CVE-2026-59993: Picotls has NULL pointer dereference during handshake, when running as a client
Vulnerability summary
CVE-2026-59993 is a medium (CVSS 6.5) NULL Pointer Dereference vulnerability in h2o picotls. EPSS estimates a 0.25% chance of exploitation in the next 30 days, higher than 16.1% of scored vulnerabilities.
Not severe (CVSS 6.5, network access); no exploitation reported by the 5 checked sources
Source freshness unavailable
How is this decided?Timeline
- 30-10-2026PublishedVulnerability first disclosed.
Description
Picotls is a TLS protocol library that allows users select different crypto backends based on their use case. In affected revisions through 44bc503, when picotls is used as a `client-side TLS stack`, a peer can send a `HelloRetryRequest` followed by a `spoofed Hello message` that causes a null-pointer dereference. This can terminate the consuming process, including a process performing other work, resulting in denial of service. This issue is fixed in commit a06ca41.
CVSS metrics
2 scores · 2 sources · 1 version- NVDNIST6.5MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
EPSS trend
Probability
0.25%
Chance of exploitation in the next 30 days
Percentile
16%
Versus all other CVEs
30-day change
Change unavailable
Change unavailable
Known exploited
Compare KEV catalogs
CISA KEVNot in the CISA KEV catalog.
ENISA CNWNot in the ENISA CNW catalog.
CIRCLNot in the CIRCL catalog.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Known conditions: network-reachable, no authentication, user interaction (CVSS vector). Other conditions: not stated in public data.
H2o1 product
- picotls< a06ca417a16f9126808bf5cabe6c5e6e0f16ecae
1 vendor · 1 product
References
3 links- github.com/h2o/picotls/security/advisories/GHSA-2xcv-h2pg-m99cX Refsource CONFIRM
- github.com/h2o/picotls/pull/608X Refsource MISC