Analysis of Mozilla NSS BigSig Vulnerability & Fuzzing issues
(CVE-2021-43527)

Today I will like to discuss the Mozilla NSS BigSig vulnerability found recently by @taviso from Google Project Zero. Inside its blogpost, Tavis Ormandy detailed the vulnerability and mentioned some really good points about why this bug was not found using fuzzing by OSS-fuzz. It’s a really interesting case to analyze if you’re writing fuzzers because there is a lot of interesting lessons to learn from it.

NOTES

Discovered by @taviso

Tavis Ormandy @taviso found a Memory corruption via DER-encoded DSA and RSA-PSS signatures inside the NSS library used by Mozilla (BUT not in Firefox)

Note: This vulnerability does NOT impact Mozilla Firefox. However, email clients and PDF viewers that use NSS for signature verification, such as Thunderbird, LibreOffice, Evolution and Evince are believed to be impacted.

links:

Vulnerability (old school Buffer overflow)

Code source: lib/cryptohi/secvfy.c

mozilla nss BigSig buffer overflow google project zero fuzzer code

Bug fixed:

Current DER fuzzer

Code source: nss/fuzz/quickder_target.cc

mozilla nss BigSig buffer overflow google project zero fuzzer

Lessons Learned by @taviso

  • Missing end-to-end testing (Only focussed on decoding using `SEC_QuickDERDecodeItem`)
  • Arbitrary size limits. (fuzzer limitation)
  • Misleading metrics. (coverage of all fuzzers combined)

My takes and advises

No fuzzer are reproducing the behavior of `vfychain` program.

–> You should always convert your programs/unittests into fuzzing harnesses. 

 

Fuzzers are between 3 and 5 years old.

–> You should review, improve and extend your fuzzer every year (or after every major change)

 

No fuzzers was calling public verifying APIs with arbitrary data

–> When you’re writing/fuzzing a library, you should fuzz ALL the public APIs of the library (even if your main program are not using them, just think that someone else library might be or even yours in the future)

 

Limitation by OSS-FUZZ config restraint the fuzzers:

–> You should use multiple fuzzers and multiple fuzzer configs (some of them might not be configured the same way by default) – That’s why I’m always advising to fuzz with multiple fuzzers

Patrick Ventuzelo / @Pat_Ventuzelo

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