Taiwan's TWCA reissues certificates after a serial-number finding

Taiwan's TWCA reissues certificates after a serial-number finding

3 November 2026: TWCA, the Taiwanese certificate authority whose TLS certificates secure government agencies, banks and businesses across the island, is replacing its customers' website certificates and chains after researchers showed that the serial numbers of its CA certificates carry only about 24 random bits. The CA/Browser Forum has required at least 64 since 2016. TWCA says the new certificates must be in place before midnight on 6 November.

Not yet independently verified. The Ministry of Digital Affairs figures (962 agencies, 9,772 certificates, 926 updated) come from iThome's report; we did not find them on a ministry page. TWCA's full incident report to Mozilla is due by 12 November. We will update this when it can be confirmed, and remove this note.

What the researchers found

The finding was filed on Mozilla's Bugzilla on 30 October by two outside researchers. TWCA's CA serial numbers are 128 bits long, but they follow a fixed layout of a date, padding, a counter and a few random bytes, so at most 24 bits come from a cryptographically secure random generator. The same structure appears in 24 CA certificates issued between June 2011 and September 2024, 18 of them after the 64-bit rule took effect on 30 September 2016.

The researchers counted 10 unrevoked, currently trusted intermediate CA certificates and two self-signed roots (TWCA CYBER Root CA and TWCA Global Root CA G2) with the structure. Eight of the ten are trusted by both Mozilla and Chrome: six issue TLS certificates and two are cross-certificates. Two S/MIME issuing CAs are trusted by Mozilla only. They also checked 1,631 subscriber TLS certificates and found no problem with their serials, and they stress that there is no evidence of key compromise, misuse or exploitation.

A second Bugzilla report, filed since, says TWCA has found subscriber certificate serials with 63 rather than 64 bits; TWCA is still working out how many certificates that covers.

Why the 2016 fix missed the CA certificates

TWCA vice president Chou Fang-kuan told iThome that in 2016 the company ran two signing systems, one for subscriber certificates and one for CA certificates, and only the subscriber code was changed to meet the 64-bit rule. A serial that looks long enough on a single certificate only shows its structure when many are compared, which he said is why internal checks missed it.

TWCA's own notice says the change is about how serial numbers are generated, not a breach or a leaked private key. Existing certificates keep working until the deadline, but customers have to install the reissued server certificate and any new intermediate certificate before 6 November, then test their sites. TWCA emailed affected customers on the evening of 1 November with the new chains and deployment notes.

Government sites and the 6 November line

According to iThome, the Ministry of Digital Affairs was alerted on 30 October and gave TWCA two days to produce compliant certificates. The ministry buys TWCA certificates for government use, and 962 agencies hold 9,772 of them. TWCA reissued those on 1 November and emailed them to the officers who applied, so agencies do not need to reapply; by the evening of 2 November, 926 had been installed. The ministry says sites still on the old certificates after 6 November may show browser security warnings, and it will commission an outside review of TWCA's issuing process and look at penalties under its contract.

Chou told iThome on the morning of 3 November that about a fifth of government and business customers had completed the update by TWCA's own tracking. Anyone running a public site, an API endpoint or a device with a TWCA certificate should check the chain it serves today, not only the leaf certificate, since it is the intermediates that are being replaced.

Company profile on TrustList: TWCA

Sources

• Mozilla Bugzilla 2087236: TWCA, CA certificate serial numbers contain fewer than 64 bits of CSPRNG output, 30 October 2026
• TWCA: website certificate specification upgrade and reissue (Chinese), 2 November 2026
• iThome: TWCA CA certificate serial entropy disclosed on Mozilla Bugzilla, Ministry of Digital Affairs orders reissue (Chinese), 3 November 2026