CVE-2026-71884 Packet CTR reports a full success length after the counter is exhausted
Attackers can recover plaintext from ciphertext when using native CTR mode with short IVs due to counter exhaustion, affecting Bouncy Castle LTS users.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-71884?
CVE-2026-71884 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. BC-LTS-JAVA, published on 9 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-323
Which versions are affected by CVE-2026-71884?
How do I fix CVE-2026-71884?
No fix has been published for CVE-2026-71884 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-71884 being exploited?
There is no known exploitation of CVE-2026-71884 in the wild and no public exploit code.
Sources
Also known as GHSA-qgfp-x9w9-pwj2.