Improper Handling of Highly Compressed Data (Data Amplification) vulnerability in the HTTP/2…

Why it is Yellow

• CVSS 8.7 high
• Remotely exploitable without logging in

References

• https://cna.erlef.org/cves/CVE-2026-109584.html
• https://github.com/mtrudel/bandit/commit/220b9dfd27f9a21305be2afc75cbe585ddc24e7a
• https://github.com/mtrudel/bandit/commit/44bf969cd59851622fb6ee4d16227fbc39a77fb4
• https://github.com/mtrudel/bandit/security/advisories/GHSA-7gfj-7gvq-3xqj
• https://osv.dev/vulnerability/EEF-CVE-2026-109584

Original description

Improper Handling of Highly Compressed Data (Data Amplification) vulnerability in the HTTP/2 implementation of mtrudel bandit allows an unauthenticated remote attacker to exhaust server memory via a small HPACK-compressed HEADERS block. The block stores one large cookie value in the HPACK dynamic table and then references it tens of thousands of times with 1-byte indexed fields. In the reporter's run, a 49 KB block made a server with default settings allocate a 183 MB cookie binary before any application code ran, and every concurrent stream can repeat the cost. Bandit.HTTP2.Connection caps only the compressed block (max_header_block_size), and its check of the decoded list against max_header_list_size never fires because that setting defaults to :infinity. combine_cookie_crumbs/1 in lib/bandit/http2/stream.ex then joins every crumb into one binary. This issue affects bandit: from 0.3.3 before 1.14.0.