The SSH transport layer advertises legacy zlib compression by default and inflates attacker-controlled payloads pre-authentication without any size limit, enabling reliable memory exhaustion DoS.
Two compression algorithms are affected:
Each SSH packet can decompress ~255 MB from 256 KB of wire data (1029:1 amplification ratio). Multiple packets can rapidly exhaust available memory, causing OOM kills in memory-constrained environments.
This vulnerability is associated with program files lib/ssh/src/ssh_transport.erl and program routines ssh_transport:decompress/2, ssh_transport:handle_packet_part/4.
This issue affects OTP from OTP 17.0 until OTP 28.4.1, 27.3.4.9 and 26.2.5.18 corresponding to ssh from 3.0.1 until 5.5.1, 5.2.11.6 and 5.1.4.14.
"}],"value":"Improper Handling of Highly Compressed Data (Compression Bomb) vulnerability in Erlang OTP ssh (ssh_transport modules) allows Denial of Service via Resource Depletion.\n\nThe SSH transport layer advertises legacy zlib compression by default and inflates attacker-controlled payloads pre-authentication without any size limit, enabling reliable memory exhaustion DoS.\n\nTwo compression algorithms are affected:\n\n* zlib: Activates immediately after key exchange, enabling unauthenticated attacks\n* zlib@openssh.com: Activates post-authentication, enabling authenticated attacks\n\nEach SSH packet can decompress ~255 MB from 256 KB of wire data (1029:1 amplification ratio). Multiple packets can rapidly exhaust available memory, causing OOM kills in memory-constrained environments.\n\nThis vulnerability is associated with program files lib/ssh/src/ssh_transport.erl and program routines ssh_transport:decompress/2, ssh_transport:handle_packet_part/4.\n\nThis issue affects OTP from OTP 17.0 until OTP 28.4.1, 27.3.4.9 and 26.2.5.18 corresponding to ssh from 3.0.1 until 5.5.1, 5.2.11.6 and 5.1.4.14."}],"impacts":[{"capecId":"CAPEC-130","descriptions":[{"lang":"en","value":"CAPEC-130 Excessive Allocation"}]},{"capecId":"CAPEC-490","descriptions":[{"lang":"en","value":"CAPEC-490 Amplification"}]}],"metrics":[{"cvssV4_0":{"Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","Safety":"NOT_DEFINED","attackComplexity":"LOW","attackRequirements":"NONE","attackVector":"NETWORK","baseScore":6.9,"baseSeverity":"MEDIUM","exploitMaturity":"NOT_DEFINED","privilegesRequired":"NONE","providerUrgency":"NOT_DEFINED","subAvailabilityImpact":"NONE","subConfidentialityImpact":"NONE","subIntegrityImpact":"NONE","userInteraction":"NONE","valueDensity":"NOT_DEFINED","vectorString":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","version":"4.0","vulnAvailabilityImpact":"LOW","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnerabilityResponseEffort":"NOT_DEFINED"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"cweId":"CWE-409","description":"CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-04-06T16:44:10.203Z","orgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","shortName":"EEF"},"references":[{"tags":["vendor-advisory","related"],"url":"https://github.com/erlang/otp/security/advisories/GHSA-c836-qprm-jw9r"},{"tags":["related"],"url":"https://cna.erlef.org/cves/CVE-2026-23943.html"},{"tags":["related"],"url":"https://osv.dev/vulnerability/EEF-CVE-2026-23943"},{"tags":["x_version-scheme"],"url":"https://www.erlang.org/doc/system/versions.html#order-of-versions"},{"tags":["patch"],"url":"https://github.com/erlang/otp/commit/43a87b949bdff12d629a8c34146711d9da93b1b1"},{"tags":["patch"],"url":"https://github.com/erlang/otp/commit/93073c3bd338c60cd2bae715ce6a1d4ffc1a8fd3"},{"tags":["patch"],"url":"https://github.com/erlang/otp/commit/0c1c04b191f6ab940e8fcfabce39eb5a8a6440a4"}],"source":{"discovery":"EXTERNAL"},"title":"Pre-auth SSH DoS via unbounded zlib inflate","workarounds":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"Best workaround - Disable all compression:
{preferred_algorithms, [{compression, ['none']}]}Alternative mitigations (less secure):
{modify_algorithms, [{rm, [{compression, ['zlib']}]}]}{max_sessions, N} % Cap total concurrent sessions (default is infinity)