malicious payload of just 11 bytes, a remote, unauthenticated attacker can force a server to allocate disproportionate chunks of memory before any security handshake even begins.
Here is the breakdown of how it works.
Trusting the Header
The TLS handshake begins with a ClientHello message wrapped in a record. Each handshake message carries a 4-byte header that declares how large the incoming message body will be.
Older versions of OpenSSL allocate a receive buffer based on that attacker-declared length before any data has actually arrived.
When a rogue header lands, the state machine triggers an unvalidated allocation. When the malicious 11-byte payload arrives, the TLS state machine reads the 4-byte handshake header and triggers an unvalidated pre-allocation based on the header's 3-byte length declaration:
Read Header⟶grow_init_buf()⟶OPENSSL_clear_realloc()⟶malloc(attacker_size)
Because there is no payload validation at this early stage, malloc() allocates up to 131 KB based solely on the untrusted packet's claims. The worker thread then blocks, waiting indefinitely for data that will never arrive.
Permanent Memory Fragmentation
Holding connections open to exhaust threads is a classic trick (like Slowloris). HollowByte introduces a nastier compounding effect due to how the GNU C Library (glibc) handles memory.
When an attacking connection drops, OpenSSL frees the buffer. However, glibc does not immediately return small-to-medium allocations to the operating system; it keeps them for potential reuse.
By launching waves of connections with randomized claimed sizes, an attacker prevents the allocator from reusing those freed chunks. The heap fragments heavily, causing the server’s Resident Set Size (RSS) to climb continuously.
Even after the attacker disconnects, the server remains permanently bloated. The only way to reclaim that memory is to terminate the process.
Real-World Testing & Ecosystem Impact
To measure the threat, we tested unpatched and patched OpenSSL instances running NGINX under various load conditions.
In a standard 1 GB RAM environment, the unpatched server was OOM-killed at 547 MB of frozen, fragmented memory. In higher-spec testing (16 GB RAM), the attack successfully locked up 25% of the system's total memory while staying safely under the connection ceiling, meaning standard connection-limiting defenses won't stop it.
Because OpenSSL is widely used and embedded, this vulnerability affects a variety of software, including web servers (Apache, NGINX), language runtimes (Node.js, Python, Ruby, PHP), and databases (MySQL, PostgreSQL).
The Fix and Next Steps
The OpenSSL team resolved this by moving to incremental buffer growth (merged in PRs #30792, #30793, and #30794). This fix was silently included as part of the OpenSSL v4.0.1 release, with silent backports to release versions 3.6.3, 3.5.7, 3.4.6, and 3.0.21. Instead of trusting the header outright, OpenSSL now grows the buffer only as bytes actually land on the wire. A claim with no follow-through now costs the server nothing.
While OpenSSL handled this as a hardening fix rather than a CVE security advisory, we recommend upgrading your distribution's OpenSSL packages immediately.