{"id":"CVE-2026-23943","state":"PUBLISHED","title":"Pre-auth SSH DoS via unbounded zlib inflate","description":"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 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to ssh from 3.0.1 before 5.5.1, 5.2.11.6 and 5.1.4.14.","publishedAt":"2026-03-13T09:11:57.794Z","updatedAt":"2026-07-24T14:15:38.635Z","severity":{"score":6.9,"label":"Medium","version":"4.0","vector":"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","source":"EEF"},"assigner":"EEF","products":["Erlang OTP"],"affected":[{"vendor":"Erlang","product":"OTP","packageName":"ssh","packageURL":"pkg:otp/ssh?repository_url=https:%2F%2Fgithub.com%2Ferlang%2Fotp&vcs_url=git%20https:%2F%2Fgithub.com%2Ferlang%2Fotp.git","defaultStatus":"unknown","versions":[{"version":"3.0.1","status":"affected","lessThan":"*","changes":[{"at":"5.5.1","status":"unaffected"},{"at":"5.2.11.6","status":"unaffected"},{"at":"5.1.4.14","status":"unaffected"}],"versionType":"otp"}],"cpes":["cpe:2.3:a:erlang:erlang\\/otp:*:*:*:*:*:*:*:*"],"modules":["ssh_transport"],"programFiles":["src/ssh_transport.erl"],"programRoutines":[{"name":"ssh_transport:decompress/2"},{"name":"ssh_transport:handle_packet_part/4"}],"repo":"https://github.com/erlang/otp"},{"vendor":"Erlang","product":"OTP","packageName":"erlang/otp","packageURL":"pkg:github/erlang/otp","defaultStatus":"unknown","versions":[{"version":"17.0","status":"affected","lessThan":"*","changes":[{"at":"28.4.1","status":"unaffected"},{"at":"27.3.4.9","status":"unaffected"},{"at":"26.2.5.18","status":"unaffected"}],"versionType":"otp"},{"version":"07b8f441ca711f9812fad9e9115bab3c3aa92f79","status":"affected","lessThan":"*","changes":[{"at":"43a87b949bdff12d629a8c34146711d9da93b1b1","status":"unaffected"},{"at":"93073c3bd338c60cd2bae715ce6a1d4ffc1a8fd3","status":"unaffected"},{"at":"0c1c04b191f6ab940e8fcfabce39eb5a8a6440a4","status":"unaffected"}],"versionType":"git"}],"collectionURL":"https://github.com","cpes":["cpe:2.3:a:erlang:erlang\\/otp:*:*:*:*:*:*:*:*"],"modules":["ssh_transport"],"programFiles":["lib/ssh/src/ssh_transport.erl"],"programRoutines":[{"name":"ssh_transport:decompress/2"},{"name":"ssh_transport:handle_packet_part/4"}],"repo":"https://github.com/erlang/otp"}],"references":["https://github.com/erlang/otp/security/advisories/GHSA-c836-qprm-jw9r","https://cna.erlef.org/cves/CVE-2026-23943.html","https://osv.dev/vulnerability/EEF-CVE-2026-23943","https://www.erlang.org/doc/system/versions.html#order-of-versions","https://github.com/erlang/otp/commit/43a87b949bdff12d629a8c34146711d9da93b1b1","https://github.com/erlang/otp/commit/93073c3bd338c60cd2bae715ce6a1d4ffc1a8fd3","https://github.com/erlang/otp/commit/0c1c04b191f6ab940e8fcfabce39eb5a8a6440a4"],"weaknesses":["CWE-409"],"ingestedAt":"2026-10-29T20:11:21.040Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/23xxx/CVE-2026-23943.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-23943","state":"PUBLISHED","datePublished":"2026-03-13T09:11:57.794Z","dateUpdated":"2026-07-24T14:15:38.635Z","assignerShortName":"EEF","assignerOrgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","dateReserved":"2026-01-19T14:23:14.343Z"},"containers":{"cna":{"title":"Pre-auth SSH DoS via unbounded zlib inflate","descriptions":[{"lang":"en","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 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to ssh from 3.0.1 before 5.5.1, 5.2.11.6 and 5.1.4.14."}],"affected":[{"vendor":"Erlang","product":"OTP","packageName":"ssh","packageURL":"pkg:otp/ssh?repository_url=https:%2F%2Fgithub.com%2Ferlang%2Fotp&vcs_url=git%20https:%2F%2Fgithub.com%2Ferlang%2Fotp.git","defaultStatus":"unknown","versions":[{"version":"3.0.1","status":"affected","lessThan":"*","changes":[{"at":"5.5.1","status":"unaffected"},{"at":"5.2.11.6","status":"unaffected"},{"at":"5.1.4.14","status":"unaffected"}],"versionType":"otp"}],"cpes":["cpe:2.3:a:erlang:erlang\\/otp:*:*:*:*:*:*:*:*"],"modules":["ssh_transport"],"programFiles":["src/ssh_transport.erl"],"programRoutines":[{"name":"ssh_transport:decompress/2"},{"name":"ssh_transport:handle_packet_part/4"}],"repo":"https://github.com/erlang/otp"},{"vendor":"Erlang","product":"OTP","packageName":"erlang/otp","packageURL":"pkg:github/erlang/otp","defaultStatus":"unknown","versions":[{"version":"17.0","status":"affected","lessThan":"*","changes":[{"at":"28.4.1","status":"unaffected"},{"at":"27.3.4.9","status":"unaffected"},{"at":"26.2.5.18","status":"unaffected"}],"versionType":"otp"},{"version":"07b8f441ca711f9812fad9e9115bab3c3aa92f79","status":"affected","lessThan":"*","changes":[{"at":"43a87b949bdff12d629a8c34146711d9da93b1b1","status":"unaffected"},{"at":"93073c3bd338c60cd2bae715ce6a1d4ffc1a8fd3","status":"unaffected"},{"at":"0c1c04b191f6ab940e8fcfabce39eb5a8a6440a4","status":"unaffected"}],"versionType":"git"}],"collectionURL":"https://github.com","cpes":["cpe:2.3:a:erlang:erlang\\/otp:*:*:*:*:*:*:*:*"],"modules":["ssh_transport"],"programFiles":["lib/ssh/src/ssh_transport.erl"],"programRoutines":[{"name":"ssh_transport:decompress/2"},{"name":"ssh_transport:handle_packet_part/4"}],"repo":"https://github.com/erlang/otp"}],"references":[{"url":"https://github.com/erlang/otp/security/advisories/GHSA-c836-qprm-jw9r","tags":["vendor-advisory","related"]},{"url":"https://cna.erlef.org/cves/CVE-2026-23943.html","tags":["related"]},{"url":"https://osv.dev/vulnerability/EEF-CVE-2026-23943","tags":["related"]},{"url":"https://www.erlang.org/doc/system/versions.html#order-of-versions","tags":["x_version-scheme"]},{"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","tags":["patch"]}],"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":{"shortName":"EEF","orgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","dateUpdated":"2026-07-24T14:15:38.635Z"},"configurations":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"The SSH server or client must advertise <tt>zlib</tt> or <tt>zlib@openssh.com</tt> compression. Both are enabled by default. With <tt>zlib</tt>, the attack is pre-authentication; with <tt>zlib@openssh.com</tt>, authentication is required first."}],"value":"The SSH server or client must advertise zlib or zlib@openssh.com compression. Both are enabled by default. With zlib, the attack is pre-authentication; with zlib@openssh.com, authentication is required first."}],"cpeApplicability":[{"nodes":[{"cpeMatch":[{"criteria":"cpe:2.3:a:erlang:erlang\\/otp:*:*:*:*:*:*:*:*","versionEndExcluding":"26.2.5.18","vulnerable":true},{"criteria":"cpe:2.3:a:erlang:erlang\\/otp:*:*:*:*:*:*:*:*","versionEndExcluding":"27.3.4.9","versionStartIncluding":"27.0","vulnerable":true},{"criteria":"cpe:2.3:a:erlang:erlang\\/otp:*:*:*:*:*:*:*:*","versionEndExcluding":"28.4.1","versionStartIncluding":"28.0","vulnerable":true}],"negate":false,"operator":"OR"}],"operator":"AND"}],"credits":[{"lang":"en","type":"finder","value":"Igor Morgenstern / Aisle Research"},{"lang":"en","type":"remediation developer","value":"Michał Wąsowski"},{"lang":"en","type":"remediation reviewer","value":"Jakub Witczak"}],"impacts":[{"capecId":"CAPEC-130","descriptions":[{"lang":"en","value":"CAPEC-130 Excessive Allocation"}]},{"capecId":"CAPEC-490","descriptions":[{"lang":"en","value":"CAPEC-490 Amplification"}]}],"source":{"discovery":"EXTERNAL"},"workarounds":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"<p><b>Best workaround - Disable all compression:</b></p><pre>{preferred_algorithms, [{compression, ['none']}]}</pre><p><b>Alternative mitigations (less secure):</b></p><ul><li>Disable only pre-auth zlib compression (authenticated users can still exploit via zlib@openssh.com):<pre>{modify_algorithms, [{rm, [{compression, ['zlib']}]}]}</pre></li><li>Limit concurrent sessions (reduces attack surface but does not prevent exploitation):<pre>{max_sessions, N}  % Cap total concurrent sessions (default is infinity)</pre></li></ul>"}],"value":"Best workaround - Disable all compression:\n\n{preferred_algorithms, [{compression, ['none']}]}\n\nAlternative mitigations (less secure):\n\n* Disable only pre-auth zlib compression (authenticated users can still exploit via zlib@openssh.com):\n  {modify_algorithms, [{rm, [{compression, ['zlib']}]}]}\n* Limit concurrent sessions (reduces attack surface but does not prevent exploitation):\n  {max_sessions, N}  % Cap total concurrent sessions (default is infinity)"}],"x_generator":{"engine":"Vulnogram 0.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-03-13T16:01:40.898658Z","id":"CVE-2026-23943","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-03-13T16:01:48.609Z"}}]}}}