CVE-2026-59248 Unbounded HPACK/QPACK prefixed-integer decoding in Cowlib causes memory-exhaustion DoS

What is CVE-2026-59248?

CVE-2026-59248 is a high-severity denial of service vulnerability in ninenines cowlib, published on 28 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Allocation of resources without limits vulnerability in ninenines cowlib allows an unauthenticated remote HTTP/2 or HTTP/3 peer to exhaust memory on the vulnerable server (or client) and cause a denial of service.

The HPACK and QPACK prefixed-integer decoder cow_hpack_common:dec_big_int/3 in src/cow_hpack_common.hrl (invoked from cow_hpack:decode/2 in src/cow_hpack.erl and from cow_qpack:decode_field_section/3 in src/cow_qpack.erl) reads continuation octets until it sees one whose high bit is clear, evaluating Int + (Value bsl M) at each step with the shift M growing by seven per octet. No limit is enforced on the number of continuation octets, on the resulting bit width, or on the value; the decoder consumes whatever encoded length the peer supplies.

Because Erlang integers are immutable, each intermediate Value bsl M and each accumulator update allocates a fresh bignum whose digit width grows linearly with the number of octets processed so far. Summed across the whole decode, the transient bignum digit materialization is on the order of the square of the encoded length. A single maximal HPACK indexed representation carried inside one HTTP/2 HEADERS plus one CONTINUATION frame at Cowboy's default max_frame_size_received can force hundreds of megabytes of transient allocation and garbage-collection churn before the resulting header-table index is rejected as invalid. Repeated or concurrent connections multiply the pressure and can drive the Erlang VM to memory exhaustion.

Cowlib is the HTTP parser used by Cowboy, RabbitMQ's management plugin, and other Erlang and Elixir HTTP/2 and HTTP/3 servers and clients, so any exposed endpoint that accepts HPACK or QPACK from an untrusted peer is reachable.

This issue affects cowlib: from 2.0.0 before 2.19.0.

Weakness: CWE-770

Which versions are affected by CVE-2026-59248?

1 open source package ship an affected version, including cowlib.

How do I fix CVE-2026-59248?

Upgrade cowlib to 2.19.0 or later.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-59248 being exploited?

There is no known exploitation of CVE-2026-59248 in the wild and no public exploit code.

Public exploit code: 1 proofs of concept on GitHub.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-94305, DEBIAN-CVE-2026-59248, EEF-CVE-2026-59248, GHSA-f37p-pc7r-rghm, UBUNTU-CVE-2026-59248, USN-8869-1.