LiveActive security incident?Get immediate response
CVE Record

CVE-2021-4442: tcp: add sanity tests to TCP_QUEUE_SEQ

In the Linux kernel, the following vulnerability has been resolved: tcp: add sanity tests to TCP_QUEUE_SEQ Qingyu Li reported a syzkaller bug where the repro changes RCV SEQ _after_ restoring data in the receive queue. mprotect(0x4aa000, 12288, PROT_READ) = 0 mmap(0x1ffff000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x1ffff000 mmap(0x20000000, 16777216, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x20000000 mmap(0x21000000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x21000000 socket(AF_INET6, SOCK_STREAM, IPPROTO_IP) = 3 setsockopt(3, SOL_TCP, TCP_REPAIR, [1], 4) = 0 connect(3, {sa_family=AF_INET6, sin6_port=htons(0), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_scope_id=0}, 28) = 0 setsockopt(3, SOL_TCP, TCP_REPAIR_QUEUE, [1], 4) = 0 sendmsg(3, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="0x0000000000000003\0\0", iov_len=20}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 20 setsockopt(3, SOL_TCP, TCP_REPAIR, [0], 4) = 0 setsockopt(3, SOL_TCP, TCP_QUEUE_SEQ, [128], 4) = 0 recvfrom(3, NULL, 20, 0, NULL, NULL) = -1 ECONNRESET (Connection reset by peer) syslog shows: [ 111.205099] TCP recvmsg seq # bug 2: copied 80, seq 0, rcvnxt 80, fl 0 [ 111.207894] WARNING: CPU: 1 PID: 356 at net/ipv4/tcp.c:2343 tcp_recvmsg_locked+0x90e/0x29a0 This should not be allowed. TCP_QUEUE_SEQ should only be used when queues are empty. This patch fixes this case, and the tx path as well.

UnknownCVSS not scoredNot KEV-listedUpdated
Discuss exposureBack to CVEs
Glexia's TakeAutomated analysisunknown

Security readout for executives and security teams

Plain-English summary

CVE-2021-4442 is a Linux kernel TCP repair bug. The kernel allowed TCP_QUEUE_SEQ to be changed after data was restored into TCP queues, which could trigger a TCP receive-path warning and connection reset. The source shows a kernel fix, but does not provide CVSS, business impact, or evidence of active exploitation.

Executive priority

Treat this as a kernel maintenance issue unless local untrusted workloads are common in your environment. There is no sourced evidence of active exploitation, but kernel defects can have broad operational impact. Patch through normal Linux update channels with higher priority for shared compute hosts.

Technical view

The flaw is in Linux TCP handling around TCP_QUEUE_SEQ and TCP_REPAIR. The fix adds sanity checks so TCP_QUEUE_SEQ is only accepted when relevant queues are empty, covering receive and transmit paths. The reported reproducer came from syzkaller and triggered tcp_recvmsg_locked warning behavior.

Likely exposure

Exposure applies to Linux systems running affected kernel versions or branches before the referenced stable fixes. The bundle lists Linux as the affected product but does not clearly express full version range semantics. Risk is most relevant where local code can reach TCP repair socket options.

Exploitation context

The bundle provides a syzkaller-derived local reproducer context and kernel warning output. It does not cite remote exploitation, privilege escalation, data theft, KEV listing, public weaponization, or active exploitation in the wild.

Researcher notes

The source evidence is narrow: a resolved upstream Linux TCP bug, syzkaller context, and stable commit references. No CVSS, CWE, exploit maturity, or precise downstream package mapping is provided. Validate affected ranges through distribution advisories before declaring exposure closed.

Mitigation direction

  • Update affected Linux kernels to vendor-supported builds containing the referenced stable fixes.
  • Prioritize systems that allow untrusted local workloads or containerized code.
  • Check Linux distribution advisories for exact fixed package versions.
  • Avoid relying on unsupported kernels with unclear backport status.

Validation and detection

  • Inventory Linux kernel versions across servers, endpoints, and container hosts.
  • Map running kernels to vendor advisories or referenced upstream stable commits.
  • Confirm patched builds include TCP_QUEUE_SEQ sanity checks.
  • Review exposure of systems running untrusted local code.
Prepared
Confidence
medium
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cve · low confidence lookup

CVE-2021-4442 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
1ADP providers
6Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxee9952831cfd0bbe834f4a26489d7dce74582e37, ee9952831cfd0bbe834f4a26489d7dce74582e37, ee9952831cfd0bbe834f4a26489d7dce74582e37, ee9952831cfd0bbe834f4a26489d7dce74582e37, ee9952831cfd0bbe834f4a26489d7dce74582e37unaffected
LinuxLinux3.5, 0, 4.19.181, 5.4.106, 5.10.24, 5.11.7, 5.12affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.