CVE-2026-71884
HighCVSS 8.2Exploitation Probability (EPSS)
Low risk34th percentile - higher than 34% of all known CVEs
Summary
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 left by the IV. With a 13- to 15-byte IV the counter is only 1 to 3 bytes, so for longer input the counter wrapped and the keystream repeated from the start of the same packet. The call still returned the full input length as though every byte had been correctly transformed, with no exception and no short length.
Risk Assessment
Two segments of the message are encrypted under the same keystream, so their plaintexts can be recovered from the ciphertext alone without the key. The caller receives no error indication and may wrongly assume the data was safely encrypted.
Recommendation
Upgrade Bouncy Castle for Java LTS to version 2.73.13 or later. Until then, avoid the native one-shot CTR path with IVs shorter than 16 bytes or use the streaming implementation, which rejects such requests.
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Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS