{"id":"CVE-2021-47245","state":"PUBLISHED","title":"netfilter: synproxy: Fix out of bounds when parsing TCP options","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnetfilter: synproxy: Fix out of bounds when parsing TCP options\n\nThe TCP option parser in synproxy (synproxy_parse_options) could read\none byte out of bounds. When the length is 1, the execution flow gets\ninto the loop, reads one byte of the opcode, and if the opcode is\nneither TCPOPT_EOL nor TCPOPT_NOP, it reads one more byte, which exceeds\nthe length of 1.\n\nThis fix is inspired by commit 9609dad263f8 (\"ipv4: tcp_input: fix stack\nout of bounds when parsing TCP options.\").\n\nv2 changes:\n\nAdded an early return when length < 0 to avoid calling\nskb_header_pointer with negative length.","publishedAt":"2024-05-21T14:19:43.648Z","updatedAt":"2026-08-05T08:46:56.392Z","severity":{"score":8.2,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"e1eb98cfeafdd85537e7e3cefe93ca9bfbcc3ea8","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"576c1526b4d83c44ad7b673cb841f36cbc6cb6c4","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"674b5f0c6a4fc5d3abce877048290cea6091fcb1","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"7d9a9a1a88a3da574e019b4de756bc73337b3b0b","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"6defc77d48eff74075b80ad5925061b2fc010d98","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"9cdf299ba4e153b5e56187648420de22c6216f02","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"f648089337cb8ed40b2bb96e244f72b9d97dc96b","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"5fc177ab759418c9537433e63301096e733fb915","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/netfilter/nf_synproxy_core.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"3.12","status":"affected"},{"version":"0","status":"unaffected","lessThan":"3.12","versionType":"semver"},{"version":"4.4.274","status":"unaffected","lessThanOrEqual":"4.4.*","versionType":"semver"},{"version":"4.9.274","status":"unaffected","lessThanOrEqual":"4.9.*","versionType":"semver"},{"version":"4.14.238","status":"unaffected","lessThanOrEqual":"4.14.*","versionType":"semver"},{"version":"4.19.196","status":"unaffected","lessThanOrEqual":"4.19.*","versionType":"semver"},{"version":"5.4.128","status":"unaffected","lessThanOrEqual":"5.4.*","versionType":"semver"},{"version":"5.10.46","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.12.13","status":"unaffected","lessThanOrEqual":"5.12.*","versionType":"semver"},{"version":"5.13","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/netfilter/nf_synproxy_core.c"]}],"references":["https://git.kernel.org/stable/c/e1eb98cfeafdd85537e7e3cefe93ca9bfbcc3ea8","https://git.kernel.org/stable/c/576c1526b4d83c44ad7b673cb841f36cbc6cb6c4","https://git.kernel.org/stable/c/674b5f0c6a4fc5d3abce877048290cea6091fcb1","https://git.kernel.org/stable/c/7d9a9a1a88a3da574e019b4de756bc73337b3b0b","https://git.kernel.org/stable/c/6defc77d48eff74075b80ad5925061b2fc010d98","https://git.kernel.org/stable/c/9cdf299ba4e153b5e56187648420de22c6216f02","https://git.kernel.org/stable/c/f648089337cb8ed40b2bb96e244f72b9d97dc96b","https://git.kernel.org/stable/c/5fc177ab759418c9537433e63301096e733fb915"],"weaknesses":[],"ingestedAt":"2026-10-29T19:49:19.404Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2021/47xxx/CVE-2021-47245.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2021-47245","state":"PUBLISHED","datePublished":"2024-05-21T14:19:43.648Z","dateUpdated":"2026-08-05T08:46:56.392Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2024-04-10T18:59:19.533Z"},"containers":{"cna":{"title":"netfilter: synproxy: Fix out of bounds when parsing TCP options","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nnetfilter: synproxy: Fix out of bounds when parsing TCP options\n\nThe TCP option parser in synproxy (synproxy_parse_options) could read\none byte out of bounds. When the length is 1, the execution flow gets\ninto the loop, reads one byte of the opcode, and if the opcode is\nneither TCPOPT_EOL nor TCPOPT_NOP, it reads one more byte, which exceeds\nthe length of 1.\n\nThis fix is inspired by commit 9609dad263f8 (\"ipv4: tcp_input: fix stack\nout of bounds when parsing TCP options.\").\n\nv2 changes:\n\nAdded an early return when length < 0 to avoid calling\nskb_header_pointer with negative length."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"e1eb98cfeafdd85537e7e3cefe93ca9bfbcc3ea8","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"576c1526b4d83c44ad7b673cb841f36cbc6cb6c4","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"674b5f0c6a4fc5d3abce877048290cea6091fcb1","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"7d9a9a1a88a3da574e019b4de756bc73337b3b0b","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"6defc77d48eff74075b80ad5925061b2fc010d98","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"9cdf299ba4e153b5e56187648420de22c6216f02","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"f648089337cb8ed40b2bb96e244f72b9d97dc96b","versionType":"git"},{"version":"48b1de4c110a7afa4b85862f6c75af817db26fad","status":"affected","lessThan":"5fc177ab759418c9537433e63301096e733fb915","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/netfilter/nf_synproxy_core.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"3.12","status":"affected"},{"version":"0","status":"unaffected","lessThan":"3.12","versionType":"semver"},{"version":"4.4.274","status":"unaffected","lessThanOrEqual":"4.4.*","versionType":"semver"},{"version":"4.9.274","status":"unaffected","lessThanOrEqual":"4.9.*","versionType":"semver"},{"version":"4.14.238","status":"unaffected","lessThanOrEqual":"4.14.*","versionType":"semver"},{"version":"4.19.196","status":"unaffected","lessThanOrEqual":"4.19.*","versionType":"semver"},{"version":"5.4.128","status":"unaffected","lessThanOrEqual":"5.4.*","versionType":"semver"},{"version":"5.10.46","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.12.13","status":"unaffected","lessThanOrEqual":"5.12.*","versionType":"semver"},{"version":"5.13","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/netfilter/nf_synproxy_core.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/e1eb98cfeafdd85537e7e3cefe93ca9bfbcc3ea8"},{"url":"https://git.kernel.org/stable/c/576c1526b4d83c44ad7b673cb841f36cbc6cb6c4"},{"url":"https://git.kernel.org/stable/c/674b5f0c6a4fc5d3abce877048290cea6091fcb1"},{"url":"https://git.kernel.org/stable/c/7d9a9a1a88a3da574e019b4de756bc73337b3b0b"},{"url":"https://git.kernel.org/stable/c/6defc77d48eff74075b80ad5925061b2fc010d98"},{"url":"https://git.kernel.org/stable/c/9cdf299ba4e153b5e56187648420de22c6216f02"},{"url":"https://git.kernel.org/stable/c/f648089337cb8ed40b2bb96e244f72b9d97dc96b"},{"url":"https://git.kernel.org/stable/c/5fc177ab759418c9537433e63301096e733fb915"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H","baseScore":8.2,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:N - synproxy_parse_options runs on received TCP packets in ipt_SYNPROXY/nft_synproxy and ipv4/ipv6_synproxy_hook; SYNPROXY is deployed on internet-facing hosts to mitigate SYN floods, so a remote peer triggers it with crafted TCP options.\nAC:L - The attacker fully controls the TCP option bytes and can reliably produce length==1 (e.g., NOPs plus a non-EOL/NOP kind) so the parser reads one byte past the end of the option data; no race or external condition is required.\nPR:N - Exploitation needs only the ability to send TCP segments to a host where SYNPROXY is enabled; no authentication or local account is required on that network path.\nUI:N - The vulnerable parser runs automatically on matching inbound TCP packets; no victim user action is required.\nS:U - Impact stays within the same kernel/host security authority; there is no VM, IOMMU, or other cross-boundary escape.\nC:L - The bug is a strictly one-byte out-of-bounds read (stack buf[40] or past copied option bytes) into a local opsize; per guidance that is a limited/bounded read (C:L), not an arbitrary read primitive.\nI:N - This is a read-only OOB; 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