Raise ValueError when an SCT list is too large to encode - #15658
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encode_scts wrote the RFC 6962 list-length prefix with a truncating `as u16` cast, so an extension whose serialized SCTs totalled more than 65535 bytes was silently encoded with a wrapped length. The result is rejected by our own parser and by OpenSSL. Use u16::try_from for both the list and per-entry lengths and raise ValueError instead. Discovered by Anthropic's OSS Scanner, as vulnerability ANT-2026-H08T92Y2. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HVLJ4Wp5FraEfACovDdSN6
alex
commented
Sep 17, 2026
alex
commented
Sep 17, 2026
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HVLJ4Wp5FraEfACovDdSN6
alex
commented
Sep 17, 2026
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HVLJ4Wp5FraEfACovDdSN6
The total-length check already bounds every entry, and the branch could never be hit, which failed the 100% coverage requirement. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HVLJ4Wp5FraEfACovDdSN6
alex
commented
Sep 17, 2026
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HVLJ4Wp5FraEfACovDdSN6
reaperhulk
approved these changes
Sep 19, 2026
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encode_sctswrote the RFC 6962SignedCertificateTimestampListlength prefix with a truncatinglength as u16cast. An extension whose serialized SCTs total more than 65535 bytes (roughly 550 typical SCTs) was therefore silently encoded with a length wrapped modulo 65536:public_bytes()andCertificateBuilder.sign()succeeded, but the result is rejected by our own parser (ValueError: Invalid SCT length) and by OpenSSL.A list that size has no valid encoding (
sct_list<1..2^16-1>), so the list length now goes throughu16::try_fromand raisesValueError. The per-entry length is bounded by that check and usestry_into().unwrap(). The regression test repeats the SCT frombadssl-sct.pem1000 times and checks thatpublic_bytes()raises for both extension classes.Discovered by Anthropic's OSS Scanner, as vulnerability ANT-2026-H08T92Y2.
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https://claude.ai/code/session_01HVLJ4Wp5FraEfACovDdSN6