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CVE-2026-71884

BC-LTS-JAVA: Packet CTR reports a full success length after the counter is exhausted

Legion of the Bouncy Castle Inc. BC-LTS-JAVA
CVSS
8.2High
ExploitationNo known exploitationNot in CISA KEV, which does not prove it is unexploitedCISA KEV
Exploit odds (EPSS)0.41%Chance of exploitation activity in the next 30 days, higher than 34% of CVEs. Scored 23 Oct 2026FIRST EPSS
Fixed in2.73.13Fixed versionCVE List
Published9 Oct 2026Updated 15 Oct 2026CVE List

What fixes it

A fixed version on one release line is not the upgrade target for every line. Check your exact version and any vendor backports.

Description

In Bouncy Castle for Java LTS before 2.73.13, the native one-shot CTR packet cipher did not check that the requested input length fitted the counter space the IV left. In CTR mode the IV and the block counter share one 16-byte block, so an IV of 13 to 15 bytes leaves a counter of only 1 to 3 bytes, addressing 256, 65536 or 16777216 blocks respectively. Given a longer input the counter wrapped and the keystream repeated from the start of the same packet, and the call then returned the full input length as though every byte had been correctly transformed. Two segments of the message were therefore encrypted under the same keystream, so their plaintexts can be recovered from the ciphertext alone, without the key, while the caller saw neither an exception nor a short length to indicate it. The streaming implementation validates at init and again while processing, and the portable AESCTRPacketCipher rejects such a request with "Counter in CTR/SIC mode out of range.", but the native one-shot path has a single entry point and performed no counter-range validation there. It now preflights the IV-derived counter range and rejects an over-long request before any output is written, so the operation is failure-atomic and never reports success for bytes it did not correctly transform. A counter of four bytes or more cannot be exhausted by a Java int length and is unaffected, as is a full 16-byte IV, where the counter range is the caller's responsibility. Bouncy Castle for Java (bcprov) is not affected, as it ships no native implementations.

Text from CVE List

Affected versions 1

  • BC-LTS-JAVAaffected: 2.73.4 before 2.73.13CVE List

References 1