EUVD-2026-39557
GHSA-c9q2-5c67-xg9p
Severity by source
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Remote pre-auth reachable (AV:N/PR:N/UI:N), but requires building a large ACK list to overflow so AC:H; impact is mainly DoS (A:H) with limited memory corruption (I:L), no confidentiality.
Primary rating from Vendor (wolfSSL).
CVSS VectorVendor: wolfSSL
Lifecycle Timeline
2DescriptionCVE.org
A heap buffer overflow could occur in the DTLS 1.3 ACK serialization path before the connecting peer is authenticated. The buffer overflow was due to an integer truncation when computing the length of the ACK record-number list, causing an undersized buffer to be allocated and then overrun. This affects builds using DTLS 1.3 and wolfSSL version 5.9.0 and earlier. A fix was added to the 5.9.1 release.
AnalysisAI
Pre-authentication heap buffer overflow in wolfSSL 5.9.0 and earlier affects builds compiled with DTLS 1.3 support, where an integer truncation in the ACK record-number list length computation (Dtls13GetAckListLength using a word16) allocates an undersized buffer that is then overrun during ACK serialization. Because the flaw is reachable before the connecting peer is authenticated, a remote unauthenticated attacker can trigger memory corruption against a DTLS 1.3 endpoint, with no public exploit identified at time of analysis. The CVSS 4.0 score of 8.8 reflects primarily a high availability (crash/DoS) impact with limited integrity impact.
Technical ContextAI
wolfSSL is a compact, widely-embedded TLS/DTLS library written in C, commonly used in IoT, embedded firmware, automotive, and resource-constrained server deployments. The defect lives in the DTLS 1.3 retransmission/ACK handling code (src/dtls13.c, functions Dtls13RtxAddAck / Dtls13GetAckListLength). DTLS 1.3 acknowledges received record numbers via an ACK message containing a list of (epoch, sequence) record numbers; wolfSSL maintains this list and computes its serialized length. The root cause is CWE-787 (Out-of-bounds Write) stemming from an integer truncation: the cumulative length of the record-number list was computed/stored in a 16-bit (word16) quantity, so a sufficiently long list wraps the length value, an undersized heap buffer is allocated from the truncated length, and the subsequent serialization writes past the allocation. The fix introduces a DTLS13_MAX_ACK_RECORDS cap of 512 records, dropping additional records to prevent the word16 overflow and bound memory consumption.
RemediationAI
Vendor-released patch: upgrade to wolfSSL 5.9.1 or later, which adds the DTLS13_MAX_ACK_RECORDS cap (512) that prevents the word16 length truncation; rebuild and redeploy all firmware/applications that statically or dynamically link wolfSSL, since many embedded deployments bundle the library. The upstream fix is in https://github.com/wolfSSL/wolfssl/pull/10116 and is referenced from https://www.wolfssl.com/docs/security-vulnerabilities/. If you cannot immediately upgrade, the most direct compensating control is to disable DTLS 1.3 in the build (use DTLS 1.2 or compile out DTLS 1.3), which removes the vulnerable code path entirely at the cost of losing DTLS 1.3 features; alternatively, restrict reachability of the DTLS listener via network ACLs/firewalling of the relevant UDP port to trusted peers only, accepting that this does not help when the service must be exposed to untrusted clients. There is no application-level configuration toggle for the ACK list cap prior to 5.9.1, so patching is strongly preferred over workarounds.
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Same weakness CWE-787 – Out-of-bounds Write
View allSame technique Buffer Overflow
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External POC / Exploit Code
Leaving vuln.today